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 options.

mainclass

Specifies the name of the class to be launched. Command-line entries following classname are the arguments for the main method.

-jar jarfile

Executes a program encapsulated in a JAR file. The jarfile argument 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.

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 arguments 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 calling 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:

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.

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.

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 determines the API that can be used. When you set --source N, you can only use the public API that was defined in JDK N.

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 compiled into memory, and the first class found in the source file is executed. Any arguments placed after the name of the source file in the original command line are passed to the compiled class when it is executed.

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:

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.

•

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 prefixing the filename with an @ character.

In source-file mode, compilation proceeds as follows:

•

Any command-line options that are relevant to the compilation environment 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 public static void main(String[]) method.

•

The compiled classes are loaded by a custom class loader, that delegates to the application class loader. This implies that classes appearing on the application class path cannot refer to any classes declared in the source file.

•

The compiled classes are executed in the context of an unnamed module, 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 [https://openjdk.org/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 determined 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_JAVA_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 wildcard 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 launcher 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 categories:

•

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 supported 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 developer options used for tuning specific areas of the Java HotSpot Virtual Machine operation that often have specific system requirements and may require privileged access to system configuration parameters. Several 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 information 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:-OptionName).

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 example, to set the size to 8 GB, you can specify either 8g, 8192m, 8388608k, or 8589934592 as the argument. If you are expected to specify 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 deprecated, obsolete, and removed:

•

Deprecated Java Options: Accepted and acted upon --- a warning is issued 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 implementations of the JVM.

-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. If the option -agentlib:foo is specified, then the JVM attempts to load the library named foo using the platform specific naming conventions and locations:

-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

Specifies a list of directories, JAR files, and ZIP archives to search for class files.

--disable-@files

Can be used anywhere on the command line, including in an argument 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.oracle.com/en/java/javase/12/language/index.html#JSLAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823] of the release.

--finalization=value

Controls whether the JVM performs finalization of objects. Valid values are "enabled" and "disabled". Finalization is enabled by default, so the value "enabled" does nothing. The value "disabled" disables finalization, so that no finalizers are invoked.

--module-path modulepath... or -p modulepath

Specifies where to find application modules with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.

--upgrade-module-path modulepath...

Specifies where to find module replacements of upgradeable modules in the runtime image with a list of path elements. The elements of a module path can be a file path to a module or a directory containing modules. Each module is either a modular JAR or an exploded-module directory.

--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[:[packagename]...|: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 assertions in the specified package and any subpackages. If the argument 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 specified class.

-disablesystemassertions or -dsa

Disables assertions in all system classes.

-enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|: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 assertions in the specified package and any subpackages. If the argument 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 specified class.

-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 java.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 automatically.

-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 option was encountered.

EXTRA OPTIONS FOR JAVA¶

The following java options are general purpose options that are specific 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 interpreter 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.

-Xcheck:jni

Performs additional checks for Java Native Interface (JNI) functions.

-Xcomp

Testing mode to exercise JIT compilers. This option should not be used in production environments.

-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 interpreter. 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 Unified 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. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. 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 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 following examples show how to set the initial and maximum size of young generation to 256 MB using various units:

-Xms size

Sets the minimum and the 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, or g or G to indicate gigabytes. The following examples show how to set the size of allocated memory to 6 MB using various units:

-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:

-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 permanently occupied which, if not used carefully, throws an out-of-memory exception.

-Xrs

Reduces the use of operating system signals by the JVM. Shutdown 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.

-Xshare:mode

Sets the class data sharing (CDS) mode.

-XshowSettings

Shows all settings and then continues.

-XshowSettings:category

Shows settings and continues. Possible category arguments for this option include the following:

-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 actual size may be rounded up to a multiple of the system page size as required by the operating system. The default value depends on the platform. For example:

--add-reads module=target-module(,target-module)*

Updates module to read the target-module, regardless of the module 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 export 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 default, this option is disabled and diagnostic options aren't available.

-XX:+UnlockExperimentalVMOptions

Unlocks the options that provide experimental features in the JVM. By default, this option is disabled and experimental features 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.

-XX:AllocateHeapAt=path

Takes a path to the file system and uses memory mapping to allocate 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.

-XX:-CompactStrings

Disables the Compact Strings feature. By default, this option is enabled. When this option is enabled, Java Strings containing 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 required 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.

-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_pidpid.log where pid is the identifier of the process that encountered the error.

-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 considered sensitive. The information can vary from release to release, and across different platforms. By default this option is disabled.

-XX:FlightRecorderOptions=parameter=value (or) -XX:FlightRecorderOptions:parameter=value

Sets the parameters that control the behavior of JFR. Multiple parameters can be specified 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 environment to have any effect. 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 will use the default large page size for the environment as the maximum size for large pages. See Large Pages.

-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. If not set, the flag is ignored and the JVM chooses the size for NIO direct-buffer allocations automatically.

-XX:-MaxFDLimit

Disables the attempt to set the soft limit for the number of open file descriptors to the hard limit. By default, this option is enabled on all platforms, but is ignored on Windows. The only time that you may need to disable this is on macOS, 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. Possible mode arguments for this option include the following:

-XX:TrimNativeHeapInterval=millis

Interval, in ms, at which the JVM will trim the native heap. Lower values will reclaim memory more eagerly at the cost of higher overhead. A value of 0 (default) disables native heap trimming. Native heap trimming is performed in a dedicated thread.

-XX:+NeverActAsServerClassMachine

Enable the "Client VM emulation" mode which only uses the C1 JIT compiler, a 32Mb CodeCache and the Serial GC. The maximum amount of memory that the JVM may use (controlled by the -XX:MaxRAM=n flag) is set to 1GB by default. The string "emulated-client" is added to the JVM version string.

-XX:ObjectAlignmentInBytes=alignment

Sets the memory alignment of Java objects (in bytes). By default, 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 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.

-XX:OnError=string

Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.

-XX:OnOutOfMemoryError=string

Sets a custom command or a series of semicolon-separated commands to run when an OutOfMemoryError exception is first thrown. If the string contains spaces, then it must be enclosed in quotation marks. For an example of a command string, see the description of the -XX:OnError option.

-XX:+PrintCommandLineFlags

Enables printing of ergonomically selected JVM flags that appeared on the command line. It can be useful to know the ergonomic 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 register (-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 construct 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:NativeMemoryTracking). 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) archive file

-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 checksums. 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 java.lang.Object and hello.Main, create a text file that contains the following two lines:

-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 parameter=value entries when starting a JFR recording:

-XX:ThreadStackSize=size

Sets the Java thread stack size (in kilobytes). Use of a scaling 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 depends on the platform. For example:

-XX:-UseCompressedOops

Disables the use of compressed pointers. By default, this option 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.

-XX:-UseContainerSupport

Linux only: 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 docker containers. It uses this information to allocate system resources. The default for this flag is true, and container support is enabled by default. It can be disabled with -XX:-UseContainerSupport.

-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.

-XX:+UseLargePages

Enables the use of large page memory. By default, this option is disabled and large page memory isn't used.

-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 option is made available for experimentation.

-XX:+AllowUserSignalHandlers

Non-Windows: Enables installation of signal handlers by the application. By default, this option is disabled and the application isn't allowed to install signal handlers.

-XX:VMOptionsFile=filename

Allows user to specify VM options in a file, for example, java -XX:VMOptionsFile=/var/my_vm_options HelloWorld.

-XX:UseBranchProtection=mode

Linux AArch64 only: Specifies the branch protection mode. All options other than none require the VM to have been built with branch protection enabled. In addition, for full protection, any native libraries provided by applications should be compiled with the same level of protection.

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 allocation pointer. By default, the number of lines to prefetch is set to 1:

-XX:AllocatePrefetchDistance=size

Sets the size (in bytes) of the prefetch distance for object allocation. Memory about to be written with the value of new objects is prefetched up to this distance starting from the address of the last allocated object. Each Java thread has its own allocation point.

-XX:AllocatePrefetchInstr=instruction

Sets the prefetch instruction to prefetch ahead of the allocation pointer. Possible values are from 0 to 3. The actual instructions behind the values depend on the platform. By default, the prefetch instruction is set to 0:

-XX:AllocatePrefetchLines=lines

Sets the number of cache lines to load after the last object allocation by using the prefetch instructions generated in compiled code. The default value is 1 if the last allocated object was an instance, and 3 if it was an array.

-XX:AllocatePrefetchStepSize=size

Sets the step size (in bytes) for sequential prefetch instructions. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the step size is set to 16 bytes:

-XX:AllocatePrefetchStyle=style

Sets the generated code style for prefetch instructions. The style argument is an integer from 0 to 3:

-XX:+BackgroundCompilation

Enables background compilation. This option is enabled by default. To disable background compilation, specify -XX:-BackgroundCompilation (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 automatically depending on the number of CPUs and memory available for compiled code. The following example shows how to set the number of threads to 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 exclude the indexOf() method of the String class from being compiled, use the following:

-XX:CompileCommandFile=filename

Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.

-XX:CompilerDirectivesFile=file

Adds directives from a file to the directives stack when a program starts. See Compiler Control [https://docs.oracle.com/en/java/javase/12/vm/compiler-control1.html#GUID-94AD8194-786A-4F19-BFFF-278F8E237F3A].

-XX:+CompilerDirectivesPrint

Prints the directives stack when the program starts or when a new directive is added.

-XX:CompileOnly=methods

Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods are compiled. 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:CompileThresholdScaling=scale

Provides unified control of first compilation. This option controls 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 values greater than 1.0 delay compilation. Setting CompileThresholdScaling 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:+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 methods that may 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 and on whether tiered compilation is enabled. In the following example it is set to 1000 bytes:

-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.

-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: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: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: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: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:MaxNodeLimit=nodes

Sets the maximum number of nodes to be used during single method compilation. By default the value depends on the features enabled. In the following example the maximum number of nodes is set to 100,000:

-XX:NonNMethodCodeHeapSize=size

Sets the size in bytes of the code segment containing nonmethod code.

-XX:NonProfiledCodeHeapSize=size

Sets the size in bytes of the code segment containing nonprofiled 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:-OptimizeStringConcat.

-XX:+PrintAssembly

Enables printing of assembly code for bytecoded and native methods 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 performance issues.

-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 default, this option is disabled and diagnostic output isn't printed.

-XX:+PrintInlining

Enables printing of inlining decisions. This let's you see which methods are getting inlined.

-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 default maximum code cache size is 240 MB; if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB. This option has a limit of 2 GB; otherwise, an error is generated. The maximum code cache size shouldn't be less than the initial code cache size; see the option -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 transactions 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 retried, 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 which the code cache consists of one large segment. With -XX:+SegmentedCodeCache, separate segments will be used for non-method, profiled method, and non-profiled method code. The segments are not resized at runtime. The advantages are better control of the memory footprint, reduced code fragmentation, and better CPU iTLB (instruction translation lookaside buffer) and instruction cache behavior due to improved locality.

-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 necessary instructions. The -XX:+UseAES is used in conjunction with UseAESIntrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseAESIntrinsics

Enables AES intrinsics. Specifying -XX:+UseAESIntrinsics is equivalent to also enabling -XX:+UseAES. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:

-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:+UnlockDiagnosticVMOptions.

-XX:+UseChaCha20Intrinsics

Enable ChaCha20 intrinsics. This option is on by default for supported platforms. To disable ChaCha20 intrinsics, specify -XX:-UseChaCha20Intrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UsePoly1305Intrinsics

Enable Poly1305 intrinsics. This option is on by default for supported platforms. To disable Poly1305 intrinsics, specify -XX:-UsePoly1305Intrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseBASE64Intrinsics

Controls the use of accelerated BASE64 encoding routines for java.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 java.util.zip.Adler32. Enabled by default on platforms that support 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:+UnlockDiagnosticVMOptions.

-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:+UnlockDiagnosticVMOptions.

-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 UseSHA512Intrinsics options.

-XX:+UseSHA1Intrinsics

Enables intrinsics for SHA-1 crypto hash function. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseSHA256Intrinsics

Enables intrinsics for SHA-224 and SHA-256 crypto hash functions. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseSHA512Intrinsics

Enables intrinsics for SHA-384 and SHA-512 crypto hash functions. 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 intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseSquareToLenIntrinsic

Enables intrinsification of BigInteger.squareToLen(). Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseMulAddIntrinsic

Enables intrinsification of BigInteger.mulAdd(). Enabled by default on platforms that support it. Flags that control intrinsics 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:+UnlockDiagnosticVMOptions.

-XX:+UseMontgomerySquareIntrinsic

Enables intrinsification of BigInteger.montgomerySquare(). Enabled by default on platforms that support it. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseCMoveUnconditionally

Generates CMove (scalar and vector) instructions regardless of profitability analysis.

-XX:+UseCodeCacheFlushing

Enables flushing of the code cache before shutting down the compiler. 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 increases the performance of Java applications that rely on concurrent 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 maximum 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 instructions are available (such as, Intel and ARM64). FMA intrinsics are generated for the java.lang.Math.fma(a, b, c) methods 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).

-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:-UseSuperWord.

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 mechanism is enabled and you can use diagnostics and troubleshooting tools such as jcmd, jstack, jmap, and jinfo.

-XX:+DTraceAllocProbes

Linux and macOS: Enable dtrace tool probes for object allocation.

-XX:+DTraceMethodProbes

Linux and macOS: Enable dtrace tool probes for method-entry and method-exit.

-XX:+DTraceMonitorProbes

Linux and macOS: Enable dtrace tool probes for monitor events.

-XX:+HeapDumpOnOutOfMemoryError

Enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath 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:+HeapDumpOnOutOfMemoryError 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 error. The following example shows how to set the default file explicitly (%p represents the current process identifier):

-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.

-XX:+PrintClassHistogram

Enables printing of a class instance histogram after one of the following events:

-XX:+PrintConcurrentLocks

Enables printing of java.util.concurrent locks after one of the following events:

-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 hsperfdata_pid, where pid is the process identifier of the Java application that you ran. Use the jstat command to display the performance data contained in this file as follows:

-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 allocation, based on the configuration of the computer (RAM and CPU). By default, the option is disabled and the heap sizes are configured less aggressively.

-XX:+AlwaysPreTouch

Requests the VM to touch every page on the Java heap after requesting 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 collection threads. The default value depends on the number of CPUs available to the JVM.

-XX:+DisableExplicitGC

Enables the option that disables processing of calls to the System.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() request. 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:InitiatingHeapOccupancyPercent. Before, G1 uses the value of -XX:InitiatingHeapOccupancyPercent directly for this purpose. The default value is 3.

-XX:G1HeapRegionSize=size

Sets the size of the regions into which the Java heap is subdivided 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 region size is determined ergonomically based on the heap size with a goal of approximately 2048 regions.

-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.

-XX:G1MixedGCCountTarget=number

Sets the target number of mixed garbage collections after a marking cycle to collect old regions with at most G1MixedGCLIveThresholdPercent 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.

-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.

-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 percent 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%.

-XX:+G1UseAdaptiveIHOP

Controls adaptive calculation of the old generation occupancy to start background work preparing for an old generation collection. If enabled, G1 uses -XX:InitiatingHeapOccupancyPercent for the first few times as specified by the value of -XX:G1AdaptiveIHOPNumInitialSamples, 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:InitiatingHeapOccupancyPercent.

-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 configuration.

-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.

-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 option, and the survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.

-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.

-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.

-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 indicate 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.

-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%.

-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.

-XX:MaxNewSize=size

Sets the maximum size (in bytes) of the heap for the young generation (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.

-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.

-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.

-XX:MaxTenuringThreshold=threshold

Sets the maximum tenuring threshold for use in adaptive GC sizing. The largest value is 15. The default value is 15 for the parallel (throughput) collector.

-XX:MetaspaceSize=size

Sets the size of the allocated class metadata space that triggers a garbage collection the first time it's exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends 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 value, then the heap is expanded. By default, this value is set to 40%.

-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 configuration.

-XX:NewRatio=ratio

Sets the ratio between young and old generation sizes. By default, this option is set to 2. The following example shows how to set the young-to-old ratio to 1:

-XX:NewSize=size

Sets the initial size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes.

-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.

-XX:+ParallelRefProcEnabled

Enables parallel reference processing. By default, this option is disabled.

-XX:+PrintAdaptiveSizePolicy

Enables printing of information about adaptive-generation sizing. 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 object is kept active on the heap after the last time it was referenced. 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.

-XX:-ShrinkHeapInSteps

Incrementally reduces the Java heap to the target size, specified 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.

-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.

-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: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%.

-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.

-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 memory allocation pool explicitly. See the -XX:SurvivorRatio option.

-XX:+UseG1GC

Enables the use of the garbage-first (G1) garbage collector. It's a server-style garbage collector, targeted for multiprocessor 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 limited 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 application's use of lower latency memory. By default, this option is disabled and no optimization for NUMA is made. The option 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.

-XX:+UseSerialGC

Enables the use of the serial garbage collector. This is generally 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.

-XX:+UseStringDeduplication

Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector.

-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 independent 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 allocation 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 reclaim 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 application. This is useful if the application is mostly idle or allocates 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 memory 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 issued when they're used.

-Xfuture

Enables strict class-file format checks that enforce close conformance 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 -verbose:gc if both are given with the same java command. -Xloggc:filename is replaced by -Xlog:gc:filename. See Enable Logging with the JVM Unified Logging Framework.

-XX:+FlightRecorder

Enables the use of Java Flight Recorder (JFR) during the runtime of the application. Since JDK 8u40 this option has not been required 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 option. The default value is 64.

-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 option. The default value is 4.

-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 option for small heaps. A small heap is a heap of approximately 125 MB. The default value is 2.

OBSOLETE JAVA OPTIONS¶

These java options are still accepted but ignored, and a warning is issued when they're used.

--illegal-access=parameter

Controlled relaxed strong encapsulation, as defined in JEP 261 [https://openjdk.org/jeps/261#Relaxed-strong-encapsulation]. This option was deprecated in JDK 16 by JEP 396 [https://openjdk.org/jeps/396] and made obsolete in JDK 17 by JEP 403 [https://openjdk.org/jeps/403].

REMOVED JAVA OPTIONS¶

These java options have been removed in JDK 21 and using them results in an error of:

-XX:+ExtendedDTraceProbes

Linux and macOS: Enables additional dtrace tool probes that affect performance. By default, this option is disabled and dtrace performs only standard probes. Use the combination of these flags instead: -XX:+DTraceMethodProbes, -XX:+DTraceAllocProbes, -XX:+DTraceMonitorProbes.

For the lists and descriptions of options removed in previous releases see the Removed Java Options section in:

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The java Command, Release 20 [https://docs.oracle.com/en/java/javase/20/docs/specs/man/java.html]

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The java Command, Release 19 [https://docs.oracle.com/en/java/javase/19/docs/specs/man/java.html]

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The java Command, Release 18 [https://docs.oracle.com/en/java/javase/18/docs/specs/man/java.html]

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The java Command, Release 17 [https://docs.oracle.com/en/java/javase/17/docs/specs/man/java.html]

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The java Command, Release 16 [https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html]

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The java Command, Release 15 [https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html]

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The java Command, Release 14 [https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html]

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The java Command, Release 13 [https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html]

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Java Platform, Standard Edition Tools Reference, Release 12 [https://docs.oracle.com/en/java/javase/12/tools/java.html#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE]

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Java Platform, Standard Edition Tools Reference, Release 11 [https://docs.oracle.com/en/java/javase/11/tools/java.html#GUID-741FC470-AA3E-494A-8D2B-1B1FE4A990D1]

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Java Platform, Standard Edition Tools Reference, Release 10 [https://docs.oracle.com/javase/10/tools/java.htm#JSWOR624]

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Java Platform, Standard Edition Tools Reference, Release 9 [https://docs.oracle.com/javase/9/tools/java.htm#JSWOR624]

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Java Platform, Standard Edition Tools Reference, Release 8 for Oracle JDK on Windows [https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html#BGBCIEFC]

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Java Platform, Standard Edition Tools Reference, Release 8 for Oracle JDK on Solaris, Linux, and macOS [https://docs.oracle.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 options and class names, which are passed to the java command. This let's you to create java commands of any length on any operating system.

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 contents of that file into an argument list just as they would be specified on the command line.

The java launcher expands the argument file contents until it encounters the --disable-@files option. You can use the --disable-@files option 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:

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The argument file must contain only ASCII characters or characters in system default encoding that's ASCII friendly, such as UTF-8.

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The argument file size must not exceed MAXINT (2,147,483,647) bytes.

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The launcher doesn't expand wildcards that are present within an argument file.

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Use white space or new line characters to separate arguments included in the file.

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White space includes a white space character, \t, \n, \r, and \f.

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Any option that contains spaces, such as a path component, must be within quotation marks using quotation ('"') characters in its entirety.

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A string within quotation marks may contain the characters \n, \r, \t, and \f. They are converted to their respective ASCII codes.

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If a file name contains embedded spaces, then put the whole file name in double quotation marks.

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File names in an argument file are relative to the current directory, not to the location of the argument file.

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Use the number sign # in the argument file to identify comments. All characters following the # are ignored until the end of line.

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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).

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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 continuation character (\) may be placed at the first column.

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Because backslash (\) is an escape character, a backslash character must be escaped with another backslash character.

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Partial quote is allowed and is closed by an end-of-file.

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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.

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Wildcards (*) aren't allowed in these lists (such as specifying *.java).

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Use of the at sign (@) to recursively interpret files isn't supported.

CODE HEAP STATE ANALYTICS¶

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.

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 java.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 Collection 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 during runtime (for example using the jcmd tool). The JVM is terminated if a value violates either the range or constraint check and an appropriate 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 automatic 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 results 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.

APPLICATION CLASS DATA SHARING¶

Application Class Data Sharing (AppCDS) stores classes used by your applications 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 application may use only a few APIs provided by a large library.

PERFORMANCE TUNING EXAMPLES¶

You can use the Java advanced runtime options to optimize the performance of your applications.

EXIT STATUS¶

The following exit values are typically returned by the launcher when the launcher is called with the wrong arguments, serious errors, or exceptions 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:

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0: Successful completion

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>0: An error occurred