JAVA(1) JDK Commands JAVA(1)
NAME
java - launch a Java application
SYNOPSIS
To launch a class file:
java [options] mainclass [args ...]
To launch the main class in a JAR file:
java [options] -jar jarfile [args ...]
To launch the main class in a module:
java [options] -m module[/mainclass] [args ...]
or
java [options] --module module[/mainclass] [args ...]
To launch a single source-file program:
java [options] source-file [args ...]
options
Optional: Specifies command-line options separated by spaces.
See Overview of Java Options for a description of available op-
tions.
mainclass
Specifies the name of the class to be launched. Command-line
entries following classname are the arguments for the main meth-
od.
-jar jarfile
Executes a program encapsulated in a JAR file. The jarfile ar-
gument is the name of a JAR file with a manifest that contains a
line in the form Main-Class:classname that defines the class
with the public static void main(String[] args) method that
serves as your application's starting point. When you use -jar,
the specified JAR file is the source of all user classes, and
other class path settings are ignored. If you're using JAR
files, then see jar.
-m or --module module[/mainclass]
Executes the main class in a module specified by mainclass if it
is given, or, if it is not given, the value in the module. In
other words, mainclass can be used when it is not specified by
the module, or to override the value when it is specified.
See Standard Options for Java.
source-file
Only used to launch a single source-file program. Specifies the
source file that contains the main class when using source-file
mode. See Using Source-File Mode to Launch Single-File
Source-Code Programs
args ...
Optional: Arguments following mainclass, source-file, -jar
jarfile, and -m or --module module/mainclass are passed as argu-
ments to the main class.
DESCRIPTION
The java command starts a Java application. It does this by starting
the Java Virtual Machine (JVM), loading the specified class, and call-
ing that class's main() method. The method must be declared public and
static, it must not return any value, and it must accept a String array
as a parameter. The method declaration has the following form:
public static void main(String[] args)
In source-file mode, the java command can launch a class declared in a
source file. See Using Source-File Mode to Launch Single-File
Source-Code Programs for a description of using the source-file mode.
Note: You can use the JDK_JAVA_OPTIONS launcher environment
variable to prepend its content to the actual command line of
the java launcher. See Using the JDK_JAVA_OPTIONS Launcher En-
vironment Variable.
By default, the first argument that isn't an option of the java command
is the fully qualified name of the class to be called. If -jar is
specified, then its argument is the name of the JAR file containing
class and resource files for the application. The startup class must
be indicated by the Main-Class manifest header in its manifest file.
Arguments after the class file name or the JAR file name are passed to
the main() method.
javaw
Windows: The javaw command is identical to java, except that with javaw
there's no associated console window. Use javaw when you don't want a
command prompt window to appear. The javaw launcher will, however,
display a dialog box with error information if a launch fails.
USING SOURCE-FILE MODE TO LAUNCH SINGLE-FILE SOURCE-CODE PROGRAMS
To launch a class declared in a source file, run the java launcher in
source-file mode. Entering source-file mode is determined by two items
on the java command line:
• The first item on the command line that is not an option or part of
an option. In other words, the item in the command line that would
otherwise be the main class name.
• The --source version option, if present.
If the class identifies an existing file that has a .java extension, or
if the --source option is specified, then source-file mode is selected.
The source file is then compiled and run. The --source option can be
used to specify the source version or N of the source code. This de-
termines the API that can be used. When you set --source N, you can
only use the public API that was defined in JDK N.
Note: The valid values of N change for each release, with new
values added and old values removed. You'll get an error mes-
sage if you use a value of N that is no longer supported. The
supported values of N are the current Java SE release (17) and a
limited number of previous releases, detailed in the com-
mand-line help for javac, under the --source and --release op-
tions.
If the file does not have the .java extension, the --source option must
be used to tell the java command to use the source-file mode. The
--source option is used for cases when the source file is a "script" to
be executed and the name of the source file does not follow the normal
naming conventions for Java source files.
In source-file mode, the effect is as though the source file is com-
piled into memory, and the first class found in the source file is exe-
cuted. Any arguments placed after the name of the source file in the
original command line are passed to the compiled class when it is exe-
cuted.
For example, if a file were named HelloWorld.java and contained a class
named hello.World, then the source-file mode command to launch the
class would be:
java HelloWorld.java
The example illustrates that the class can be in a named package, and
does not need to be in the unnamed package. This use of source-file
mode is informally equivalent to using the following two commands where
hello.World is the name of the class in the package:
javac -d <memory> HelloWorld.java
java -cp <memory> hello.World
In source-file mode, any additional command-line options are processed
as follows:
• The launcher scans the options specified before the source file for
any that are relevant in order to compile the source file.
This includes: --class-path, --module-path, --add-exports, --add-mod-
ules, --limit-modules, --patch-module, --upgrade-module-path, and any
variant forms of those options. It also includes the new --en-
able-preview option, described in JEP 12.
• No provision is made to pass any additional options to the compiler,
such as -processor or -Werror.
• Command-line argument files (@-files) may be used in the standard
way. Long lists of arguments for either the VM or the program being
invoked may be placed in files specified on the command-line by pre-
fixing the filename with an @ character.
In source-file mode, compilation proceeds as follows:
• Any command-line options that are relevant to the compilation envi-
ronment are taken into account.
• No other source files are found and compiled, as if the source path
is set to an empty value.
• Annotation processing is disabled, as if -proc:none is in effect.
• If a version is specified, via the --source option, the value is used
as the argument for an implicit --release option for the compilation.
This sets both the source version accepted by compiler and the system
API that may be used by the code in the source file.
• The source file is compiled in the context of an unnamed module.
• The source file should contain one or more top-level classes, the
first of which is taken as the class to be executed.
• The compiler does not enforce the optional restriction defined at the
end of JLS ??7.6, that a type in a named package should exist in a
file whose name is composed from the type name followed by the .java
extension.
• If the source file contains errors, appropriate error messages are
written to the standard error stream, and the launcher exits with a
non-zero exit code.
In source-file mode, execution proceeds as follows:
• The class to be executed is the first top-level class found in the
source file. It must contain a declaration of the standard pub-
lic static void main(String[]) method.
• The compiled classes are loaded by a custom class loader, that dele-
gates to the application class loader. This implies that classes ap-
pearing on the application class path cannot refer to any classes de-
clared in the source file.
• The compiled classes are executed in the context of an unnamed mod-
ule, as though --add-modules=ALL-DEFAULT is in effect. This is in
addition to any other --add-module options that may be have been
specified on the command line.
• Any arguments appearing after the name of the file on the command
line are passed to the standard main method in the obvious way.
• It is an error if there is a class on the application class path
whose name is the same as that of the class to be executed.
See JEP 330: Launch Single-File Source-Code Programs [http://open-
jdk.java.net/jeps/330] for complete details.
USING THE JDK_JAVA_OPTIONS LAUNCHER ENVIRONMENT VARIABLE
JDK_JAVA_OPTIONS prepends its content to the options parsed from the
command line. The content of the JDK_JAVA_OPTIONS environment variable
is a list of arguments separated by white-space characters (as deter-
mined by isspace()). These are prepended to the command line arguments
passed to java launcher. The encoding requirement for the environment
variable is the same as the java command line on the system. JDK_JA-
VA_OPTIONS environment variable content is treated in the same manner
as that specified in the command line.
Single (') or double (") quotes can be used to enclose arguments that
contain whitespace characters. All content between the open quote and
the first matching close quote are preserved by simply removing the
pair of quotes. In case a matching quote is not found, the launcher
will abort with an error message. @-files are supported as they are
specified in the command line. However, as in @-files, use of a wild-
card is not supported. In order to mitigate potential misuse of
JDK_JAVA_OPTIONS behavior, options that specify the main class (such as
-jar) or cause the java launcher to exit without executing the main
class (such as -h) are disallowed in the environment variable. If any
of these options appear in the environment variable, the launcher will
abort with an error message. When JDK_JAVA_OPTIONS is set, the launch-
er prints a message to stderr as a reminder.
Example:
$ export JDK_JAVA_OPTIONS='-g @file1 -Dprop=value @file2 -Dws.prop="white spaces"'
$ java -Xint @file3
is equivalent to the command line:
java -g @file1 -Dprop=value @file2 -Dws.prop="white spaces" -Xint @file3
OVERVIEW OF JAVA OPTIONS
The java command supports a wide range of options in the following cat-
egories:
• Standard Options for Java: Options guaranteed to be supported by all
implementations of the Java Virtual Machine (JVM). They're used for
common actions, such as checking the version of the JRE, setting the
class path, enabling verbose output, and so on.
• Extra Options for Java: General purpose options that are specific to
the Java HotSpot Virtual Machine. They aren't guaranteed to be sup-
ported by all JVM implementations, and are subject to change. These
options start with -X.
The advanced options aren't recommended for casual use. These are de-
veloper options used for tuning specific areas of the Java HotSpot Vir-
tual Machine operation that often have specific system requirements and
may require privileged access to system configuration parameters. Sev-
eral examples of performance tuning are provided in Performance Tuning
Examples. These options aren't guaranteed to be supported by all JVM
implementations and are subject to change. Advanced options start with
-XX.
• Advanced Runtime Options for Java: Control the runtime behavior of
the Java HotSpot VM.
• Advanced JIT Compiler Options for java: Control the dynamic
just-in-time (JIT) compilation performed by the Java HotSpot VM.
• Advanced Serviceability Options for Java: Enable gathering system in-
formation and performing extensive debugging.
• Advanced Garbage Collection Options for Java: Control how garbage
collection (GC) is performed by the Java HotSpot
Boolean options are used to either enable a feature that's disabled by
default or disable a feature that's enabled by default. Such options
don't require a parameter. Boolean -XX options are enabled using the
plus sign (-XX:+OptionName) and disabled using the minus sign (-XX:-Op-
tionName).
For options that require an argument, the argument may be separated
from the option name by a space, a colon (:), or an equal sign (=), or
the argument may directly follow the option (the exact syntax differs
for each option). If you're expected to specify the size in bytes,
then you can use no suffix, or use the suffix k or K for kilobytes
(KB), m or M for megabytes (MB), or g or G for gigabytes (GB). For ex-
ample, to set the size to 8 GB, you can specify either 8g, 8192m,
8388608k, or 8589934592 as the argument. If you are expected to speci-
fy the percentage, then use a number from 0 to 1. For example, specify
0.25 for 25%.
The following sections describe the options that are obsolete, depre-
cated, and removed:
• Deprecated Java Options: Accepted and acted upon --- a warning is is-
sued when they're used.
• Obsolete Java Options: Accepted but ignored --- a warning is issued
when they're used.
• Removed Java Options: Removed --- using them results in an error.
STANDARD OPTIONS FOR JAVA
These are the most commonly used options supported by all implementa-
tions of the JVM.
Note: To specify an argument for a long option, you can use ei-
ther --name=value or --name value.
-agentlib:libname[=options]
Loads the specified native agent library. After the library
name, a comma-separated list of options specific to the library
can be used.
• Linux and macOS: If the option -agentlib:foo is specified,
then the JVM attempts to load the library named libfoo.so in
the location specified by the LD_LIBRARY_PATH system variable
(on macOS this variable is DYLD_LIBRARY_PATH).
• Windows: If the option -agentlib:foo is specified, then the
JVM attempts to load the library named foo.dll in the location
specified by the PATH system variable.
The following example shows how to load the Java Debug Wire
Protocol (JDWP) library and listen for the socket connection
on port 8000, suspending the JVM before the main class loads:
-agentlib:jdwp=transport=dt_socket,server=y,ad-
dress=8000
-agentpath:pathname[=options]
Loads the native agent library specified by the absolute path
name. This option is equivalent to -agentlib but uses the full
path and file name of the library.
--class-path classpath, -classpath classpath, or -cp classpath
A semicolon (;) separated list of directories, JAR archives, and
ZIP archives to search for class files.
Specifying classpath overrides any setting of the CLASSPATH en-
vironment variable. If the class path option isn't used and
classpath isn't set, then the user class path consists of the
current directory (.).
As a special convenience, a class path element that contains a
base name of an asterisk (*) is considered equivalent to speci-
fying a list of all the files in the directory with the exten-
sion .jar or .JAR . A Java program can't tell the difference
between the two invocations. For example, if the directory my-
dir contains a.jar and b.JAR, then the class path element my-
dir/* is expanded to A.jar:b.JAR, except that the order of JAR
files is unspecified. All .jar files in the specified directo-
ry, even hidden ones, are included in the list. A class path
entry consisting of an asterisk (*) expands to a list of all the
jar files in the current directory. The CLASSPATH environment
variable, where defined, is similarly expanded. Any class path
wildcard expansion that occurs before the Java VM is started.
Java programs never see wildcards that aren't expanded except by
querying the environment, such as by calling Sys-
tem.getenv("CLASSPATH").
--disable-@files
Can be used anywhere on the command line, including in an argu-
ment file, to prevent further @filename expansion. This option
stops expanding @-argfiles after the option.
--enable-preview
Allows classes to depend on preview features [https://docs.ora-
cle.com/en/java/javase/12/language/index.html#JS-
LAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823] of the release.
--module-path modulepath... or -p modulepath
A semicolon (;) separated list of directories in which each di-
rectory is a directory of modules.
--upgrade-module-path modulepath...
A semicolon (;) separated list of directories in which each di-
rectory is a directory of modules that replace upgradeable mod-
ules in the runtime image.
--add-modules module[,module...]
Specifies the root modules to resolve in addition to the initial
module. module also can be ALL-DEFAULT, ALL-SYSTEM, and
ALL-MODULE-PATH.
--list-modules
Lists the observable modules and then exits.
-d module_name or --describe-module module_name
Describes a specified module and then exits.
--dry-run
Creates the VM but doesn't execute the main method. This
--dry-run option might be useful for validating the command-line
options such as the module system configuration.
--validate-modules
Validates all modules and exit. This option is helpful for
finding conflicts and other errors with modules on the module
path.
-Dproperty=value
Sets a system property value. The property variable is a string
with no spaces that represents the name of the property. The
value variable is a string that represents the value of the
property. If value is a string with spaces, then enclose it in
quotation marks (for example -Dfoo="foo bar").
-disableassertions[:[packagename]...|:classname] or -da[:[package-
name]...|:classname]
Disables assertions. By default, assertions are disabled in all
packages and classes. With no arguments, -disableassertions
(-da) disables assertions in all packages and classes. With the
packagename argument ending in ..., the switch disables asser-
tions in the specified package and any subpackages. If the ar-
gument is simply ..., then the switch disables assertions in the
unnamed package in the current working directory. With the
classname argument, the switch disables assertions in the speci-
fied class.
The -disableassertions (-da) option applies to all class loaders
and to system classes (which don't have a class loader).
There's one exception to this rule: If the option is provided
with no arguments, then it doesn't apply to system classes.
This makes it easy to disable assertions in all classes except
for system classes. The -disablesystemassertions option enables
you to disable assertions in all system classes. To explicitly
enable assertions in specific packages or classes, use the -en-
ableassertions (-ea) option. Both options can be used at the
same time. For example, to run the MyClass application with as-
sertions enabled in the package com.wombat.fruitbat (and any
subpackages) but disabled in the class com.wombat.fruit-
bat.Brickbat, use the following command:
java -ea:com.wombat.fruitbat... -da:com.wombat.fruit-
bat.Brickbat MyClass
-disablesystemassertions or -dsa
Disables assertions in all system classes.
-enableassertions[:[packagename]...|:classname] or -ea[:[package-
name]...|:classname]
Enables assertions. By default, assertions are disabled in all
packages and classes. With no arguments, -enableassertions
(-ea) enables assertions in all packages and classes. With the
packagename argument ending in ..., the switch enables asser-
tions in the specified package and any subpackages. If the ar-
gument is simply ..., then the switch enables assertions in the
unnamed package in the current working directory. With the
classname argument, the switch enables assertions in the speci-
fied class.
The -enableassertions (-ea) option applies to all class loaders
and to system classes (which don't have a class loader).
There's one exception to this rule: If the option is provided
with no arguments, then it doesn't apply to system classes.
This makes it easy to enable assertions in all classes except
for system classes. The -enablesystemassertions option provides
a separate switch to enable assertions in all system classes.
To explicitly disable assertions in specific packages or class-
es, use the -disableassertions (-da) option. If a single com-
mand contains multiple instances of these switches, then they're
processed in order, before loading any classes. For example, to
run the MyClass application with assertions enabled only in the
package com.wombat.fruitbat (and any subpackages) but disabled
in the class com.wombat.fruitbat.Brickbat, use the following
command:
java -ea:com.wombat.fruitbat... -da:com.wombat.fruit-
bat.Brickbat MyClass
-enablesystemassertions or -esa
Enables assertions in all system classes.
-help, -h, or -?
Prints the help message to the error stream.
--help Prints the help message to the output stream.
-javaagent:jarpath[=options]
Loads the specified Java programming language agent. See ja-
va.lang.instrument.
--show-version
Prints the product version to the output stream and continues.
-showversion
Prints the product version to the error stream and continues.
--show-module-resolution
Shows module resolution output during startup.
-splash:imagepath
Shows the splash screen with the image specified by imagepath.
HiDPI scaled images are automatically supported and used if
available. The unscaled image file name, such as image.ext,
should always be passed as the argument to the -splash option.
The most appropriate scaled image provided is picked up automat-
ically.
For example, to show the splash.gif file from the images direc-
tory when starting your application, use the following option:
-splash:images/splash.gif
See the SplashScreen API documentation for more information.
-verbose:class
Displays information about each loaded class.
-verbose:gc
Displays information about each garbage collection (GC) event.
-verbose:jni
Displays information about the use of native methods and other
Java Native Interface (JNI) activity.
-verbose:module
Displays information about the modules in use.
--version
Prints product version to the output stream and exits.
-version
Prints product version to the error stream and exits.
-X Prints the help on extra options to the error stream.
--help-extra
Prints the help on extra options to the output stream.
@argfile
Specifies one or more argument files prefixed by @ used by the
java command. It isn't uncommon for the java command line to be
very long because of the .jar files needed in the classpath.
The @argfile option overcomes command-line length limitations by
enabling the launcher to expand the contents of argument files
after shell expansion, but before argument processing. Contents
in the argument files are expanded because otherwise, they would
be specified on the command line until the --disable-@files op-
tion was encountered.
The argument files can also contain the main class name and all
options. If an argument file contains all of the options re-
quired by the java command, then the command line could simply
be:
java @argfile
See java Command-Line Argument Files for a description and exam-
ples of using @-argfiles.
EXTRA OPTIONS FOR JAVA
The following java options are general purpose options that are specif-
ic to the Java HotSpot Virtual Machine.
-Xbatch
Disables background compilation. By default, the JVM compiles
the method as a background task, running the method in inter-
preter mode until the background compilation is finished. The
-Xbatch flag disables background compilation so that compilation
of all methods proceeds as a foreground task until completed.
This option is equivalent to -XX:-BackgroundCompilation.
-Xbootclasspath/a:directories|zip|JAR-files
Specifies a list of directories, JAR files, and ZIP archives to
append to the end of the default bootstrap class path.
Linux and macOS: Colons (:) separate entities in this list.
Windows: Semicolons (;) separate entities in this list.
-Xcheck:jni
Performs additional checks for Java Native Interface (JNI) func-
tions.
The following checks are considered indicative of significant
problems with the native code, and the JVM terminates with an
irrecoverable error in such cases:
• The thread doing the call is not attached to the JVM.
• The thread doing the call is using the JNIEnv belonging to an-
other thread.
• A parameter validation check fails:
• A jfieldID, or jmethodID, is detected as being invalid. For
example:
• Of the wrong type
• Associated with the wrong class
• A parameter of the wrong type is detected.
• An invalid parameter value is detected. For example:
• NULL where not permitted
• An out-of-bounds array index, or frame capacity
• A non-UTF-8 string
• An invalid JNI reference
• An attempt to use a ReleaseXXX function on a parameter not
produced by the corresponding GetXXX function
The following checks only result in warnings being printed:
• A JNI call was made without checking for a pending exception
from a previous JNI call, and the current call is not safe
when an exception may be pending.
• The number of JNI local references existing when a JNI func-
tion terminates exceeds the number guaranteed to be available.
See the EnsureLocalcapacity function.
• A class descriptor is in decorated format (Lname;) when it
should not be.
• A NULL parameter is allowed, but its use is questionable.
• Calling other JNI functions in the scope of Get/ReleasePrimi-
tiveArrayCritical or Get/ReleaseStringCritical
Expect a performance degradation when this option is used.
-Xdebug
Does nothing. Provided for backward compatibility.
-Xdiag Shows additional diagnostic messages.
-Xint Runs the application in interpreted-only mode. Compilation to
native code is disabled, and all bytecode is executed by the in-
terpreter. The performance benefits offered by the just-in-time
(JIT) compiler aren't present in this mode.
-Xinternalversion
Displays more detailed JVM version information than the -version
option, and then exits.
-Xlog:option
Configure or enable logging with the Java Virtual Machine (JVM)
unified logging framework. See Enable Logging with the JVM Uni-
fied Logging Framework.
-Xmixed
Executes all bytecode by the interpreter except for hot methods,
which are compiled to native code. On by default. Use -Xint to
switch off.
-Xmn size
Sets the initial and maximum size (in bytes) of the heap for the
young generation (nursery) in the generational collectors. Ap-
pend the letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. The young genera-
tion region of the heap is used for new objects. GC is per-
formed in this region more often than in other regions. If the
size for the young generation is too small, then a lot of minor
garbage collections are performed. If the size is too large,
then only full garbage collections are performed, which can take
a long time to complete. It is recommended that you do not set
the size for the young generation for the G1 collector, and keep
the size for the young generation greater than 25% and less than
50% of the overall heap size for other collectors. The follow-
ing examples show how to set the initial and maximum size of
young generation to 256 MB using various units:
-Xmn256m
-Xmn262144k
-Xmn268435456
Instead of the -Xmn option to set both the initial and maximum
size of the heap for the young generation, you can use -XX:New-
Size to set the initial size and -XX:MaxNewSize to set the maxi-
mum size.
-Xms size
Sets the minimum and initial size (in bytes) of the heap. This
value must be a multiple of 1024 and greater than 1 MB. Append
the letter k or K to indicate kilobytes, m or M to indicate
megabytes, g or G to indicate gigabytes. The following examples
show how to set the size of allocated memory to 6 MB using vari-
ous units:
-Xms6291456
-Xms6144k
-Xms6m
Instead of the -Xms option to set both the minimum and initial
size of the heap, you can use -XX:MinHeapSize to set the minimum
size and -XX:InitialHeapSize to set the initial size.
If you don't set this option, the initial size is set as the sum
of the sizes allocated for the old generation and the young gen-
eration. The initial size of the heap for the young generation
can be set using the -Xmn option or the -XX:NewSize option.
-Xmx size
Specifies the maximum size (in bytes) of the heap. This value
must be a multiple of 1024 and greater than 2 MB. Append the
letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. The default value
is chosen at runtime based on system configuration. For server
deployments, -Xms and -Xmx are often set to the same value. The
following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
-Xmx83886080
-Xmx81920k
-Xmx80m
The -Xmx option is equivalent to -XX:MaxHeapSize.
-Xnoclassgc
Disables garbage collection (GC) of classes. This can save some
GC time, which shortens interruptions during the application
run. When you specify -Xnoclassgc at startup, the class objects
in the application are left untouched during GC and are always
be considered live. This can result in more memory being perma-
nently occupied which, if not used carefully, throws an
out-of-memory exception.
-Xrs Reduces the use of operating system signals by the JVM. Shut-
down hooks enable the orderly shutdown of a Java application by
running user cleanup code (such as closing database connections)
at shutdown, even if the JVM terminates abruptly.
• Linux and macOS:
• The JVM catches signals to implement shutdown hooks for un-
expected termination. The JVM uses SIGHUP, SIGINT, and
SIGTERM to initiate the running of shutdown hooks.
• Applications embedding the JVM frequently need to trap sig-
nals such as SIGINT or SIGTERM, which can lead to interfer-
ence with the JVM signal handlers. The -Xrs option is
available to address this issue. When -Xrs is used, the
signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT aren't
changed by the JVM, and signal handlers for these signals
aren't installed.
• Windows:
• The JVM watches for console control events to implement
shutdown hooks for unexpected termination. Specifically,
the JVM registers a console control handler that begins
shutdown-hook processing and returns TRUE for CTRL_C_EVENT,
CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, and CTRL_SHUT-
DOWN_EVENT.
• The JVM uses a similar mechanism to implement the feature of
dumping thread stacks for debugging purposes. The JVM uses
CTRL_BREAK_EVENT to perform thread dumps.
• If the JVM is run as a service (for example, as a servlet
engine for a web server), then it can receive CTRL_LO-
GOFF_EVENT but shouldn't initiate shutdown because the oper-
ating system doesn't actually terminate the process. To
avoid possible interference such as this, the -Xrs option
can be used. When the -Xrs option is used, the JVM doesn't
install a console control handler, implying that it doesn't
watch for or process CTRL_C_EVENT, CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.
There are two consequences of specifying -Xrs:
• Linux and macOS: SIGQUIT thread dumps aren't available.
• Windows: Ctrl + Break thread dumps aren't available.
User code is responsible for causing shutdown hooks to run, for
example, by calling the System.exit() when the JVM is to be ter-
minated.
-Xshare:mode
Sets the class data sharing (CDS) mode.
Possible mode arguments for this option include the following:
auto Use shared class data if possible (default).
on Require using shared class data, otherwise fail.
Note: The -Xshare:on option is used for testing purposes
only. It may cause the VM to unexpectedly exit during
start-up when the CDS archive cannot be used (for exam-
ple, when certain VM parameters are changed, or when a
different JDK is used). This option should not be used
in production environments.
off Do not attempt to use shared class data.
-XshowSettings
Shows all settings and then continues.
-XshowSettings:category
Shows settings and continues. Possible category arguments for
this option include the following:
all Shows all categories of settings. This is the default
value.
locale Shows settings related to locale.
properties
Shows settings related to system properties.
vm Shows the settings of the JVM.
system Linux: Shows host system or container configuration and
continues.
-Xss size
Sets the thread stack size (in bytes). Append the letter k or K
to indicate KB, m or M to indicate MB, or g or G to indicate GB.
The default value depends on the platform:
• Linux/x64 (64-bit): 1024 KB
• macOS (64-bit): 1024 KB
• Windows: The default value depends on virtual memory
The following examples set the thread stack size to 1024 KB in
different units:
-Xss1m
-Xss1024k
-Xss1048576
This option is similar to -XX:ThreadStackSize.
--add-reads module=target-module(,target-module)*
Updates module to read the target-module, regardless of the mod-
ule declaration. target-module can be all unnamed to read all
unnamed modules.
--add-exports module/package=target-module(,target-module)*
Updates module to export package to target-module, regardless of
module declaration. The target-module can be all unnamed to ex-
port to all unnamed modules.
--add-opens module/package=target-module(,target-module)*
Updates module to open package to target-module, regardless of
module declaration.
--limit-modules module[,module...]
Specifies the limit of the universe of observable modules.
--patch-module module=file(;file)*
Overrides or augments a module with classes and resources in JAR
files or directories.
--source version
Sets the version of the source in source-file mode.
EXTRA OPTIONS FOR MACOS
The following extra options are macOS specific.
-XstartOnFirstThread
Runs the main() method on the first (AppKit) thread.
-Xdock:name=application_name
Overrides the default application name displayed in dock.
-Xdock:icon=path_to_icon_file
Overrides the default icon displayed in dock.
ADVANCED OPTIONS FOR JAVA
These java options can be used to enable other advanced options.
-XX:+UnlockDiagnosticVMOptions
Unlocks the options intended for diagnosing the JVM. By de-
fault, this option is disabled and diagnostic options aren't
available.
Command line options that are enabled with the use of this op-
tion are not supported. If you encounter issues while using any
of these options, it is very likely that you will be required to
reproduce the problem without using any of these unsupported op-
tions before Oracle Support can assist with an investigation.
It is also possible that any of these options may be removed or
their behavior changed without any warning.
-XX:+UnlockExperimentalVMOptions
Unlocks the options that provide experimental features in the
JVM. By default, this option is disabled and experimental fea-
tures aren't available.
ADVANCED RUNTIME OPTIONS FOR JAVA
These java options control the runtime behavior of the Java HotSpot VM.
-XX:ActiveProcessorCount=x
Overrides the number of CPUs that the VM will use to calculate
the size of thread pools it will use for various operations such
as Garbage Collection and ForkJoinPool.
The VM normally determines the number of available processors
from the operating system. This flag can be useful for parti-
tioning CPU resources when running multiple Java processes in
docker containers. This flag is honored even if UseContainer-
Support is not enabled. See -XX:-UseContainerSupport for a de-
scription of enabling and disabling container support.
-XX:AllocateHeapAt=path
Takes a path to the file system and uses memory mapping to allo-
cate the object heap on the memory device. Using this option
enables the HotSpot VM to allocate the Java object heap on an
alternative memory device, such as an NV-DIMM, specified by the
user.
Alternative memory devices that have the same semantics as DRAM,
including the semantics of atomic operations, can be used in-
stead of DRAM for the object heap without changing the existing
application code. All other memory structures (such as the code
heap, metaspace, and thread stacks) continue to reside in DRAM.
Some operating systems expose non-DRAM memory through the file
system. Memory-mapped files in these file systems bypass the
page cache and provide a direct mapping of virtual memory to the
physical memory on the device. The existing heap related flags
(such as -Xmx and -Xms) and garbage-collection related flags
continue to work as before.
-XX:-CompactStrings
Disables the Compact Strings feature. By default, this option
is enabled. When this option is enabled, Java Strings contain-
ing only single-byte characters are internally represented and
stored as single-byte-per-character Strings using ISO-8859-1 /
Latin-1 encoding. This reduces, by 50%, the amount of space re-
quired for Strings containing only single-byte characters. For
Java Strings containing at least one multibyte character: these
are represented and stored as 2 bytes per character using UTF-16
encoding. Disabling the Compact Strings feature forces the use
of UTF-16 encoding as the internal representation for all Java
Strings.
Cases where it may be beneficial to disable Compact Strings in-
clude the following:
• When it's known that an application overwhelmingly will be al-
locating multibyte character Strings
• In the unexpected event where a performance regression is ob-
served in migrating from Java SE 8 to Java SE 9 and an analy-
sis shows that Compact Strings introduces the regression
In both of these scenarios, disabling Compact Strings makes
sense.
-XX:ErrorFile=filename
Specifies the path and file name to which error data is written
when an irrecoverable error occurs. By default, this file is
created in the current working directory and named hs_err_pid-
pid.log where pid is the identifier of the process that encoun-
tered the error.
The following example shows how to set the default log file
(note that the identifier of the process is specified as %p):
-XX:ErrorFile=./hs_err_pid%p.log
• Linux and macOS: The following example shows how to set the
error log to /var/log/java/java_error.log:
-XX:ErrorFile=/var/log/java/java_error.log
• Windows: The following example shows how to set the error log
file to C:/log/java/java_error.log:
-XX:ErrorFile=C:/log/java/java_error.log
If the file exists, and is writeable, then it will be overwrit-
ten. Otherwise, if the file can't be created in the specified
directory (due to insufficient space, permission problem, or an-
other issue), then the file is created in the temporary directo-
ry for the operating system:
• Linux and macOS: The temporary directory is /tmp.
• Windows: The temporary directory is specified by the value of
the TMP environment variable; if that environment variable
isn't defined, then the value of the TEMP environment variable
is used.
-XX:+ExtensiveErrorReports
Enables the reporting of more extensive error information in the
ErrorFile. This option can be turned on in environments where
maximal information is desired - even if the resulting logs may
be quite large and/or contain information that might be consid-
ered sensitive. The information can vary from release to re-
lease, and across different platforms. By default this option
is disabled.
-XX:FlightRecorderOptions=parameter=value (or)-XX:FlightRecorderOp-
tions:parameter=value
Sets the parameters that control the behavior of JFR.
The following list contains the available JFR parameter=value
entries:
globalbuffersize=size
Specifies the total amount of primary memory used for da-
ta retention. The default value is based on the value
specified for memorysize. Change the memorysize parame-
ter to alter the size of global buffers.
maxchunksize=size
Specifies the maximum size (in bytes) of the data chunks
in a recording. Append m or M to specify the size in
megabytes (MB), or g or G to specify the size in giga-
bytes (GB). By default, the maximum size of data chunks
is set to 12 MB. The minimum allowed is 1 MB.
memorysize=size
Determines how much buffer memory should be used, and
sets the globalbuffersize and numglobalbuffers parameters
based on the size specified. Append m or M to specify
the size in megabytes (MB), or g or G to specify the size
in gigabytes (GB). By default, the memory size is set to
10 MB.
numglobalbuffers
Specifies the number of global buffers used. The default
value is based on the memory size specified. Change the
memorysize parameter to alter the number of global buf-
fers.
old-object-queue-size=number-of-objects
Maximum number of old objects to track. By default, the
number of objects is set to 256.
repository=path
Specifies the repository (a directory) for temporary disk
storage. By default, the system's temporary directory is
used.
retransform={true|false}
Specifies whether event classes should be retransformed
using JVMTI. If false, instrumentation is added when
event classes are loaded. By default, this parameter is
enabled.
samplethreads={true|false}
Specifies whether thread sampling is enabled. Thread
sampling occurs only if the sampling event is enabled
along with this parameter. By default, this parameter is
enabled.
stackdepth=depth
Stack depth for stack traces. By default, the depth is
set to 64 method calls. The maximum is 2048. Values
greater than 64 could create significant overhead and re-
duce performance.
threadbuffersize=size
Specifies the per-thread local buffer size (in bytes).
By default, the local buffer size is set to 8 kilobytes,
with a minimum value of 4 kilobytes. Overriding this pa-
rameter could reduce performance and is not recommended.
You can specify values for multiple parameters by separating
them with a comma.
-XX:LargePageSizeInBytes=size
Sets the maximum large page size (in bytes) used by the JVM.
The size argument must be a valid page size supported by the en-
vironment to have any effect. Append the letter k or K to indi-
cate kilobytes, m or M to indicate megabytes, or g or G to indi-
cate gigabytes. By default, the size is set to 0, meaning that
the JVM will use the default large page size for the environment
as the maximum size for large pages. See Large Pages.
The following example describes how to set the large page size
to 1 gigabyte (GB):
-XX:LargePageSizeInBytes=1g
-XX:MaxDirectMemorySize=size
Sets the maximum total size (in bytes) of the java.nio package,
direct-buffer allocations. Append the letter k or K to indicate
kilobytes, m or M to indicate megabytes, or g or G to indicate
gigabytes. By default, the size is set to 0, meaning that the
JVM chooses the size for NIO direct-buffer allocations automati-
cally.
The following examples illustrate how to set the NIO size to
1024 KB in different units:
-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576
-XX:-MaxFDLimit
Disables the attempt to set the soft limit for the number of
open file descriptors to the hard limit. By default, this op-
tion is enabled on all platforms, but is ignored on Windows.
The only time that you may need to disable this is on Mac OS,
where its use imposes a maximum of 10240, which is lower than
the actual system maximum.
-XX:NativeMemoryTracking=mode
Specifies the mode for tracking JVM native memory usage. Possi-
ble mode arguments for this option include the following:
off Instructs not to track JVM native memory usage. This is
the default behavior if you don't specify the -XX:Native-
MemoryTracking option.
summary
Tracks memory usage only by JVM subsystems, such as Java
heap, class, code, and thread.
detail In addition to tracking memory usage by JVM subsystems,
track memory usage by individual CallSite, individual
virtual memory region and its committed regions.
-XX:ObjectAlignmentInBytes=alignment
Sets the memory alignment of Java objects (in bytes). By de-
fault, the value is set to 8 bytes. The specified value should
be a power of 2, and must be within the range of 8 and 256 (in-
clusive). This option makes it possible to use compressed
pointers with large Java heap sizes.
The heap size limit in bytes is calculated as:
4GB * ObjectAlignmentInBytes
Note: As the alignment value increases, the unused space
between objects also increases. As a result, you may not
realize any benefits from using compressed pointers with
large Java heap sizes.
-XX:OnError=string
Sets a custom command or a series of semicolon-separated com-
mands to run when an irrecoverable error occurs. If the string
contains spaces, then it must be enclosed in quotation marks.
• Linux and macOS: The following example shows how the -XX:On-
Error option can be used to run the gcore command to create a
core image, and start the gdb debugger to attach to the
process in case of an irrecoverable error (the %p designates
the current process identifier):
-XX:OnError="gcore %p;gdb -p %p"
• Windows: The following example shows how the -XX:OnError op-
tion can be used to run the userdump.exe utility to obtain a
crash dump in case of an irrecoverable error (the %p desig-
nates the current process identifier). This example assumes
that the path to the userdump.exe utility is specified in the
PATH environment variable:
-XX:OnError="userdump.exe %p"
-XX:OnOutOfMemoryError=string
Sets a custom command or a series of semicolon-separated com-
mands to run when an OutOfMemoryError exception is first thrown.
If the string contains spaces, then it must be enclosed in quo-
tation marks. For an example of a command string, see the de-
scription of the -XX:OnError option.
-XX:+PrintCommandLineFlags
Enables printing of ergonomically selected JVM flags that ap-
peared on the command line. It can be useful to know the ergo-
nomic values set by the JVM, such as the heap space size and the
selected garbage collector. By default, this option is disabled
and flags aren't printed.
-XX:+PreserveFramePointer
Selects between using the RBP register as a general purpose reg-
ister (-XX:-PreserveFramePointer) and using the RBP register to
hold the frame pointer of the currently executing method
(-XX:+PreserveFramePointer . If the frame pointer is available,
then external profiling tools (for example, Linux perf) can con-
struct more accurate stack traces.
-XX:+PrintNMTStatistics
Enables printing of collected native memory tracking data at JVM
exit when native memory tracking is enabled (see -XX:NativeMemo-
ryTracking). By default, this option is disabled and native
memory tracking data isn't printed.
-XX:SharedArchiveFile=path
Specifies the path and name of the class data sharing (CDS) ar-
chive file
See Application Class Data Sharing.
-XX:+VerifySharedSpaces
If this option is specified, the JVM will load a CDS archive
file only if it passes an integrity check based on CRC32 check-
sums. The purpose of this flag is to check for unintentional
damage to CDS archive files in transmission or storage. To
guarantee the security and proper operation of CDS, the user
must ensure that the CDS archive files used by Java applications
cannot be modified without proper authorization.
-XX:SharedArchiveConfigFile=shared_config_file
Specifies additional shared data added to the archive file.
-XX:SharedClassListFile=file_name
Specifies the text file that contains the names of the classes
to store in the class data sharing (CDS) archive. This file
contains the full name of one class per line, except slashes (/)
replace dots (.). For example, to specify the classes ja-
va.lang.Object and hello.Main, create a text file that contains
the following two lines:
java/lang/Object
hello/Main
The classes that you specify in this text file should include
the classes that are commonly used by the application. They may
include any classes from the application, extension, or boot-
strap class paths.
See Application Class Data Sharing.
-XX:+ShowCodeDetailsInExceptionMessages
Enables printing of improved NullPointerException messages.
When an application throws a NullPointerException, the option
enables the JVM to analyze the program's bytecode instructions
to determine precisely which reference is null, and describes
the source with a null-detail message. The null-detail message
is calculated and returned by NullPointerException.getMessage(),
and will be printed as the exception message along with the
method, filename, and line number. By default, this option is
enabled.
-XX:+ShowMessageBoxOnError
Enables the display of a dialog box when the JVM experiences an
irrecoverable error. This prevents the JVM from exiting and
keeps the process active so that you can attach a debugger to it
to investigate the cause of the error. By default, this option
is disabled.
-XX:StartFlightRecording=parameter=value
Starts a JFR recording for the Java application. This option is
equivalent to the JFR.start diagnostic command that starts a
recording during runtime. You can set the following parame-
ter=value entries when starting a JFR recording:
delay=time
Specifies the delay between the Java application launch
time and the start of the recording. Append s to specify
the time in seconds, m for minutes, h for hours, or d for
days (for example, specifying 10m means 10 minutes). By
default, there's no delay, and this parameter is set to
0.
disk={true|false}
Specifies whether to write data to disk while recording.
By default, this parameter is enabled.
dumponexit={true|false}
Specifies if the running recording is dumped when the JVM
shuts down. If enabled and a filename is not entered,
the recording is written to a file in the directory where
the process was started. The file name is a system-gen-
erated name that contains the process ID, recording ID,
and current timestamp, similar to
hotspot-pid-47496-id-1-2018_01_25_19_10_41.jfr. By de-
fault, this parameter is disabled.
duration=time
Specifies the duration of the recording. Append s to
specify the time in seconds, m for minutes, h for hours,
or d for days (for example, specifying 5h means 5 hours).
By default, the duration isn't limited, and this parame-
ter is set to 0.
filename=path
Specifies the path and name of the file to which the
recording is written when the recording is stopped, for
example:
• recording.jfr
• /home/user/recordings/recording.jfr
• c:\recordings\recording.jfr
name=identifier
Takes both the name and the identifier of a recording.
maxage=time
Specifies the maximum age of disk data to keep for the
recording. This parameter is valid only when the disk
parameter is set to true. Append s to specify the time
in seconds, m for minutes, h for hours, or d for days
(for example, specifying 30s means 30 seconds). By de-
fault, the maximum age isn't limited, and this parameter
is set to 0s.
maxsize=size
Specifies the maximum size (in bytes) of disk data to
keep for the recording. This parameter is valid only
when the disk parameter is set to true. The value must
not be less than the value for the maxchunksize parameter
set with -XX:FlightRecorderOptions. Append m or M to
specify the size in megabytes, or g or G to specify the
size in gigabytes. By default, the maximum size of disk
data isn't limited, and this parameter is set to 0.
path-to-gc-roots={true|false}
Specifies whether to collect the path to garbage collec-
tion (GC) roots at the end of a recording. By default,
this parameter is disabled.
The path to GC roots is useful for finding memory leaks,
but collecting it is time-consuming. Enable this option
only when you start a recording for an application that
you suspect has a memory leak. If the settings parameter
is set to profile, the stack trace from where the poten-
tial leaking object was allocated is included in the in-
formation collected.
settings=path
Specifies the path and name of the event settings file
(of type JFC). By default, the default.jfc file is used,
which is located in JAVA_HOME/lib/jfr. This default set-
tings file collects a predefined set of information with
low overhead, so it has minimal impact on performance and
can be used with recordings that run continuously.
A second settings file is also provided, profile.jfc,
which provides more data than the default configuration,
but can have more overhead and impact performance. Use
this configuration for short periods of time when more
information is needed.
You can specify values for multiple parameters by separating
them with a comma. Event settings and .jfc options can be spec-
ified using the following syntax:
option=value
Specifies the option value to modify. To list available
options, use the JAVA_HOME/bin/jfr tool.
event-setting=value
Specifies the event setting value to modify. Use the
form: #= To add a new event setting, prefix the event
name with '+'.
You can specify values for multiple event settings and .jfc op-
tions by separating them with a comma. In case of a conflict
between a parameter and a .jfc option, the parameter will take
precedence. The whitespace delimiter can be omitted for times-
pan values, i.e. 20ms. For more information about the settings
syntax, see Javadoc of the jdk.jfr package.
-XX:ThreadStackSize=size
Sets the Java thread stack size (in kilobytes). Use of a scal-
ing suffix, such as k, results in the scaling of the kilobytes
value so that -XX:ThreadStackSize=1k sets the Java thread stack
size to 1024*1024 bytes or 1 megabyte. The default value de-
pends on the platform:
• Linux/x64 (64-bit): 1024 KB
• macOS (64-bit): 1024 KB
• Windows: The default value depends on virtual memory
The following examples show how to set the thread stack size to
1 megabyte in different units:
-XX:ThreadStackSize=1k
-XX:ThreadStackSize=1024
This option is similar to -Xss.
-XX:-UseCompressedOops
Disables the use of compressed pointers. By default, this op-
tion is enabled, and compressed pointers are used. This will
automatically limit the maximum ergonomically determined Java
heap size to the maximum amount of memory that can be covered by
compressed pointers. By default this range is 32 GB.
With compressed oops enabled, object references are represented
as 32-bit offsets instead of 64-bit pointers, which typically
increases performance when running the application with Java
heap sizes smaller than the compressed oops pointer range. This
option works only for 64-bit JVMs.
It's possible to use compressed pointers with Java heap sizes
greater than 32 GB. See the -XX:ObjectAlignmentInBytes option.
-XX:-UseContainerSupport
The VM now provides automatic container detection support, which
allows the VM to determine the amount of memory and number of
processors that are available to a Java process running in dock-
er containers. It uses this information to allocate system re-
sources. This support is only available on Linux x64 platforms.
If supported, the default for this flag is true, and container
support is enabled by default. It can be disabled with
-XX:-UseContainerSupport.
Unified Logging is available to help to diagnose issues related
to this support.
Use -Xlog:os+container=trace for maximum logging of container
information. See Enable Logging with the JVM Unified Logging
Framework for a description of using Unified Logging.
-XX:+UseHugeTLBFS
Linux only: This option is the equivalent of specifying
-XX:+UseLargePages. This option is disabled by default. This
option pre-allocates all large pages up-front, when memory is
reserved; consequently the JVM can't dynamically grow or shrink
large pages memory areas; see -XX:UseTransparentHugePages if you
want this behavior.
See Large Pages.
-XX:+UseLargePages
Enables the use of large page memory. By default, this option
is disabled and large page memory isn't used.
See Large Pages.
-XX:+UseTransparentHugePages
Linux only: Enables the use of large pages that can dynamically
grow or shrink. This option is disabled by default. You may
encounter performance problems with transparent huge pages as
the OS moves other pages around to create huge pages; this op-
tion is made available for experimentation.
-XX:+AllowUserSignalHandlers
Enables installation of signal handlers by the application. By
default, this option is disabled and the application isn't al-
lowed to install signal handlers.
-XX:VMOptionsFile=filename
Allows user to specify VM options in a file, for example, ja-
va -XX:VMOptionsFile=/var/my_vm_options HelloWorld.
ADVANCED JIT COMPILER OPTIONS FOR JAVA
These java options control the dynamic just-in-time (JIT) compilation
performed by the Java HotSpot VM.
-XX:AllocateInstancePrefetchLines=lines
Sets the number of lines to prefetch ahead of the instance allo-
cation pointer. By default, the number of lines to prefetch is
set to 1:
-XX:AllocateInstancePrefetchLines=1
-XX:AllocatePrefetchDistance=size
Sets the size (in bytes) of the prefetch distance for object al-
location. Memory about to be written with the value of new ob-
jects is prefetched up to this distance starting from the ad-
dress of the last allocated object. Each Java thread has its
own allocation point.
Negative values denote that prefetch distance is chosen based on
the platform. Positive values are bytes to prefetch. Append
the letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. The default value
is set to -1.
The following example shows how to set the prefetch distance to
1024 bytes:
-XX:AllocatePrefetchDistance=1024
-XX:AllocatePrefetchInstr=instruction
Sets the prefetch instruction to prefetch ahead of the alloca-
tion pointer. Possible values are from 0 to 3. The actual in-
structions behind the values depend on the platform. By de-
fault, the prefetch instruction is set to 0:
-XX:AllocatePrefetchInstr=0
-XX:AllocatePrefetchLines=lines
Sets the number of cache lines to load after the last object al-
location by using the prefetch instructions generated in com-
piled code. The default value is 1 if the last allocated object
was an instance, and 3 if it was an array.
The following example shows how to set the number of loaded
cache lines to 5:
-XX:AllocatePrefetchLines=5
-XX:AllocatePrefetchStepSize=size
Sets the step size (in bytes) for sequential prefetch instruc-
tions. Append the letter k or K to indicate kilobytes, m or M
to indicate megabytes, g or G to indicate gigabytes. By de-
fault, the step size is set to 16 bytes:
-XX:AllocatePrefetchStepSize=16
-XX:AllocatePrefetchStyle=style
Sets the generated code style for prefetch instructions. The
style argument is an integer from 0 to 3:
0 Don't generate prefetch instructions.
1 Execute prefetch instructions after each allocation.
This is the default setting.
2 Use the thread-local allocation block (TLAB) watermark
pointer to determine when prefetch instructions are exe-
cuted.
3 Generate one prefetch instruction per cache line.
-XX:+BackgroundCompilation
Enables background compilation. This option is enabled by de-
fault. To disable background compilation, specify -XX:-Back-
groundCompilation (this is equivalent to specifying -Xbatch).
-XX:CICompilerCount=threads
Sets the number of compiler threads to use for compilation. By
default, the number of compiler threads is selected automatical-
ly depending on the number of CPUs and memory available for com-
piled code. The following example shows how to set the number
of threads to 2:
-XX:CICompilerCount=2
-XX:+UseDynamicNumberOfCompilerThreads
Dynamically create compiler thread up to the limit specified by
-XX:CICompilerCount. This option is enabled by default.
-XX:CompileCommand=command,method[,option]
Specifies a command to perform on a method. For example, to ex-
clude the indexOf() method of the String class from being com-
piled, use the following:
-XX:CompileCommand=exclude,java/lang/String.indexOf
Note that the full class name is specified, including all pack-
ages and subpackages separated by a slash (/). For easier
cut-and-paste operations, it's also possible to use the method
name format produced by the -XX:+PrintCompilation and -XX:+Log-
Compilation options:
-XX:CompileCommand=exclude,java.lang.String::indexOf
If the method is specified without the signature, then the com-
mand is applied to all methods with the specified name. Howev-
er, you can also specify the signature of the method in the
class file format. In this case, you should enclose the argu-
ments in quotation marks, because otherwise the shell treats the
semicolon as a command end. For example, if you want to exclude
only the indexOf(String) method of the String class from being
compiled, use the following:
-XX:CompileCommand="exclude,java/lang/String.index-
Of,(Ljava/lang/String;)I"
You can also use the asterisk (*) as a wildcard for class and
method names. For example, to exclude all indexOf() methods in
all classes from being compiled, use the following:
-XX:CompileCommand=exclude,*.indexOf
The commas and periods are aliases for spaces, making it easier
to pass compiler commands through a shell. You can pass argu-
ments to -XX:CompileCommand using spaces as separators by en-
closing the argument in quotation marks:
-XX:CompileCommand="exclude java/lang/String indexOf"
Note that after parsing the commands passed on the command line
using the -XX:CompileCommand options, the JIT compiler then
reads commands from the .hotspot_compiler file. You can add
commands to this file or specify a different file using the
-XX:CompileCommandFile option.
To add several commands, either specify the -XX:CompileCommand
option multiple times, or separate each argument with the new
line separator (\n). The following commands are available:
break Sets a breakpoint when debugging the JVM to stop at the
beginning of compilation of the specified method.
compileonly
Excludes all methods from compilation except for the
specified method. As an alternative, you can use the
-XX:CompileOnly option, which lets you specify several
methods.
dontinline
Prevents inlining of the specified method.
exclude
Excludes the specified method from compilation.
help Prints a help message for the -XX:CompileCommand option.
inline Attempts to inline the specified method.
log Excludes compilation logging (with the -XX:+LogCompila-
tion option) for all methods except for the specified
method. By default, logging is performed for all com-
piled methods.
option Passes a JIT compilation option to the specified method
in place of the last argument (option). The compilation
option is set at the end, after the method name. For ex-
ample, to enable the BlockLayoutByFrequency option for
the append() method of the StringBuffer class, use the
following:
-XX:CompileCommand=option,java/lang/String-
Buffer.append,BlockLayoutByFrequency
You can specify multiple compilation options, separated
by commas or spaces.
print Prints generated assembler code after compilation of the
specified method.
quiet Instructs not to print the compile commands. By default,
the commands that you specify with the -XX:CompileCommand
option are printed; for example, if you exclude from com-
pilation the indexOf() method of the String class, then
the following is printed to standard output:
CompilerOracle: exclude java/lang/String.indexOf
You can suppress this by specifying the -XX:CompileCom-
mand=quiet option before other -XX:CompileCommand op-
tions.
-XX:CompileCommandFile=filename
Sets the file from which JIT compiler commands are read. By de-
fault, the .hotspot_compiler file is used to store commands per-
formed by the JIT compiler.
Each line in the command file represents a command, a class
name, and a method name for which the command is used. For ex-
ample, this line prints assembly code for the toString() method
of the String class:
print java/lang/String toString
If you're using commands for the JIT compiler to perform on
methods, then see the -XX:CompileCommand option.
-XX:CompilerDirectivesFile=file
Adds directives from a file to the directives stack when a pro-
gram starts. See Compiler Control [https://docs.ora-
cle.com/en/java/javase/12/vm/compiler-con-
trol1.html#GUID-94AD8194-786A-4F19-BFFF-278F8E237F3A].
The -XX:CompilerDirectivesFile option has to be used together
with the -XX:UnlockDiagnosticVMOptions option that unlocks diag-
nostic JVM options.
-XX:+CompilerDirectivesPrint
Prints the directives stack when the program starts or when a
new directive is added.
The -XX:+CompilerDirectivesPrint option has to be used together
with the -XX:UnlockDiagnosticVMOptions option that unlocks diag-
nostic JVM options.
-XX:CompileOnly=methods
Sets the list of methods (separated by commas) to which compila-
tion should be restricted. Only the specified methods are com-
piled. Specify each method with the full class name (including
the packages and subpackages). For example, to compile only the
length() method of the String class and the size() method of the
List class, use the following:
-XX:CompileOnly=java/lang/String.length,ja-
va/util/List.size
Note that the full class name is specified, including all pack-
ages and subpackages separated by a slash (/). For easier cut
and paste operations, it's also possible to use the method name
format produced by the -XX:+PrintCompilation and -XX:+LogCompi-
lation options:
-XX:CompileOnly=java.lang.String::length,ja-
va.util.List::size
Although wildcards aren't supported, you can specify only the
class or package name to compile all methods in that class or
package, as well as specify just the method to compile methods
with this name in any class:
-XX:CompileOnly=java/lang/String
-XX:CompileOnly=java/lang
-XX:CompileOnly=.length
-XX:CompileThresholdScaling=scale
Provides unified control of first compilation. This option con-
trols when methods are first compiled for both the tiered and
the nontiered modes of operation. The CompileThresholdScaling
option has a floating point value between 0 and +Inf and scales
the thresholds corresponding to the current mode of operation
(both tiered and nontiered). Setting CompileThresholdScaling to
a value less than 1.0 results in earlier compilation while val-
ues greater than 1.0 delay compilation. Setting CompileThresh-
oldScaling to 0 is equivalent to disabling compilation.
-XX:+DoEscapeAnalysis
Enables the use of escape analysis. This option is enabled by
default. To disable the use of escape analysis, specify
-XX:-DoEscapeAnalysis.
-XX:InitialCodeCacheSize=size
Sets the initial code cache size (in bytes). Append the letter
k or K to indicate kilobytes, m or M to indicate megabytes, or g
or G to indicate gigabytes. The default value depends on the
platform. The initial code cache size shouldn't be less than
the system's minimal memory page size. The following example
shows how to set the initial code cache size to 32 KB:
-XX:InitialCodeCacheSize=32k
-XX:+Inline
Enables method inlining. This option is enabled by default to
increase performance. To disable method inlining, specify
-XX:-Inline.
-XX:InlineSmallCode=size
Sets the maximum code size (in bytes) for already compiled meth-
ods that may be inlined. This flag only applies to the C2 com-
piler. Append the letter k or K to indicate kilobytes, m or M
to indicate megabytes, or g or G to indicate gigabytes. The de-
fault value depends on the platform and on whether tiered compi-
lation is enabled. In the following example it is set to 1000
bytes:
-XX:InlineSmallCode=1000
-XX:+LogCompilation
Enables logging of compilation activity to a file named
hotspot.log in the current working directory. You can specify a
different log file path and name using the -XX:LogFile option.
By default, this option is disabled and compilation activity
isn't logged. The -XX:+LogCompilation option has to be used to-
gether with the -XX:UnlockDiagnosticVMOptions option that un-
locks diagnostic JVM options.
You can enable verbose diagnostic output with a message printed
to the console every time a method is compiled by using the
-XX:+PrintCompilation option.
-XX:FreqInlineSize=size
Sets the maximum bytecode size (in bytes) of a hot method to be
inlined. This flag only applies to the C2 compiler. Append the
letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. The default value
depends on the platform. In the following example it is set to
325 bytes:
-XX:FreqInlineSize=325
-XX:MaxInlineSize=size
Sets the maximum bytecode size (in bytes) of a cold method to be
inlined. This flag only applies to the C2 compiler. Append the
letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. By default, the
maximum bytecode size is set to 35 bytes:
-XX:MaxInlineSize=35
-XX:C1MaxInlineSize=size
Sets the maximum bytecode size (in bytes) of a cold method to be
inlined. This flag only applies to the C1 compiler. Append the
letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. By default, the
maximum bytecode size is set to 35 bytes:
-XX:MaxInlineSize=35
-XX:MaxTrivialSize=size
Sets the maximum bytecode size (in bytes) of a trivial method to
be inlined. This flag only applies to the C2 compiler. Append
the letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. By default, the
maximum bytecode size of a trivial method is set to 6 bytes:
-XX:MaxTrivialSize=6
-XX:C1MaxTrivialSize=size
Sets the maximum bytecode size (in bytes) of a trivial method to
be inlined. This flag only applies to the C1 compiler. Append
the letter k or K to indicate kilobytes, m or M to indicate
megabytes, or g or G to indicate gigabytes. By default, the
maximum bytecode size of a trivial method is set to 6 bytes:
-XX:MaxTrivialSize=6
-XX:MaxNodeLimit=nodes
Sets the maximum number of nodes to be used during single method
compilation. By default the value depends on the features en-
abled. In the following example the maximum number of nodes is
set to 100,000:
-XX:MaxNodeLimit=100000
-XX:NonNMethodCodeHeapSize=size
Sets the size in bytes of the code segment containing nonmethod
code.
A nonmethod code segment containing nonmethod code, such as com-
piler buffers and the bytecode interpreter. This code type
stays in the code cache forever. This flag is used only if
-XX:SegmentedCodeCache is enabled.
-XX:NonProfiledCodeHeapSize=size
Sets the size in bytes of the code segment containing nonpro-
filed methods. This flag is used only if -XX:SegmentedCodeCache
is enabled.
-XX:+OptimizeStringConcat
Enables the optimization of String concatenation operations.
This option is enabled by default. To disable the optimization
of String concatenation operations, specify -XX:-OptimizeString-
Concat.
-XX:+PrintAssembly
Enables printing of assembly code for bytecoded and native meth-
ods by using the external hsdis-<arch>.so or .dll library. For
64-bit VM on Windows, it's hsdis-amd64.dll. This lets you to
see the generated code, which may help you to diagnose perfor-
mance issues.
By default, this option is disabled and assembly code isn't
printed. The -XX:+PrintAssembly option has to be used together
with the -XX:UnlockDiagnosticVMOptions option that unlocks diag-
nostic JVM options.
-XX:ProfiledCodeHeapSize=size
Sets the size in bytes of the code segment containing profiled
methods. This flag is used only if -XX:SegmentedCodeCache is
enabled.
-XX:+PrintCompilation
Enables verbose diagnostic output from the JVM by printing a
message to the console every time a method is compiled. This
lets you to see which methods actually get compiled. By de-
fault, this option is disabled and diagnostic output isn't
printed.
You can also log compilation activity to a file by using the
-XX:+LogCompilation option.
-XX:+PrintInlining
Enables printing of inlining decisions. This let's you see
which methods are getting inlined.
By default, this option is disabled and inlining information
isn't printed. The -XX:+PrintInlining option has to be used to-
gether with the -XX:+UnlockDiagnosticVMOptions option that un-
locks diagnostic JVM options.
-XX:ReservedCodeCacheSize=size
Sets the maximum code cache size (in bytes) for JIT-compiled
code. Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, or g or G to indicate gigabytes. The de-
fault maximum code cache size is 240 MB; if you disable tiered
compilation with the option -XX:-TieredCompilation, then the de-
fault size is 48 MB. This option has a limit of 2 GB; other-
wise, an error is generated. The maximum code cache size
shouldn't be less than the initial code cache size; see the op-
tion -XX:InitialCodeCacheSize.
-XX:RTMAbortRatio=abort_ratio
Specifies the RTM abort ratio is specified as a percentage (%)
of all executed RTM transactions. If a number of aborted trans-
actions becomes greater than this ratio, then the compiled code
is deoptimized. This ratio is used when the -XX:+UseRTMDeopt
option is enabled. The default value of this option is 50.
This means that the compiled code is deoptimized if 50% of all
transactions are aborted.
-XX:RTMRetryCount=number_of_retries
Specifies the number of times that the RTM locking code is re-
tried, when it is aborted or busy, before falling back to the
normal locking mechanism. The default value for this option is
5. The -XX:UseRTMLocking option must be enabled.
-XX:+SegmentedCodeCache
Enables segmentation of the code cache. Without the -XX:+Seg-
mentedCodeCache, the code cache consists of one large segment.
With -XX:+SegmentedCodeCache, we have separate segments for non-
method, profiled method, and nonprofiled method code. These
segments aren't resized at runtime. The feature is enabled by
default if tiered compilation is enabled (-XX:+TieredCompilation
) and -XX:ReservedCodeCacheSize >= 240 MB. The advantages are
better control of the memory footprint, reduced code fragmenta-
tion, and better iTLB/iCache behavior due to improved locality.
iTLB/iCache is a CPU-specific term meaning Instruction Transla-
tion Lookaside Buffer (ITLB). ICache is an instruction cache in
theCPU. The implementation of the code cache can be found in
the file: /share/vm/code/codeCache.cpp.
-XX:StartAggressiveSweepingAt=percent
Forces stack scanning of active methods to aggressively remove
unused code when only the given percentage of the code cache is
free. The default value is 10%.
-XX:-TieredCompilation
Disables the use of tiered compilation. By default, this option
is enabled.
-XX:UseSSE=version
Enables the use of SSE instruction set of a specified version.
Is set by default to the highest supported version available
(x86 only).
-XX:UseAVX=version
Enables the use of AVX instruction set of a specified version.
Is set by default to the highest supported version available
(x86 only).
-XX:+UseAES
Enables hardware-based AES intrinsics for hardware that supports
it. This option is on by default on hardware that has the nec-
essary instructions. The -XX:+UseAES is used in conjunction
with UseAESIntrinsics. Flags that control intrinsics now re-
quire the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseAESIntrinsics
Enables AES intrinsics. Specifying-XX:+UseAESIntrinsics is
equivalent to also enabling -XX:+UseAES. To disable hard-
ware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIn-
trinsics. For example, to enable hardware AES, use the follow-
ing flags:
-XX:+UseAES -XX:+UseAESIntrinsics
Flags that control intrinsics now require the option -XX:+Un-
lockDiagnosticVMOptions.
-XX:+UseAESCTRIntrinsics
Analogous to -XX:+UseAESIntrinsics enables AES/CTR intrinsics.
-XX:+UseGHASHIntrinsics
Controls the use of GHASH intrinsics. Enabled by default on
platforms that support the corresponding instructions. Flags
that control intrinsics now require the option -XX:+UnlockDiag-
nosticVMOptions.
-XX:+UseBASE64Intrinsics
Controls the use of accelerated BASE64 encoding routines for ja-
va.util.Base64. Enabled by default on platforms that support
it. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseAdler32Intrinsics
Controls the use of Adler32 checksum algorithm intrinsic for ja-
va.util.zip.Adler32. Enabled by default on platforms that sup-
port it. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseCRC32Intrinsics
Controls the use of CRC32 intrinsics for java.util.zip.CRC32.
Enabled by default on platforms that support it. Flags that
control intrinsics now require the option -XX:+UnlockDiagnos-
ticVMOptions.
-XX:+UseCRC32CIntrinsics
Controls the use of CRC32C intrinsics for java.util.zip.CRC32C.
Enabled by default on platforms that support it. Flags that
control intrinsics now require the option -XX:+UnlockDiagnos-
ticVMOptions.
-XX:+UseSHA
Enables hardware-based intrinsics for SHA crypto hash functions
for some hardware. The UseSHA option is used in conjunction
with the UseSHA1Intrinsics, UseSHA256Intrinsics, and Use-
SHA512Intrinsics options.
The UseSHA and UseSHA*Intrinsics flags are enabled by default on
machines that support the corresponding instructions.
This feature is applicable only when using the sun.securi-
ty.provider.Sun provider for SHA operations. Flags that control
intrinsics now require the option -XX:+UnlockDiagnosticVMOp-
tions.
To disable all hardware-based SHA intrinsics, specify the
-XX:-UseSHA. To disable only a particular SHA intrinsic, use
the appropriate corresponding option. For example: -XX:-Use-
SHA256Intrinsics.
-XX:+UseSHA1Intrinsics
Enables intrinsics for SHA-1 crypto hash function. Flags that
control intrinsics now require the option -XX:+UnlockDiagnos-
ticVMOptions.
-XX:+UseSHA256Intrinsics
Enables intrinsics for SHA-224 and SHA-256 crypto hash func-
tions. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseSHA512Intrinsics
Enables intrinsics for SHA-384 and SHA-512 crypto hash func-
tions. Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-XX:+UseMathExactIntrinsics
Enables intrinsification of various java.lang.Math.*Exact()
functions. Enabled by default. Flags that control intrinsics
now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMultiplyToLenIntrinsic
Enables intrinsification of BigInteger.multiplyToLen(). Enabled
by default on platforms that support it. Flags that control in-
trinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseSquareToLenIntrinsic
Enables intrinsification of BigInteger.squareToLen(). Enabled
by default on platforms that support it. Flags that control in-
trinsics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMulAddIntrinsic
Enables intrinsification of BigInteger.mulAdd(). Enabled by de-
fault on platforms that support it. Flags that control intrin-
sics now require the option -XX:+UnlockDiagnosticVMOptions.
-XX:+UseMontgomeryMultiplyIntrinsic
Enables intrinsification of BigInteger.montgomeryMultiply().
Enabled by default on platforms that support it. Flags that
control intrinsics now require the option -XX:+UnlockDiagnos-
ticVMOptions.
-XX:+UseMontgomerySquareIntrinsic
Enables intrinsification of BigInteger.montgomerySquare(). En-
abled by default on platforms that support it. Flags that con-
trol intrinsics now require the option -XX:+UnlockDiagnosticV-
MOptions.
-XX:+UseCMoveUnconditionally
Generates CMove (scalar and vector) instructions regardless of
profitability analysis.
-XX:+UseCodeCacheFlushing
Enables flushing of the code cache before shutting down the com-
piler. This option is enabled by default. To disable flushing
of the code cache before shutting down the compiler, specify
-XX:-UseCodeCacheFlushing.
-XX:+UseCondCardMark
Enables checking if the card is already marked before updating
the card table. This option is disabled by default. It should
be used only on machines with multiple sockets, where it in-
creases the performance of Java applications that rely on con-
current operations.
-XX:+UseCountedLoopSafepoints
Keeps safepoints in counted loops. Its default value depends on
whether the selected garbage collector requires low latency
safepoints.
-XX:LoopStripMiningIter=number_of_iterations
Controls the number of iterations in the inner strip mined loop.
Strip mining transforms counted loops into two level nested
loops. Safepoints are kept in the outer loop while the inner
loop can execute at full speed. This option controls the maxi-
mum number of iterations in the inner loop. The default value
is 1,000.
-XX:LoopStripMiningIterShortLoop=number_of_iterations
Controls loop strip mining optimization. Loops with the number
of iterations less than specified will not have safepoints in
them. Default value is 1/10th of -XX:LoopStripMiningIter.
-XX:+UseFMA
Enables hardware-based FMA intrinsics for hardware where FMA in-
structions are available (such as, Intel and ARM64). FMA in-
trinsics are generated for the java.lang.Math.fma(a, b, c) meth-
ods that calculate the value of ( a * b + c ) expressions.
-XX:+UseRTMDeopt
Autotunes RTM locking depending on the abort ratio. This ratio
is specified by the -XX:RTMAbortRatio option. If the number of
aborted transactions exceeds the abort ratio, then the method
containing the lock is deoptimized and recompiled with all locks
as normal locks. This option is disabled by default. The
-XX:+UseRTMLocking option must be enabled.
-XX:+UseRTMLocking
Generates Restricted Transactional Memory (RTM) locking code for
all inflated locks, with the normal locking mechanism as the
fallback handler. This option is disabled by default. Options
related to RTM are available only on x86 CPUs that support
Transactional Synchronization Extensions (TSX).
RTM is part of Intel's TSX, which is an x86 instruction set ex-
tension and facilitates the creation of multithreaded applica-
tions. RTM introduces the new instructions XBEGIN, XABORT,
XEND, and XTEST. The XBEGIN and XEND instructions enclose a set
of instructions to run as a transaction. If no conflict is
found when running the transaction, then the memory and register
modifications are committed together at the XEND instruction.
The XABORT instruction can be used to explicitly abort a trans-
action and the XTEST instruction checks if a set of instructions
is being run in a transaction.
A lock on a transaction is inflated when another thread tries to
access the same transaction, thereby blocking the thread that
didn't originally request access to the transaction. RTM re-
quires that a fallback set of operations be specified in case a
transaction aborts or fails. An RTM lock is a lock that has
been delegated to the TSX's system.
RTM improves performance for highly contended locks with low
conflict in a critical region (which is code that must not be
accessed by more than one thread concurrently). RTM also im-
proves the performance of coarse-grain locking, which typically
doesn't perform well in multithreaded applications.
(Coarse-grain locking is the strategy of holding locks for long
periods to minimize the overhead of taking and releasing locks,
while fine-grained locking is the strategy of trying to achieve
maximum parallelism by locking only when necessary and unlocking
as soon as possible.) Also, for lightly contended locks that are
used by different threads, RTM can reduce false cache line shar-
ing, also known as cache line ping-pong. This occurs when mul-
tiple threads from different processors are accessing different
resources, but the resources share the same cache line. As a
result, the processors repeatedly invalidate the cache lines of
other processors, which forces them to read from main memory in-
stead of their cache.
-XX:+UseSuperWord
Enables the transformation of scalar operations into superword
operations. Superword is a vectorization optimization. This
option is enabled by default. To disable the transformation of
scalar operations into superword operations, specify -XX:-UseSu-
perWord.
ADVANCED SERVICEABILITY OPTIONS FOR JAVA
These java options provide the ability to gather system information and
perform extensive debugging.
-XX:+DisableAttachMechanism
Disables the mechanism that lets tools attach to the JVM. By
default, this option is disabled, meaning that the attach mecha-
nism is enabled and you can use diagnostics and troubleshooting
tools such as jcmd, jstack, jmap, and jinfo.
Note: The tools such as jcmd, jinfo, jmap, and jstack
shipped with the JDK aren't supported when using the
tools from one JDK version to troubleshoot a different
JDK version.
-XX:+ExtendedDTraceProbes
Linux and macOS: Enables additional dtrace tool probes that af-
fect the performance. By default, this option is disabled and
dtrace performs only standard probes.
-XX:+HeapDumpOnOutOfMemoryError
Enables the dumping of the Java heap to a file in the current
directory by using the heap profiler (HPROF) when a ja-
va.lang.OutOfMemoryError exception is thrown. You can explicit-
ly set the heap dump file path and name using the -XX:HeapDump-
Path option. By default, this option is disabled and the heap
isn't dumped when an OutOfMemoryError exception is thrown.
-XX:HeapDumpPath=path
Sets the path and file name for writing the heap dump provided
by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemo-
ryError option is set. By default, the file is created in the
current working directory, and it's named java_pid<pid>.hprof
where <pid> is the identifier of the process that caused the er-
ror. The following example shows how to set the default file
explicitly (%p represents the current process identifier):
-XX:HeapDumpPath=./java_pid%p.hprof
• Linux and macOS: The following example shows how to set the
heap dump file to /var/log/java/java_heapdump.hprof:
-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
• Windows: The following example shows how to set the heap dump
file to C:/log/java/java_heapdump.log:
-XX:HeapDumpPath=C:/log/java/java_heapdump.log
-XX:LogFile=path
Sets the path and file name to where log data is written. By
default, the file is created in the current working directory,
and it's named hotspot.log.
• Linux and macOS: The following example shows how to set the
log file to /var/log/java/hotspot.log:
-XX:LogFile=/var/log/java/hotspot.log
• Windows: The following example shows how to set the log file
to C:/log/java/hotspot.log:
-XX:LogFile=C:/log/java/hotspot.log
-XX:+PrintClassHistogram
Enables printing of a class instance histogram after one of the
following events:
• Linux and macOS: Control+Break
• Windows: Control+C (SIGTERM)
By default, this option is disabled.
Setting this option is equivalent to running the jmap -histo
command, or the jcmd pid GC.class_histogram command, where pid
is the current Java process identifier.
-XX:+PrintConcurrentLocks
Enables printing of java.util.concurrent locks after one of the
following events:
• Linux and macOS: Control+Break
• Windows: Control+C (SIGTERM)
By default, this option is disabled.
Setting this option is equivalent to running the jstack -l com-
mand or the jcmd pid Thread.print -l command, where pid is the
current Java process identifier.
-XX:+PrintFlagsRanges
Prints the range specified and allows automatic testing of the
values. See Validate Java Virtual Machine Flag Arguments.
-XX:+PerfDataSaveToFile
If enabled, saves jstat binary data when the Java application
exits. This binary data is saved in a file named hsperfda-
ta_pid, where pid is the process identifier of the Java applica-
tion that you ran. Use the jstat command to display the perfor-
mance data contained in this file as follows:
jstat -class file:///path/hsperfdata_pid
jstat -gc file:///path/hsperfdata_pid
-XX:+UsePerfData
Enables the perfdata feature. This option is enabled by default
to allow JVM monitoring and performance testing. Disabling it
suppresses the creation of the hsperfdata_userid directories.
To disable the perfdata feature, specify -XX:-UsePerfData.
ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA
These java options control how garbage collection (GC) is performed by
the Java HotSpot VM.
-XX:+AggressiveHeap
Enables Java heap optimization. This sets various parameters to
be optimal for long-running jobs with intensive memory alloca-
tion, based on the configuration of the computer (RAM and CPU).
By default, the option is disabled and the heap sizes are con-
figured less aggressively.
-XX:+AlwaysPreTouch
Requests the VM to touch every page on the Java heap after re-
questing it from the operating system and before handing memory
out to the application. By default, this option is disabled and
all pages are committed as the application uses the heap space.
-XX:ConcGCThreads=threads
Sets the number of threads used for concurrent GC. Sets threads
to approximately 1/4 of the number of parallel garbage collec-
tion threads. The default value depends on the number of CPUs
available to the JVM.
For example, to set the number of threads for concurrent GC to
2, specify the following option:
-XX:ConcGCThreads=2
-XX:+DisableExplicitGC
Enables the option that disables processing of calls to the Sys-
tem.gc() method. This option is disabled by default, meaning
that calls to System.gc() are processed. If processing of calls
to System.gc() is disabled, then the JVM still performs GC when
necessary.
-XX:+ExplicitGCInvokesConcurrent
Enables invoking of concurrent GC by using the System.gc() re-
quest. This option is disabled by default and can be enabled
only with the -XX:+UseG1GC option.
-XX:G1AdaptiveIHOPNumInitialSamples=number
When -XX:UseAdaptiveIHOP is enabled, this option sets the number
of completed marking cycles used to gather samples until G1
adaptively determines the optimum value of -XX:InitiatingHeapOc-
cupancyPercent. Before, G1 uses the value of -XX:Initiat-
ingHeapOccupancyPercent directly for this purpose. The default
value is 3.
-XX:G1HeapRegionSize=size
Sets the size of the regions into which the Java heap is subdi-
vided when using the garbage-first (G1) collector. The value is
a power of 2 and can range from 1 MB to 32 MB. The default re-
gion size is determined ergonomically based on the heap size
with a goal of approximately 2048 regions.
The following example sets the size of the subdivisions to 16
MB:
-XX:G1HeapRegionSize=16m
-XX:G1HeapWastePercent=percent
Sets the percentage of heap that you're willing to waste. The
Java HotSpot VM doesn't initiate the mixed garbage collection
cycle when the reclaimable percentage is less than the heap
waste percentage. The default is 5 percent.
-XX:G1MaxNewSizePercent=percent
Sets the percentage of the heap size to use as the maximum for
the young generation size. The default value is 60 percent of
your Java heap.
This is an experimental flag. This setting replaces the -XX:De-
faultMaxNewGenPercent setting.
-XX:G1MixedGCCountTarget=number
Sets the target number of mixed garbage collections after a
marking cycle to collect old regions with at most G1MixedG-
CLIveThresholdPercent live data. The default is 8 mixed garbage
collections. The goal for mixed collections is to be within
this target number.
-XX:G1MixedGCLiveThresholdPercent=percent
Sets the occupancy threshold for an old region to be included in
a mixed garbage collection cycle. The default occupancy is 85
percent.
This is an experimental flag. This setting replaces the
-XX:G1OldCSetRegionLiveThresholdPercent setting.
-XX:G1NewSizePercent=percent
Sets the percentage of the heap to use as the minimum for the
young generation size. The default value is 5 percent of your
Java heap.
This is an experimental flag. This setting replaces the -XX:De-
faultMinNewGenPercent setting.
-XX:G1OldCSetRegionThresholdPercent=percent
Sets an upper limit on the number of old regions to be collected
during a mixed garbage collection cycle. The default is 10 per-
cent of the Java heap.
-XX:G1ReservePercent=percent
Sets the percentage of the heap (0 to 50) that's reserved as a
false ceiling to reduce the possibility of promotion failure for
the G1 collector. When you increase or decrease the percentage,
ensure that you adjust the total Java heap by the same amount.
By default, this option is set to 10%.
The following example sets the reserved heap to 20%:
-XX:G1ReservePercent=20
-XX:+G1UseAdaptiveIHOP
Controls adaptive calculation of the old generation occupancy to
start background work preparing for an old generation collec-
tion. If enabled, G1 uses -XX:InitiatingHeapOccupancyPercent
for the first few times as specified by the value of -XX:G1Adap-
tiveIHOPNumInitialSamples, and after that adaptively calculates
a new optimum value for the initiating occupancy automatically.
Otherwise, the old generation collection process always starts
at the old generation occupancy determined by -XX:Initiat-
ingHeapOccupancyPercent.
The default is enabled.
-XX:InitialHeapSize=size
Sets the initial size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater
than 1 MB. Append the letter k or K to indicate kilobytes, m or
M to indicate megabytes, or g or G to indicate gigabytes. The
default value is selected at run time based on the system con-
figuration.
The following examples show how to set the size of allocated
memory to 6 MB using various units:
-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6m
If you set this option to 0, then the initial size is set as the
sum of the sizes allocated for the old generation and the young
generation. The size of the heap for the young generation can
be set using the -XX:NewSize option.
-XX:InitialRAMPercentage=percent
Sets the initial amount of memory that the JVM will use for the
Java heap before applying ergonomics heuristics as a percentage
of the maximum amount determined as described in the -XX:MaxRAM
option. The default value is 1.5625 percent.
The following example shows how to set the percentage of the
initial amount of memory used for the Java heap:
-XX:InitialRAMPercentage=5
-XX:InitialSurvivorRatio=ratio
Sets the initial survivor space ratio used by the throughput
garbage collector (which is enabled by the -XX:+UseParallelGC
option). Adaptive sizing is enabled by default with the
throughput garbage collector by using the -XX:+UseParallelGC op-
tion, and the survivor space is resized according to the appli-
cation behavior, starting with the initial value. If adaptive
sizing is disabled (using the -XX:-UseAdaptiveSizePolicy op-
tion), then the -XX:SurvivorRatio option should be used to set
the size of the survivor space for the entire execution of the
application.
The following formula can be used to calculate the initial size
of survivor space (S) based on the size of the young generation
(Y), and the initial survivor space ratio (R):
S=Y/(R+2)
The 2 in the equation denotes two survivor spaces. The larger
the value specified as the initial survivor space ratio, the
smaller the initial survivor space size.
By default, the initial survivor space ratio is set to 8. If
the default value for the young generation space size is used (2
MB), then the initial size of the survivor space is 0.2 MB.
The following example shows how to set the initial survivor
space ratio to 4:
-XX:InitialSurvivorRatio=4
-XX:InitiatingHeapOccupancyPercent=percent
Sets the percentage of the old generation occupancy (0 to 100)
at which to start the first few concurrent marking cycles for
the G1 garbage collector.
By default, the initiating value is set to 45%. A value of 0
implies nonstop concurrent GC cycles from the beginning until G1
adaptively sets this value.
See also the -XX:G1UseAdaptiveIHOP and -XX:G1AdaptiveIHOPNumIni-
tialSamples options.
The following example shows how to set the initiating heap occu-
pancy to 75%:
-XX:InitiatingHeapOccupancyPercent=75
-XX:MaxGCPauseMillis=time
Sets a target for the maximum GC pause time (in milliseconds).
This is a soft goal, and the JVM will make its best effort to
achieve it. The specified value doesn't adapt to your heap
size. By default, for G1 the maximum pause time target is 200
milliseconds. The other generational collectors do not use a
pause time goal by default.
The following example shows how to set the maximum target pause
time to 500 ms:
-XX:MaxGCPauseMillis=500
-XX:MaxHeapSize=size
Sets the maximum size (in byes) of the memory allocation pool.
This value must be a multiple of 1024 and greater than 2 MB.
Append the letter k or K to indicate kilobytes, m or M to indi-
cate megabytes, or g or G to indicate gigabytes. The default
value is selected at run time based on the system configuration.
For server deployments, the options -XX:InitialHeapSize and
-XX:MaxHeapSize are often set to the same value.
The following examples show how to set the maximum allowed size
of allocated memory to 80 MB using various units:
-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80m
The -XX:MaxHeapSize option is equivalent to -Xmx.
-XX:MaxHeapFreeRatio=percent
Sets the maximum allowed percentage of free heap space (0 to
100) after a GC event. If free heap space expands above this
value, then the heap is shrunk. By default, this value is set
to 70%.
Minimize the Java heap size by lowering the values of the param-
eters MaxHeapFreeRatio (default value is 70%) and MinHeapFreeRa-
tio (default value is 40%) with the command-line options
-XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Lowering Max-
HeapFreeRatio to as low as 10% and MinHeapFreeRatio to 5% has
successfully reduced the heap size without too much performance
regression; however, results may vary greatly depending on your
application. Try different values for these parameters until
they're as low as possible yet still retain acceptable perfor-
mance.
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Customers trying to keep the heap small should also add the op-
tion -XX:-ShrinkHeapInSteps. See Performance Tuning Examples
for a description of using this option to keep the Java heap
small by reducing the dynamic footprint for embedded applica-
tions.
-XX:MaxMetaspaceSize=size
Sets the maximum amount of native memory that can be allocated
for class metadata. By default, the size isn't limited. The
amount of metadata for an application depends on the application
itself, other running applications, and the amount of memory
available on the system.
The following example shows how to set the maximum class metada-
ta size to 256 MB:
-XX:MaxMetaspaceSize=256m
-XX:MaxNewSize=size
Sets the maximum size (in bytes) of the heap for the young gen-
eration (nursery). The default value is set ergonomically.
-XX:MaxRAM=size
Sets the maximum amount of memory that the JVM may use for the
Java heap before applying ergonomics heuristics. The default
value is the maximum amount of available memory to the JVM
process or 128 GB, whichever is lower.
The maximum amount of available memory to the JVM process is the
minimum of the machine's physical memory and any constraints set
by the environment (e.g. container).
Specifying this option disables automatic use of compressed oops
if the combined result of this and other options influencing the
maximum amount of memory is larger than the range of memory ad-
dressable by compressed oops. See -XX:UseCompressedOops for
further information about compressed oops.
The following example shows how to set the maximum amount of
available memory for sizing the Java heap to 2 GB:
-XX:MaxRAM=2G
-XX:MaxRAMPercentage=percent
Sets the maximum amount of memory that the JVM may use for the
Java heap before applying ergonomics heuristics as a percentage
of the maximum amount determined as described in the -XX:MaxRAM
option. The default value is 25 percent.
Specifying this option disables automatic use of compressed oops
if the combined result of this and other options influencing the
maximum amount of memory is larger than the range of memory ad-
dressable by compressed oops. See -XX:UseCompressedOops for
further information about compressed oops.
The following example shows how to set the percentage of the
maximum amount of memory used for the Java heap:
-XX:MaxRAMPercentage=75
-XX:MinRAMPercentage=percent
Sets the maximum amount of memory that the JVM may use for the
Java heap before applying ergonomics heuristics as a percentage
of the maximum amount determined as described in the -XX:MaxRAM
option for small heaps. A small heap is a heap of approximately
125 MB. The default value is 50 percent.
The following example shows how to set the percentage of the
maximum amount of memory used for the Java heap for small heaps:
-XX:MinRAMPercentage=75
-XX:MaxTenuringThreshold=threshold
Sets the maximum tenuring threshold for use in adaptive GC siz-
ing. The largest value is 15. The default value is 15 for the
parallel (throughput) collector.
The following example shows how to set the maximum tenuring
threshold to 10:
-XX:MaxTenuringThreshold=10
-XX:MetaspaceSize=size
Sets the size of the allocated class metadata space that trig-
gers a garbage collection the first time it's exceeded. This
threshold for a garbage collection is increased or decreased de-
pending on the amount of metadata used. The default size de-
pends on the platform.
-XX:MinHeapFreeRatio=percent
Sets the minimum allowed percentage of free heap space (0 to
100) after a GC event. If free heap space falls below this val-
ue, then the heap is expanded. By default, this value is set to
40%.
Minimize Java heap size by lowering the values of the parameters
MaxHeapFreeRatio (default value is 70%) and MinHeapFreeRatio
(default value is 40%) with the command-line options -XX:Max-
HeapFreeRatio and -XX:MinHeapFreeRatio. Lowering MaxHeapFreeRa-
tio to as low as 10% and MinHeapFreeRatio to 5% has successfully
reduced the heap size without too much performance regression;
however, results may vary greatly depending on your application.
Try different values for these parameters until they're as low
as possible, yet still retain acceptable performance.
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Customers trying to keep the heap small should also add the op-
tion -XX:-ShrinkHeapInSteps. See Performance Tuning Examples
for a description of using this option to keep the Java heap
small by reducing the dynamic footprint for embedded applica-
tions.
-XX:MinHeapSize=size
Sets the minimum size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater
than 1 MB. Append the letter k or K to indicate kilobytes, m or
M to indicate megabytes, or g or G to indicate gigabytes. The
default value is selected at run time based on the system con-
figuration.
The following examples show how to set the mimimum size of allo-
cated memory to 6 MB using various units:
-XX:MinHeapSize=6291456
-XX:MinHeapSize=6144k
-XX:MinHeapSize=6m
If you set this option to 0, then the minimum size is set to the
same value as the initial size.
-XX:NewRatio=ratio
Sets the ratio between young and old generation sizes. By de-
fault, this option is set to 2. The following example shows how
to set the young-to-old ratio to 1:
-XX:NewRatio=1
-XX:NewSize=size
Sets the initial size (in bytes) of the heap for the young gen-
eration (nursery). Append the letter k or K to indicate kilo-
bytes, m or M to indicate megabytes, or g or G to indicate giga-
bytes.
The young generation region of the heap is used for new objects.
GC is performed in this region more often than in other regions.
If the size for the young generation is too low, then a large
number of minor GCs are performed. If the size is too high,
then only full GCs are performed, which can take a long time to
complete. It is recommended that you keep the size for the
young generation greater than 25% and less than 50% of the over-
all heap size.
The following examples show how to set the initial size of the
young generation to 256 MB using various units:
-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456
The -XX:NewSize option is equivalent to -Xmn.
-XX:ParallelGCThreads=threads
Sets the number of the stop-the-world (STW) worker threads. The
default value depends on the number of CPUs available to the JVM
and the garbage collector selected.
For example, to set the number of threads for G1 GC to 2, speci-
fy the following option:
-XX:ParallelGCThreads=2
-XX:+ParallelRefProcEnabled
Enables parallel reference processing. By default, this option
is disabled.
-XX:+PrintAdaptiveSizePolicy
Enables printing of information about adaptive-generation siz-
ing. By default, this option is disabled.
-XX:+ScavengeBeforeFullGC
Enables GC of the young generation before each full GC. This
option is enabled by default. It is recommended that you don't
disable it, because scavenging the young generation before a
full GC can reduce the number of objects reachable from the old
generation space into the young generation space. To disable GC
of the young generation before each full GC, specify the option
-XX:-ScavengeBeforeFullGC.
-XX:SoftRefLRUPolicyMSPerMB=time
Sets the amount of time (in milliseconds) a softly reachable ob-
ject is kept active on the heap after the last time it was ref-
erenced. The default value is one second of lifetime per free
megabyte in the heap. The -XX:SoftRefLRUPolicyMSPerMB option
accepts integer values representing milliseconds per one
megabyte of the current heap size (for Java HotSpot Client VM)
or the maximum possible heap size (for Java HotSpot Server VM).
This difference means that the Client VM tends to flush soft
references rather than grow the heap, whereas the Server VM
tends to grow the heap rather than flush soft references. In
the latter case, the value of the -Xmx option has a significant
effect on how quickly soft references are garbage collected.
The following example shows how to set the value to 2.5 seconds:
-XX:SoftRefLRUPolicyMSPerMB=2500
-XX:-ShrinkHeapInSteps
Incrementally reduces the Java heap to the target size, speci-
fied by the option -XX:MaxHeapFreeRatio. This option is enabled
by default. If disabled, then it immediately reduces the Java
heap to the target size instead of requiring multiple garbage
collection cycles. Disable this option if you want to minimize
the Java heap size. You will likely encounter performance
degradation when this option is disabled.
See Performance Tuning Examples for a description of using the
MaxHeapFreeRatio option to keep the Java heap small by reducing
the dynamic footprint for embedded applications.
-XX:StringDeduplicationAgeThreshold=threshold
Identifies String objects reaching the specified age that are
considered candidates for deduplication. An object's age is a
measure of how many times it has survived garbage collection.
This is sometimes referred to as tenuring.
Note: String objects that are promoted to an old heap re-
gion before this age has been reached are always consid-
ered candidates for deduplication. The default value for
this option is 3. See the -XX:+UseStringDeduplication
option.
-XX:SurvivorRatio=ratio
Sets the ratio between eden space size and survivor space size.
By default, this option is set to 8. The following example
shows how to set the eden/survivor space ratio to 4:
-XX:SurvivorRatio=4
-XX:TargetSurvivorRatio=percent
Sets the desired percentage of survivor space (0 to 100) used
after young garbage collection. By default, this option is set
to 50%.
The following example shows how to set the target survivor space
ratio to 30%:
-XX:TargetSurvivorRatio=30
-XX:TLABSize=size
Sets the initial size (in bytes) of a thread-local allocation
buffer (TLAB). Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or G to indicate gigabytes.
If this option is set to 0, then the JVM selects the initial
size automatically.
The following example shows how to set the initial TLAB size to
512 KB:
-XX:TLABSize=512k
-XX:+UseAdaptiveSizePolicy
Enables the use of adaptive generation sizing. This option is
enabled by default. To disable adaptive generation sizing,
specify -XX:-UseAdaptiveSizePolicy and set the size of the memo-
ry allocation pool explicitly. See the -XX:SurvivorRatio op-
tion.
-XX:+UseG1GC
Enables the use of the garbage-first (G1) garbage collector.
It's a server-style garbage collector, targeted for multiproces-
sor machines with a large amount of RAM. This option meets GC
pause time goals with high probability, while maintaining good
throughput. The G1 collector is recommended for applications
requiring large heaps (sizes of around 6 GB or larger) with lim-
ited GC latency requirements (a stable and predictable pause
time below 0.5 seconds). By default, this option is enabled and
G1 is used as the default garbage collector.
-XX:+UseGCOverheadLimit
Enables the use of a policy that limits the proportion of time
spent by the JVM on GC before an OutOfMemoryError exception is
thrown. This option is enabled, by default, and the parallel GC
will throw an OutOfMemoryError if more than 98% of the total
time is spent on garbage collection and less than 2% of the heap
is recovered. When the heap is small, this feature can be used
to prevent applications from running for long periods of time
with little or no progress. To disable this option, specify the
option -XX:-UseGCOverheadLimit.
-XX:+UseNUMA
Enables performance optimization of an application on a machine
with nonuniform memory architecture (NUMA) by increasing the ap-
plication's use of lower latency memory. By default, this op-
tion is disabled and no optimization for NUMA is made. The op-
tion is available only when the parallel garbage collector is
used (-XX:+UseParallelGC).
-XX:+UseParallelGC
Enables the use of the parallel scavenge garbage collector (also
known as the throughput collector) to improve the performance of
your application by leveraging multiple processors.
By default, this option is disabled and the default collector is
used.
-XX:+UseSerialGC
Enables the use of the serial garbage collector. This is gener-
ally the best choice for small and simple applications that
don't require any special functionality from garbage collection.
By default, this option is disabled and the default collector is
used.
-XX:+UseSHM
Linux only: Enables the JVM to use shared memory to set up large
pages.
See Large Pages for setting up large pages.
-XX:+UseStringDeduplication
Enables string deduplication. By default, this option is dis-
abled. To use this option, you must enable the garbage-first
(G1) garbage collector.
String deduplication reduces the memory footprint of String ob-
jects on the Java heap by taking advantage of the fact that many
String objects are identical. Instead of each String object
pointing to its own character array, identical String objects
can point to and share the same character array.
-XX:+UseTLAB
Enables the use of thread-local allocation blocks (TLABs) in the
young generation space. This option is enabled by default. To
disable the use of TLABs, specify the option -XX:-UseTLAB.
-XX:+UseZGC
Enables the use of the Z garbage collector (ZGC). This is a low
latency garbage collector, providing max pause times of a few
milliseconds, at some throughput cost. Pause times are indepen-
dent of what heap size is used. Supports heap sizes from 8MB to
16TB.
-XX:ZAllocationSpikeTolerance=factor
Sets the allocation spike tolerance for ZGC. By default, this
option is set to 2.0. This factor describes the level of allo-
cation spikes to expect. For example, using a factor of 3.0
means the current allocation rate can be expected to triple at
any time.
-XX:ZCollectionInterval=seconds
Sets the maximum interval (in seconds) between two GC cycles
when using ZGC. By default, this option is set to 0 (disabled).
-XX:ZFragmentationLimit=percent
Sets the maximum acceptable heap fragmentation (in percent) for
ZGC. By default, this option is set to 25. Using a lower value
will cause the heap to be compacted more aggressively, to re-
claim more memory at the cost of using more CPU time.
-XX:+ZProactive
Enables proactive GC cycles when using ZGC. By default, this
option is enabled. ZGC will start a proactive GC cycle if doing
so is expected to have minimal impact on the running applica-
tion. This is useful if the application is mostly idle or allo-
cates very few objects, but you still want to keep the heap size
down and allow reference processing to happen even when there
are a lot of free space on the heap.
-XX:+ZUncommit
Enables uncommitting of unused heap memory when using ZGC. By
default, this option is enabled. Uncommitting unused heap memo-
ry will lower the memory footprint of the JVM, and make that
memory available for other processes to use.
-XX:ZUncommitDelay=seconds
Sets the amount of time (in seconds) that heap memory must have
been unused before being uncommitted. By default, this option
is set to 300 (5 minutes). Committing and uncommitting memory
are relatively expensive operations. Using a lower value will
cause heap memory to be uncommitted earlier, at the risk of soon
having to commit it again.
DEPRECATED JAVA OPTIONS
These java options are deprecated and might be removed in a future JDK
release. They're still accepted and acted upon, but a warning is is-
sued when they're used.
-Xfuture
Enables strict class-file format checks that enforce close con-
formance to the class-file format specification. Developers
should use this flag when developing new code. Stricter checks
may become the default in future releases.
-Xloggc:filename
Sets the file to which verbose GC events information should be
redirected for logging. The -Xloggc option overrides -ver-
bose:gc if both are given with the same java command. -Xlog-
gc:filename is replaced by -Xlog:gc:filename. See Enable Log-
ging with the JVM Unified Logging Framework.
Example:
-Xlog:gc:garbage-collection.log
-XX:+FlightRecorder
Enables the use of Java Flight Recorder (JFR) during the runtime
of the application. Since JDK 8u40 this option has not been re-
quired to use JFR.
-XX:InitialRAMFraction=ratio
Sets the initial amount of memory that the JVM may use for the
Java heap before applying ergonomics heuristics as a ratio of
the maximum amount determined as described in the -XX:MaxRAM op-
tion. The default value is 64.
Use the option -XX:InitialRAMPercentage instead.
-XX:MaxRAMFraction=ratio
Sets the maximum amount of memory that the JVM may use for the
Java heap before applying ergonomics heuristics as a fraction of
the maximum amount determined as described in the -XX:MaxRAM op-
tion. The default value is 4.
Specifying this option disables automatic use of compressed oops
if the combined result of this and other options influencing the
maximum amount of memory is larger than the range of memory ad-
dressable by compressed oops. See -XX:UseCompressedOops for
further information about compressed oops.
Use the option -XX:MaxRAMPercentage instead.
-XX:MinRAMFraction=ratio
Sets the maximum amount of memory that the JVM may use for the
Java heap before applying ergonomics heuristics as a fraction of
the maximum amount determined as described in the -XX:MaxRAM op-
tion for small heaps. A small heap is a heap of approximately
125 MB. The default value is 2.
Use the option -XX:MinRAMPercentage instead.
-XX:+UseBiasedLocking
Enables the use of biased locking. Some applications with sig-
nificant amounts of uncontended synchronization may attain sig-
nificant speedups with this flag enabled, but applications with
certain patterns of locking may see slowdowns.
By default, this option is disabled.
OBSOLETE JAVA OPTIONS
These java options are still accepted but ignored, and a warning is is-
sued when they're used.
--illegal-access=parameter
Controlled relaxed strong encapsulation, as defined in JEP 261
[https://openjdk.java.net/jeps/261#Relaxed-strong-encapsula-
tion]. This option was deprecated in JDK 16 by JEP 396
[https://openjdk.java.net/jeps/396] and made obsolete in JDK 17
by JEP 403 [https://openjdk.java.net/jeps/403].
REMOVED JAVA OPTIONS
These java options have been removed in JDK 17 and using them results
in an error of:
Unrecognized VM option option-name
-XX:+UseMembar
Enabled issuing membars on thread-state transitions. This op-
tion was disabled by default on all platforms except ARM
servers, where it was enabled.
-XX:MaxPermSize=size
Sets the maximum permanent generation space size (in bytes).
This option was deprecated in JDK 8 and superseded by the
-XX:MaxMetaspaceSize option.
-XX:PermSize=size
Sets the space (in bytes) allocated to the permanent generation
that triggers a garbage collection if it's exceeded. This op-
tion was deprecated in JDK 8 and superseded by the -XX:Metas-
paceSize option.
-XX:+TraceClassLoading
Enables tracing of classes as they are loaded. By default, this
option is disabled and classes aren't traced.
The replacement Unified Logging syntax is -Xlog:class+load=lev-
el. See Enable Logging with the JVM Unified Logging Framework
Use level=info for regular information, or level=debug for addi-
tional information. In Unified Logging syntax, -verbose:class
equals -Xlog:class+load=info,class+unload=info.
-XX:+TraceClassLoadingPreorder
Enables tracing of all loaded classes in the order in which
they're referenced. By default, this option is disabled and
classes aren't traced.
The replacement Unified Logging syntax is -Xlog:class+pre-
order=debug. See Enable Logging with the JVM Unified Logging
Framework.
-XX:+TraceClassResolution
Enables tracing of constant pool resolutions. By default, this
option is disabled and constant pool resolutions aren't traced.
The replacement Unified Logging syntax is -Xlog:class+re-
solve=debug. See Enable Logging with the JVM Unified Logging
Framework.
-XX:+TraceLoaderConstraints
Enables tracing of the loader constraints recording. By de-
fault, this option is disabled and loader constraints recording
isn't traced.
The replacement Unified Logging syntax is -Xlog:class+load-
er+constraints=info. See Enable Logging with the JVM Unified
Logging Framework.
For the lists and descriptions of options removed in previous releases
see the Removed Java Options section in:
• Java Platform, Standard Edition Tools Reference, Release 16
[https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html]
• Java Platform, Standard Edition Tools Reference, Release 15
[https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html]
• Java Platform, Standard Edition Tools Reference, Release 14
[https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html]
• Java Platform, Standard Edition Tools Reference, Release 13
[https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html]
• Java Platform, Standard Edition Tools Reference, Release 12
[https://docs.oracle.com/en/java/javase/12/tools/ja-
va.html#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE]
• Java Platform, Standard Edition Tools Reference, Release 11
[https://docs.oracle.com/en/java/javase/11/tools/ja-
va.html#GUID-741FC470-AA3E-494A-8D2B-1B1FE4A990D1]
• Java Platform, Standard Edition Tools Reference, Release 10
[https://docs.oracle.com/javase/10/tools/java.htm#JSWOR624]
• Java Platform, Standard Edition Tools Reference, Release 9
[https://docs.oracle.com/javase/9/tools/java.htm#JSWOR624]
• Java Platform, Standard Edition Tools Reference, Release 8 for Oracle
JDK on Windows [https://docs.oracle.com/javase/8/docs/tech-
notes/tools/windows/java.html#BGBCIEFC]
• Java Platform, Standard Edition Tools Reference, Release 8 for Oracle
JDK on Solaris, Linux, and macOS [https://docs.ora-
cle.com/javase/8/docs/technotes/tools/unix/java.html#BGBCIEFC]
JAVA COMMAND-LINE ARGUMENT FILES
You can shorten or simplify the java command by using @ argument files
to specify one or more text files that contain arguments, such as op-
tions and class names, which are passed to the java command. This
let's you to create java commands of any length on any operating sys-
tem.
In the command line, use the at sign (@) prefix to identify an argument
file that contains java options and class names. When the java command
encounters a file beginning with the at sign (@), it expands the con-
tents of that file into an argument list just as they would be speci-
fied on the command line.
The java launcher expands the argument file contents until it encoun-
ters the --disable-@files option. You can use the --disable-@files op-
tion anywhere on the command line, including in an argument file, to
stop @ argument files expansion.
The following items describe the syntax of java argument files:
• The argument file must contain only ASCII characters or characters in
system default encoding that's ASCII friendly, such as UTF-8.
• The argument file size must not exceed MAXINT (2,147,483,647) bytes.
• The launcher doesn't expand wildcards that are present within an ar-
gument file.
• Use white space or new line characters to separate arguments included
in the file.
• White space includes a white space character, \t, \n, \r, and \f.
For example, it is possible to have a path with a space, such as
c:\Program Files that can be specified as either "c:\\Program Files"
or, to avoid an escape, c:\Program" "Files.
• Any option that contains spaces, such as a path component, must be
within quotation marks using quotation ('"') characters in its en-
tirety.
• A string within quotation marks may contain the characters \n, \r,
\t, and \f. They are converted to their respective ASCII codes.
• If a file name contains embedded spaces, then put the whole file name
in double quotation marks.
• File names in an argument file are relative to the current directory,
not to the location of the argument file.
• Use the number sign # in the argument file to identify comments. All
characters following the # are ignored until the end of line.
• Additional at sign @ prefixes to @ prefixed options act as an escape,
(the first @ is removed and the rest of the arguments are presented
to the launcher literally).
• Lines may be continued using the continuation character (\) at the
end-of-line. The two lines are concatenated with the leading white
spaces trimmed. To prevent trimming the leading white spaces, a con-
tinuation character (\) may be placed at the first column.
• Because backslash (\) is an escape character, a backslash character
must be escaped with another backslash character.
• Partial quote is allowed and is closed by an end-of-file.
• An open quote stops at end-of-line unless \ is the last character,
which then joins the next line by removing all leading white space
characters.
• Wildcards (*) aren't allowed in these lists (such as specifying *.ja-
va).
• Use of the at sign (@) to recursively interpret files isn't support-
ed.
Example of Open or Partial Quotes in an Argument File
In the argument file,
-cp "lib/
cool/
app/
jars
this is interpreted as:
-cp lib/cool/app/jars
Example of a Backslash Character Escaped with Another Backslash
Character in an Argument File
To output the following:
-cp c:\Program Files (x86)\Java\jre\lib\ext;c:\Program Files\Ja-
va\jre9\lib\ext
The backslash character must be specified in the argument file as:
-cp "c:\\Program Files (x86)\\Java\\jre\\lib\\ext;c:\\Pro-
gram Files\\Java\\jre9\\lib\\ext"
Example of an EOL Escape Used to Force Concatenation of Lines in an
Argument File
In the argument file,
-cp "/lib/cool app/jars:\
/lib/another app/jars"
This is interpreted as:
-cp /lib/cool app/jars:/lib/another app/jars
Example of Line Continuation with Leading Spaces in an Argument File
In the argument file,
-cp "/lib/cool\
\app/jars???
This is interpreted as:
-cp /lib/cool app/jars
Examples of Using Single Argument File
You can use a single argument file, such as myargumentfile in the fol-
lowing example, to hold all required java arguments:
java @myargumentfile
Examples of Using Argument Files with Paths
You can include relative paths in argument files; however, they're rel-
ative to the current working directory and not to the paths of the ar-
gument files themselves. In the following example, path1/options and
path2/options represent argument files with different paths. Any rela-
tive paths that they contain are relative to the current working direc-
tory and not to the argument files:
java @path1/options @path2/classes
CODE HEAP STATE ANALYTICS
Overview
There are occasions when having insight into the current state of the
JVM code heap would be helpful to answer questions such as:
• Why was the JIT turned off and then on again and again?
• Where has all the code heap space gone?
• Why is the method sweeper not working effectively?
To provide this insight, a code heap state analytics feature has been
implemented that enables on-the-fly analysis of the code heap. The an-
alytics process is divided into two parts. The first part examines the
entire code heap and aggregates all information that is believed to be
useful or important. The second part consists of several independent
steps that print the collected information with an emphasis on differ-
ent aspects of the data. Data collection and printing are done on an
"on request" basis.
Syntax
Requests for real-time, on-the-fly analysis can be issued with the fol-
lowing command:
jcmd pid Compiler.CodeHeap_Analytics [function] [granularity]
If you are only interested in how the code heap looks like after run-
ning a sample workload, you can use the command line option:
-Xlog:codecache=Trace
To see the code heap state when a "CodeCache full" condition exists,
start the VM with the command line option:
-Xlog:codecache=Debug
See CodeHeap State Analytics (OpenJDK) [https://bugs.openjdk.ja-
va.net/secure/attachment/75649/JVM_CodeHeap_StateAnalytics_V2.pdf] for
a detailed description of the code heap state analytics feature, the
supported functions, and the granularity options.
ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK
You use the -Xlog option to configure or enable logging with the Java
Virtual Machine (JVM) unified logging framework.
Synopsis
-Xlog[:[what][:[output][:[decorators][:output-options[,...]]]]]
-Xlog:directive
what Specifies a combination of tags and levels of the form
tag1[+tag2...][*][=level][,...]. Unless the wildcard (*) is
specified, only log messages tagged with exactly the tags speci-
fied are matched. See -Xlog Tags and Levels.
output Sets the type of output. Omitting the output type defaults to
stdout. See -Xlog Output.
decorators
Configures the output to use a custom set of decorators. Omit-
ting decorators defaults to uptime, level, and tags. See Deco-
rations.
output-options
Sets the -Xlog logging output options.
directive
A global option or subcommand: help, disable, async
Description
The Java Virtual Machine (JVM) unified logging framework provides a
common logging system for all components of the JVM. GC logging for
the JVM has been changed to use the new logging framework. The mapping
of old GC flags to the corresponding new Xlog configuration is de-
scribed in Convert GC Logging Flags to Xlog. In addition, runtime log-
ging has also been changed to use the JVM unified logging framework.
The mapping of legacy runtime logging flags to the corresponding new
Xlog configuration is described in Convert Runtime Logging Flags to
Xlog.
The following provides quick reference to the -Xlog command and syntax
for options:
-Xlog Enables JVM logging on an info level.
-Xlog:help
Prints -Xlog usage syntax and available tags, levels, and deco-
rators along with example command lines with explanations.
-Xlog:disable
Turns off all logging and clears all configuration of the log-
ging framework including the default configuration for warnings
and errors.
-Xlog[:option]
Applies multiple arguments in the order that they appear on the
command line. Multiple -Xlog arguments for the same output
override each other in their given order.
The option is set as:
[tag-selection][:[output][:[decorators][:output-op-
tions]]]
Omitting the tag-selection defaults to a tag-set of all and a
level of info.
tag[+...] all
The all tag is a meta tag consisting of all tag-sets available.
The asterisk * in a tag set definition denotes a wildcard tag
match. Matching with a wildcard selects all tag sets that con-
tain at least the specified tags. Without the wildcard, only
exact matches of the specified tag sets are selected.
output-options is
filecount=file-count filesize=file size with optional K,
M or G suffix
Default Configuration
When the -Xlog option and nothing else is specified on the command
line, the default configuration is used. The default configuration
logs all messages with a level that matches either warning or error re-
gardless of what tags the message is associated with. The default con-
figuration is equivalent to entering the following on the command line:
-Xlog:all=warning:stdout:uptime,level,tags
Controlling Logging at Runtime
Logging can also be controlled at run time through Diagnostic Commands
(with the jcmd utility). Everything that can be specified on the com-
mand line can also be specified dynamically with the VM.log command.
As the diagnostic commands are automatically exposed as MBeans, you can
use JMX to change logging configuration at run time.
-Xlog Tags and Levels
Each log message has a level and a tag set associated with it. The
level of the message corresponds to its details, and the tag set corre-
sponds to what the message contains or which JVM component it involves
(such as, gc, jit, or os). Mapping GC flags to the Xlog configuration
is described in Convert GC Logging Flags to Xlog. Mapping legacy run-
time logging flags to the corresponding Xlog configuration is described
in Convert Runtime Logging Flags to Xlog.
Available log levels:
• off
• trace
• debug
• info
• warning
• error
Available log tags:
There are literally dozens of log tags, which in the right combina-
tions, will enable a range of logging output. The full set of avail-
able log tags can be seen using -Xlog:help. Specifying all instead of
a tag combination matches all tag combinations.
-Xlog Output
The -Xlog option supports the following types of outputs:
• stdout --- Sends output to stdout
• stderr --- Sends output to stderr
• file=filename --- Sends output to text file(s).
When using file=filename, specifying %p and/or %t in the file name ex-
pands to the JVM's PID and startup timestamp, respectively. You can
also configure text files to handle file rotation based on file size
and a number of files to rotate. For example, to rotate the log file
every 10 MB and keep 5 files in rotation, specify the options file-
size=10M, filecount=5. The target size of the files isn't guaranteed
to be exact, it's just an approximate value. Files are rotated by de-
fault with up to 5 rotated files of target size 20 MB, unless config-
ured otherwise. Specifying filecount=0 means that the log file
shouldn't be rotated. There's a possibility of the pre-existing log
file getting overwritten.
-Xlog Output Mode
By default logging messages are output synchronously - each log message
is written to the designated output when the logging call is made. But
you can instead use asynchronous logging mode by specifying:
-Xlog:async
Write all logging asynchronously.
In asynchronous logging mode, log sites enqueue all logging messages to
an intermediate buffer and a standalone thread is responsible for
flushing them to the corresponding outputs. The intermediate buffer is
bounded and on buffer exhaustion the enqueuing message is discarded.
Log entry write operations are guaranteed non-blocking.
The option -XX:AsyncLogBufferSize=N specifies the memory budget in
bytes for the intermediate buffer. The default value should be big
enough to cater for most cases. Users can provide a custom value to
trade memory overhead for log accuracy if they need to.
Decorations
Logging messages are decorated with information about the message. You
can configure each output to use a custom set of decorators. The order
of the output is always the same as listed in the table. You can con-
figure the decorations to be used at run time. Decorations are
prepended to the log message. For example:
[6.567s][info][gc,old] Old collection complete
Omitting decorators defaults to uptime, level, and tags. The none dec-
orator is special and is used to turn off all decorations.
time (t), utctime (utc), uptime (u), timemillis (tm), uptimemillis
(um), timenanos (tn), uptimenanos (un), hostname (hn), pid (p), tid
(ti), level (l), tags (tg) decorators can also be specified as none for
no decoration.
Decorations Description
──────────────────────────────────────────────────────────────────────────
time or t Current time and date in ISO-8601 format.
utctime or utc Universal Time Coordinated or Coordinated Universal
Time.
uptime or u Time since the start of the JVM in seconds and mil-
liseconds. For example, 6.567s.
timemillis or The same value as generated by System.currentTimeMil-
tm lis()
uptimemillis or Milliseconds since the JVM started.
um
timenanos or tn The same value generated by System.nanoTime().
uptimenanos or Nanoseconds since the JVM started.
un
hostname or hn The host name.
pid or p The process identifier.
tid or ti The thread identifier.
level or l The level associated with the log message.
tags or tg The tag-set associated with the log message.
Convert GC Logging Flags to Xlog
Legacy Garbage Collec- Xlog Configura- Comment
tion (GC) Flag tion
───────────────────────────────────────────────────────────────────────────────────────
G1PrintHeapRegions -Xlog:gc+re- Not Applicable
gion=trace
GCLogFileSize No configuration Log rotation is handled by the
available framework.
NumberOfGCLogFiles Not Applicable Log rotation is handled by the
framework.
PrintAdaptiveSizePoli- -Xlog:gc+er- Use a level of debug for most
cy go*=level of the information, or a level
of trace for all of what was
logged for PrintAdaptive-
SizePolicy.
PrintGC -Xlog:gc Not Applicable
PrintGCApplicationCon- -Xlog:safepoint Note that PrintGCApplication-
currentTime ConcurrentTime and PrintGCAp-
plicationStoppedTime are logged
on the same tag and aren't sep-
arated in the new logging.
PrintGCApplication- -Xlog:safepoint Note that PrintGCApplication-
StoppedTime ConcurrentTime and PrintGCAp-
plicationStoppedTime are logged
on the same tag and not sepa-
rated in the new logging.
PrintGCCause Not Applicable GC cause is now always logged.
PrintGCDateStamps Not Applicable Date stamps are logged by the
framework.
PrintGCDetails -Xlog:gc* Not Applicable
PrintGCID Not Applicable GC ID is now always logged.
PrintGCTaskTimeStamps -Xlog:gc+task*=de- Not Applicable
bug
PrintGCTimeStamps Not Applicable Time stamps are logged by the
framework.
PrintHeapAtGC -Xlog:gc+heap=trace Not Applicable
PrintReferenceGC -Xlog:gc+ref*=debug Note that in the old logging,
PrintReferenceGC had an effect
only if PrintGCDetails was also
enabled.
PrintStringDeduplica- `-Xlog:gc+stringdedup*=de- ` Not Applicable
tionStatistics bug
PrintTenuringDistribu- -Xlog:gc+age*=level Use a level of debug for the
tion most relevant information, or a
level of trace for all of what
was logged for PrintTenur-
ingDistribution.
UseGCLogFileRotation Not Applicable What was logged for PrintTenur-
ingDistribution.
Convert Runtime Logging Flags to Xlog
These legacy flags are no longer recognized and will cause an error if
used directly. Use their unified logging equivalent instead.
Legacy Runtime Xlog Configuration Comment
Flag
──────────────────────────────────────────────────────────────────────────────
TraceExceptions -Xlog:exceptions=in- Not Applicable
fo
TraceClassLoad- -Xlog:class+load=lev- Use level=info for regular informa-
ing el tion, or level=debug for additional
information. In Unified Logging
syntax, -verbose:class equals
-Xlog:class+load=info,class+un-
load=info.
TraceClassLoad- -Xlog:class+pre- Not Applicable
ingPreorder order=debug
TraceClassUn- -Xlog:class+un- Use level=info for regular informa-
loading load=level tion, or level=trace for additional
information. In Unified Logging
syntax, -verbose:class equals
-Xlog:class+load=info,class+un-
load=info.
VerboseVerifi- -Xlog:verifica- Not Applicable
cation tion=info
TraceClassPaths -Xlog:class+path=info Not Applicable
TraceClassReso- -Xlog:class+re- Not Applicable
lution solve=debug
TraceClassIni- -Xlog:class+init=info Not Applicable
tialization
TraceLoaderCon- -Xlog:class+load- Not Applicable
straints er+constraints=info
TraceClassLoad- -Xlog:class+load- Use level=debug for regular infor-
erData er+data=level mation or level=trace for addition-
al information.
TraceSafepoint- -Xlog:safe- Not Applicable
CleanupTime point+cleanup=info
TraceSafepoint -Xlog:safepoint=debug Not Applicable
TraceMonitorIn- -Xlog:monitorinfla- Not Applicable
flation tion=debug
TraceBiased- -Xlog:biasedlock- Use level=info for regular informa-
Locking ing=level tion, or level=trace for additional
information.
TraceRede- -Xlog:rede- level=info, debug, and trace pro-
fineClasses fine+class*=level vide increasing amounts of informa-
tion.
-Xlog Usage Examples
The following are -Xlog examples.
-Xlog Logs all messages by using the info level to stdout with uptime,
levels, and tags decorations. This is equivalent to using:
-Xlog:all=info:stdout:uptime,levels,tags
-Xlog:gc
Logs messages tagged with the gc tag using info level to stdout.
The default configuration for all other messages at level warn-
ing is in effect.
-Xlog:gc,safepoint
Logs messages tagged either with the gc or safepoint tags, both
using the info level, to stdout, with default decorations. Mes-
sages tagged with both gc and safepoint won't be logged.
-Xlog:gc+ref=debug
Logs messages tagged with both gc and ref tags, using the debug
level to stdout, with default decorations. Messages tagged only
with one of the two tags won't be logged.
-Xlog:gc=debug:file=gc.txt:none
Logs messages tagged with the gc tag using the debug level to a
file called gc.txt with no decorations. The default configura-
tion for all other messages at level warning is still in effect.
-Xlog:gc=trace:file=gctrace.txt:uptimemillis,pids:filecount=5,file-
size=1024
Logs messages tagged with the gc tag using the trace level to a
rotating file set with 5 files with size 1 MB with the base name
gctrace.txt and uses decorations uptimemillis and pid.
The default configuration for all other messages at level warn-
ing is still in effect.
-Xlog:gc::uptime,tid
Logs messages tagged with the gc tag using the default 'info'
level to default the output stdout and uses decorations uptime
and tid. The default configuration for all other messages at
level warning is still in effect.
-Xlog:gc*=info,safepoint*=off
Logs messages tagged with at least gc using the info level, but
turns off logging of messages tagged with safepoint. Messages
tagged with both gc and safepoint won't be logged.
-Xlog:disable -Xlog:safepoint=trace:safepointtrace.txt
Turns off all logging, including warnings and errors, and then
enables messages tagged with safepointusing tracelevel to the
file safepointtrace.txt. The default configuration doesn't ap-
ply, because the command line started with -Xlog:disable.
Complex -Xlog Usage Examples
The following describes a few complex examples of using the -Xlog op-
tion.
-Xlog:gc+class*=debug
Logs messages tagged with at least gc and class tags using the
debug level to stdout. The default configuration for all other
messages at the level warning is still in effect
-Xlog:gc+meta*=trace,class*=off:file=gcmetatrace.txt
Logs messages tagged with at least the gc and meta tags using
the trace level to the file metatrace.txt but turns off all mes-
sages tagged with class. Messages tagged with gc, meta, and
class aren't be logged as class* is set to off. The default
configuration for all other messages at level warning is in ef-
fect except for those that include class.
-Xlog:gc+meta=trace
Logs messages tagged with exactly the gc and meta tags using the
trace level to stdout. The default configuration for all other
messages at level warning is still be in effect.
-Xlog:gc+class+heap*=debug,meta*=warning,threads*=off
Logs messages tagged with at least gc, class, and heap tags us-
ing the trace level to stdout but only log messages tagged with
meta with level. The default configuration for all other mes-
sages at the level warning is in effect except for those that
include threads.
VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS
You use values provided to all Java Virtual Machine (JVM) command-line
flags for validation and, if the input value is invalid or
out-of-range, then an appropriate error message is displayed.
Whether they're set ergonomically, in a command line, by an input tool,
or through the APIs (for example, classes contained in the package ja-
va.lang.management) the values provided to all Java Virtual Machine
(JVM) command-line flags are validated. Ergonomics are described in
Java Platform, Standard Edition HotSpot Virtual Machine Garbage Collec-
tion Tuning Guide.
Range and constraints are validated either when all flags have their
values set during JVM initialization or a flag's value is changed dur-
ing runtime (for example using the jcmd tool). The JVM is terminated
if a value violates either the range or constraint check and an appro-
priate error message is printed on the error stream.
For example, if a flag violates a range or a constraint check, then the
JVM exits with an error:
java -XX:AllocatePrefetchStyle=5 -version
intx AllocatePrefetchStyle=5 is outside the allowed range [ 0 ... 3 ]
Improperly specified VM option 'AllocatePrefetchStyle=5'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
The flag -XX:+PrintFlagsRanges prints the range of all the flags. This
flag allows automatic testing of the flags by the values provided by
the ranges. For the flags that have the ranges specified, the type,
name, and the actual range is printed in the output.
For example,
intx ThreadStackSize [ 0 ... 9007199254740987 ] {pd product}
For the flags that don't have the range specified, the values aren't
displayed in the print out. For example:
size_t NewSize [ ... ] {product}
This helps to identify the flags that need to be implemented. The au-
tomatic testing framework can skip those flags that don't have values
and aren't implemented.
LARGE PAGES
You use large pages, also known as huge pages, as memory pages that are
significantly larger than the standard memory page size (which varies
depending on the processor and operating system). Large pages optimize
processor Translation-Lookaside Buffers.
A Translation-Lookaside Buffer (TLB) is a page translation cache that
holds the most-recently used virtual-to-physical address translations.
A TLB is a scarce system resource. A TLB miss can be costly because
the processor must then read from the hierarchical page table, which
may require multiple memory accesses. By using a larger memory page
size, a single TLB entry can represent a larger memory range. This re-
sults in less pressure on a TLB, and memory-intensive applications may
have better performance.
However, using large pages can negatively affect system performance.
For example, when a large amount of memory is pinned by an application,
it may create a shortage of regular memory and cause excessive paging
in other applications and slow down the entire system. Also, a system
that has been up for a long time could produce excessive fragmentation,
which could make it impossible to reserve enough large page memory.
When this happens, either the OS or JVM reverts to using regular pages.
Linux and Windows support large pages.
Large Pages Support for Linux
Linux supports large pages since version 2.6. To check if your envi-
ronment supports large pages, try the following:
# cat /proc/meminfo | grep Huge
HugePages_Total: 0
HugePages_Free: 0
...
Hugepagesize: 2048 kB
If the output contains items prefixed with "Huge", then your system
supports large pages. The values may vary depending on environment.
The Hugepagesize field shows the default large page size in your envi-
ronment, and the other fields show details for large pages of this
size. Newer kernels have support for multiple large page sizes. To
list the supported page sizes, run this:
# ls /sys/kernel/mm/hugepages/
hugepages-1048576kB hugepages-2048kB
The above environment supports 2 MB and 1 GB large pages, but they need
to be configured so that the JVM can use them. When using large pages
and not enabling transparent huge pages (option -XX:+UseTransparen-
tHugePages), the number of large pages must be pre-allocated. For ex-
ample, to enable 8 GB of memory to be backed by 2 MB large pages, login
as root and run:
# echo 4096 > /sys/ker-
nel/mm/hugepages/hugepages-2048kB/nr_hugepages
It is always recommended to check the value of nr_hugepages after the
request to make sure the kernel was able to allocate the requested num-
ber of large pages.
When using the option -XX:+UseSHM to enable large pages you also need
to make sure the SHMMAX parameter is configured to allow large enough
shared memory segments to be allocated. To allow a maximum shared seg-
ment of 8 GB, login as root and run:
# echo 8589934592 > /proc/sys/kernel/shmmax
In some environments this is not needed since the default value is
large enough, but it is important to make sure the value is large
enough to fit the amount of memory intended to be backed by large
pages.
Note: The values contained in /proc and /sys reset after you re-
boot your system, so may want to set them in an initialization
script (for example, rc.local or sysctl.conf).
If you configure the OS kernel parameters to enable use of large pages,
the Java processes may allocate large pages for the Java heap as well
as other internal areas, for example:
• Code cache
• Marking bitmaps
Consequently, if you configure the nr_hugepages parameter to the size
of the Java heap, then the JVM can still fail to allocate the heap us-
ing large pages because other areas such as the code cache might al-
ready have used some of the configured large pages.
Large Pages Support for Windows
To use large pages support on Windows, the administrator must first as-
sign additional privileges to the user who is running the application:
1. Select Control Panel, Administrative Tools, and then Local Security
Policy.
2. Select Local Policies and then User Rights Assignment.
3. Double-click Lock pages in memory, then add users and/or groups.
4. Reboot your system.
Note that these steps are required even if it's the administrator who's
running the application, because administrators by default don't have
the privilege to lock pages in memory.
APPLICATION CLASS DATA SHARING
Application Class Data Sharing (AppCDS) stores classes used by your ap-
plications in an archive file. Since these classes are stored in a
format that can be loaded very quickly (compared to classes stored in a
JAR file), AppCDS can improve the start-up time of your applications.
In addition, AppCDS can reduce the runtime memory footprint by sharing
parts of these classes across multiple processes.
Classes in the CDS archive are stored in an optimized format that's
about 2 to 5 times larger than classes stored in JAR files or the JDK
runtime image. Therefore, it's a good idea to archive only those
classes that are actually used by your application. These usually are
just a small portion of all available classes. For example, your ap-
plication may use only a few APIs provided by a large library.
Using CDS Archives
By default, in most JDK distributions, unless -Xshare:off is specified,
the JVM starts up with a default CDS archive, which is usually located
in JAVA_HOME/lib/server/classes.jsa (or JAVA_HOME\bin\server\class-
es.jsa on Windows). This archive contains about 1300 core library
classes that are used by most applications.
To use CDS for the exact set of classes used by your application, you
can use the -XX:SharedArchiveFile option, which has the general form:
-XX:SharedArchiveFile=<static_archive>:<dynamic_archive>
• The <static_archive> overrides the default CDS archive.
• The <dynamic_archive> provides additional classes that can be loaded
on top of those in the <static_archive>.
• On Windows, the above path delimiter : should be replaced with ;
(The names "static" and "dyanmic" are used for historical reasons. The
only significance is that the "static" archive is loaded first and the
"dynamic" archive is loaded second).
The JVM can use up to two archives. To use only a single <static_ar-
chive>, you can omit the <dynamic_archive> portion:
-XX:SharedArchiveFile=<static_archive>
For convenience, the <dynamic_archive> records the location of the
<static_archive>. Therefore, you can omit the <static_archive> by say-
ing only:
-XX:SharedArchiveFile=<dynamic_archive>
Creating CDS Archives
CDS archives can be created with several methods:
• -Xshare:dump
• -XX:ArchiveClassesAtExit
• jcmd VM.cds
One common operation in all these methods is a "trial run", where you
run the application once to determine the classes that should be stored
in the archive.
Creating a Static CDS Archive File with -Xshare:dump
The following steps create a static CDS archive file that contains all
the classes used by the test.Hello application.
1. Create a list of all classes used by the test.Hello application.
The following command creates a file named hello.classlist that con-
tains a list of all classes used by this application:
java -Xshare:off -XX:DumpLoadedClassList=hel-
lo.classlist -cp hello.jar test.Hello
The classpath specified by the -cp parameter must contain only JAR
files.
2. Create a static archive, named hello.jsa, that contains all the
classes in hello.classlist:
java -Xshare:dump -XX:SharedArchiveFile=hel-
lo.jsa -XX:SharedClassListFile=hello.classlist -cp hello.jar
3. Run the application test.Hello with the archive hello.jsa:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hel-
lo
4. Optional Verify that the test.Hello application is using the class
contained in the hello.jsa shared archive:
java -XX:SharedArchiveFile=hello.jsa -cp hel-
lo.jar -Xlog:class+load test.Hello
The output of this command should contain the following text:
[info][class,load] test.Hello source: shared objects file
Creating a Dynamic CDS Archive File with -XX:SharedArchiveFile
Advantages of dynamic CDS archives are:
• They usually use less disk space, since they don't need to store the
classes that are already in the static archive.
• They are created with one fewer step than the comparable static ar-
chive.
The following steps create a dynamic CDS archive file that contains the
classes that are used by the test.Hello application, excluding those
that are already in the default CDS archive.
1. Create a dynamic CDS archive, named hello.jsa, that contains all the
classes in hello.jar loaded by the application test.Hello:
java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello
2. Run the application test.Hello with the shared archive hello.jsa:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hel-
lo
3. Optional Repeat step 4 of the previous section to verify that the
test.Hello application is using the class contained in the hello.jsa
shared archive.
It's also possible to create a dynamic CDS archive with a non-default
static CDS archive. E.g.,
java -XX:SharedArchiveFile=base.jsa -XX:ArchiveClassesAtEx-
it=hello.jsa -cp hello.jar Hello
To run the application using this dynamic CDS archive:
java -XX:SharedArchiveFile=base.jsa:hello.jsa -cp hello.jar Hel-
lo
(On Windows, the above path delimiter : should be replaced with ;)
As mention above, the name of the static archive can be skipped:
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello
Creating CDS Archive Files with jcmd
The previous two sections require you to modify the application's
start-up script in order to create a CDS archive. Sometimes this could
be difficult, for example, if the application's class path is set up by
complex routines.
The jcmd VM.cds command provides a less intrusive way for creating a
CDS archive by connecting to a running JVM process. You can create ei-
ther a static:
jcmd <pid> VM.cds static_dump my_static_archive.jsa
or a dynamic archive:
jcmd <pid> VM.cds dynamic_dump my_dynamic_archive.jsa
To use the resulting archive file in a subsequent run of the applica-
tion without modifying the application's start-up script, you can use
the following technique:
env JAVA_TOOL_OPTIONS=-XX:SharedArchiveFile=my_static_ar-
chive.jsa bash app_start.sh
Note: to use jcmd <pid> VM.cds dynamic_dump, the JVM process identified
by <pid> must be started with -XX:+RecordDynamicDumpInfo, which can al-
so be passed to the application start-up script with the same tech-
nique:
env JAVA_TOOL_OPTIONS=-XX:+RecordDynamicDumpIn-
fo bash app_start.sh
Restrictions on Class Path and Module Path
• Neither the class path (-classpath and -Xbootclasspath/a) nor the
module path (--module-path) can contain non-empty directories.
• Only modular JAR files are supported in --module-path. Exploded mod-
ules are not supported.
• The class path used at archive creation time must be the same as (or
a prefix of) the class path used at run time. (There's no such re-
quirement for the module path.)
• The CDS archive cannot be loaded if any JAR files in the class path
or module path are modified after the archive is generated.
• If any of the VM options --upgrade-module-path, --patch-module or
--limit-modules are specified, CDS is disabled. This means that the
JVM will execute without loading any CDS archives. In addition, if
you try to create a CDS archive with any of these 3 options speci-
fied, the JVM will report an error.
PERFORMANCE TUNING EXAMPLES
You can use the Java advanced runtime options to optimize the perfor-
mance of your applications.
Tuning for Higher Throughput
Use the following commands and advanced options to achieve higher
throughput performance for your application:
java -server -XX:+UseParallelGC -XX:+Use-
LargePages -Xmn10g -Xms26g -Xmx26g
Tuning for Lower Response Time
Use the following commands and advanced options to achieve lower re-
sponse times for your application:
java -XX:+UseG1GC -XX:MaxGCPauseMillis=100
Keeping the Java Heap Small and Reducing the Dynamic Footprint of
Embedded Applications
Use the following advanced runtime options to keep the Java heap small
and reduce the dynamic footprint of embedded applications:
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Note: The defaults for these two options are 70% and 40% respec-
tively. Because performance sacrifices can occur when using
these small settings, you should optimize for a small footprint
by reducing these settings as much as possible without introduc-
ing unacceptable performance degradation.
EXIT STATUS
The following exit values are typically returned by the launcher when
the launcher is called with the wrong arguments, serious errors, or ex-
ceptions thrown by the JVM. However, a Java application may choose to
return any value by using the API call System.exit(exitValue). The
values are:
• 0: Successful completion
• >0: An error occurred
JDK 17 2021 JAVA(1)
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