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You can use thejava command to launch a Java application.

Synopsis

Windows: Thejavaw command is identical tojava, except that withjavaw there’s no associated console window. Usejavaw when you don’t want a command prompt window to appear. Thejavaw launcher will, however, display a dialog box with error information if a launch fails.

To launch a class file:

java [options]mainclass [args...]

To launch the main class in a JAR file:

java [options] -jarjarfile [args...]

To launch the main class in a module:

java [options] -mmodule[/mainclass] [args...]

java [options] --modulemodule[/mainclass] [args...]

To launch a single source-file program:

java [options]source-file [args...]

[options]

Optional: Specifies command-line options separated by spaces. SeeOverview of Java Options for a description of available options.

mainclass

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

-jarjarfile

Executes a program encapsulated in a JAR file. Thejarfile argument is the name of a JAR file with a manifest that contains a line in the formMain-Class:classname that defines the class with thepublic 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 seejar.

-m or--modulemodule[/mainclass]

Executes the main class in a module specified bymainclass if it is given, or, if it is not given, the value in themodule. In other words,mainclass can be used when it is not specified by the module, or to override the value when it is specified.

SeeStandard 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. SeeUsing Source-File Mode to Launch Single-File Source-Code Programs

[args...]

Optional: Arguments followingmainclass,source-file,-jarjarfile, and-m or--modulemodule/mainclass are passed as arguments to the main class.

Description

Thejava command starts a Java application. It does this by starting the Java Runtime Environment (JRE), loading the specified class, and calling that class'smain() method. The method must be declaredpublic andstatic, it must not return any value, and it must accept aString array as a parameter. The method declaration has the following form:

public static void main(String[] args)

In source-file mode, thejava command can launch a class declared in a source file. SeeUsing Source-File Mode to Launch Single-File Source-Code Programs for a description of using the source-file mode.

Note:

You can use theJDK_JAVA_OPTIONS launcher environment variable to prepend its content to the actual command line of thejava launcher. SeeUsing the JDK_JAVA_OPTIONS Launcher Environment Variable.

By default, the first argument that isn’t an option of thejava 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 the class and resource files for the application. The startup class must be indicated by theMain-Class manifest header in its manifest file.

Arguments after the class file name or the JAR file name are passed to themain() method.

Using Source-File Mode to Launch Single-File Source-Code Programs

To launch a class declared in a source file, run thejava launcher in source-file mode. Entering source-file mode is determined by two items on thejava 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--sourceversion 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 sourceversion orN of the source code. This determines the API that can be used. When you set--sourceN, you can only use the public API that was defined in JDKN.

Note:

The valid values ofN change for each release, with new values added and old values removed. You'll get an error message if you use a value ofN that is no longer supported. Supported values ofN for this release are6,7,8,9,10, and11.

If the file does not have the.java extension, the--source option must be used to tell thejava 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 namedHelloWorld.java and contained a class namedhello.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 wherehello.World is the name of the class in the package:

javac -dmemory HelloWorld.javajava -cpmemory 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-modules,--limit-modules,--patch-module,--upgrade-module-path, and any variant forms of those options. It also includes the--enable-preview option, described inJEP 12: Preview Language and VM Features.
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 standardpublic 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.

SeeJEP 330: Launch Single-File Source-Code Programs 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 theJDK_JAVA_OPTIONS environment variable is a list of arguments separated by white-space characters (as determined byisspace()). These are prepended to the command line arguments passed tojava launcher. The encoding requirement for the environment variable is the same as thejava 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 ofJDK_JAVA_OPTIONS behavior, options that specify the main class (such as-jar) or cause thejava 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. WhenJDK_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

Thejava 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 inPerformance 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 suffixk orK for kilobytes (KB),m orM for megabytes (MB), org orG for gigabytes (GB). For example, to set the size to 8 GB, you can specify either8g,8192m,8388608k, or8589934592 as the argument. If you are expected to specify the percentage, then use a number from 0 to 1. For example, specify0.25 for 25%.

The following sections describe the options that are obsolete, deprecated, 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.

Note:

To specify an argument for a long option, you can use either--name=value or--namevalue.

-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.

Oracle Solaris, Linux, and macOS: If the option-agentlib:foo is specified, then the JVM attempts to load the library namedlibfoo.so in the location specified by theLD_LIBRARY_PATH system variable (on macOS this variable isDYLD_LIBRARY_PATH).
Windows: If the option-agentlib:foo is specified, then the JVM attempts to load the library namedfoo.dll in the location specified by thePATH 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,address=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-pathclasspath,-classpathclasspath, or-cpclasspath

A semicolon (;) separated list of directories, JAR archives, and ZIP archives to search for class files.

Specifyingclasspath overrides any setting of theCLASSPATH environment variable. If the class path option isn’t used andclasspath 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 specifying a list of all the files in the directory with the extension.jar or.JAR. A Java program can’t tell the difference between the two invocations. For example, if the directorymydir containsa.jar andb.JAR, then the class path elementmydir/* is expanded toA.jar:b.JAR, except that the order of JAR files is unspecified. All.jar files in the specified directory, 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. TheCLASSPATH environment variable, where defined, is similarly expanded. Any class path wildcard expansion occurs before the Java VM is started. Java programs never see wildcards that aren’t expanded except by querying the environment, such as by callingSystem.getenv("CLASSPATH").

--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 of the release.

--module-pathmodulepath... or-pmodulepath

A semicolon (;) separated list of directories in which each directory is a directory of modules.

--upgrade-module-pathmodulepath...

A semicolon (;) separated list of directories in which each directory is a directory of modules that replace upgradeable modules in the runtime image.

--add-modulesmodule[,module...]

Specifies the root modules to resolve in addition to the initial module.module also can beALL-DEFAULT,ALL-SYSTEM, andALL-MODULE-PATH.

--list-modules

Lists the observable modules and then exits.

-dmodule name or--describe-modulemodule_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 exits. This option is helpful for finding conflicts and other errors with modules on the module path.

-Dproperty=value

Sets a system property value. Theproperty variable is a string with no spaces that represents the name of the property. Thevalue variable is a string that represents the value of the property. Ifvalue 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 thepackagename 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 theclassname argument, the switch disables assertions in the specified 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-enableassertions (-ea) option. Both options can be used at the same time. For example, to run theMyClass application with assertions enabled in the packagecom.wombat.fruitbat (and any subpackages) but disabled in the classcom.wombat.fruitbat.Brickbat, use the following command:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

-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 thepackagename 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 theclassname argument, the switch enables assertions in the specified 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 classes, use the-disableassertions (-da) option. If a single command contains multiple instances of these switches, then they’re processed in order, before loading any classes. For example, to run theMyClass application with assertions enabled only in the packagecom.wombat.fruitbat (and any subpackages) but disabled in the classcom.wombat.fruitbat.Brickbat, use the following command:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.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.

--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 byimagepath. HiDPI scaled images are automatically supported and used if available. The unscaled image file name, such asimage.ext, should always be passed as the argument to the-splash option. The most appropriate scaled image provided is picked up automatically.

For example, to show thesplash.gif file from theimages directory 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 error stream and exits.

-version

Prints product version to the output 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 thejava command. It isn’t uncommon for thejava 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-Xdisable-@files option was encountered.

The argument files can also contain the main class name and all options. If an argument file contains all of the options required by thejava command, then the command line could simply be:

java @argfile

Seejava Command-Line Argument Files for a description and examples of using@argfile .

Extra Options for Java

The followingjava 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.

Oracle Solaris, 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) functions. Specifically, it validates the parameters passed to the JNI function and the runtime environment data before processing the JNI request. It also checks for pending exceptions between JNI calls. Any invalid data encountered indicates a problem in the native code, and the JVM terminates with an irrecoverable error in such cases. Expect a performance degradation when this option is used.

-Xcomp

Forces compilation of methods on first invocation. By default, the Client VM (-client) performs 1,000 interpreted method invocations and the Server VM (-server) performs 10,000 interpreted method invocations to gather information for efficient compilation. Specifying the-Xcomp option disables interpreted method invocations to increase compilation performance at the expense of efficiency. You can also change the number of interpreted method invocations before compilation by using the-XX:CompileThreshold option.

-Xdebug

Does nothing. Provided for backward compatibility.

-Xdiag

Shows additional diagnostic messages.

-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.

-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.

-Xloggc:option

Enables the JVM unified logging framework. Logs GC status to a file with time stamps.

-Xlog:option

Configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. SeeEnable Logging with the JVM Unified Logging Framework.

-Xmixed

Executes all bytecode by the interpreter except for hot methods, which are compiled to native code.

-Xmnsize

Sets the initial and maximum size (in bytes) of the heap for the young generation (nursery). Append the letterk orK to indicate kilobytes,m orM to indicate megabytes, org orG 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. Oracle recommends that you keep the size for the young generation greater than 25% and less than 50% of the overall heap size. The following 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:NewSize to set the initial size and-XX:MaxNewSize to set the maximum size.

-Xmssize

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 letterk orK to indicate kilobytes,m orM to indicate megabytes,g orG to indicate gigabytes. The following examples show how to set the size of allocated memory to 6 MB by using various units:

-Xms6291456-Xms6144k-Xms6m

If you don’t set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set by using the-Xmn option or the-XX:NewSize option.

Note:

The-XX:InitalHeapSize option can also be used to set the initial heap size. If it appears after-Xms on the command line, then the initial heap size gets set to the value specified with-XX:InitalHeapSize.

-Xmxsize

Specifies the maximum size (in bytes) of the memory allocation pool in bytes. This value must be a multiple of 1024 and greater than 2 MB. Append the letterk orK to indicate kilobytes,m orM to indicate megabytes, andg orG 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 by 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 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.

Oracle Solaris, Linux, and macOS:
- The JVM catches signals to implement shutdown hooks for unexpected termination. The JVM usesSIGHUP,SIGINT, andSIGTERM to initiate the running of shutdown hooks.
- Applications embedding the JVM frequently need to trap signals such asSIGINT orSIGTERM, which can lead to interference with the JVM signal handlers. The-Xrs option is available to address this issue. When-Xrs is used, the signal masks forSIGINT,SIGTERM,SIGHUP, andSIGQUIT 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 returnsTRUE forCTRL_C_EVENT,CTRL_CLOSE_EVENT,CTRL_LOGOFF_EVENT, andCTRL_SHUTDOWN_EVENT.
- The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes. The JVM usesCTRL_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 receiveCTRL_LOGOFF_EVENT but shouldn’t initiate shutdown because the operating 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 processCTRL_C_EVENT,CTRL_CLOSE_EVENT,CTRL_LOGOFF_EVENT, orCTRL_SHUTDOWN_EVENT.

There are two consequences of specifying-Xrs:

Oracle Solaris, 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 theSystem.exit() when the JVM is to be terminated.

-Xshare:mode

Sets the class data sharing (CDS) mode.

Possiblemode arguments for this option include the following:

auto: Uses CDS if possible. This is the default value for Java HotSpot 32-Bit Client VM.
on: Requires the use of CDS. This option prints an error message and exits if class data sharing can’t be used.
Note:
-Xshare:on is used for testing purposes only and can cause intermittent failures due to the use of address space layout randomization by the operation system. This option should not be used in production environments.
off: Instructs not to use CDS.

-XshowSettings

Shows all settings and then continues.

-XshowSettings:category

Shows settings and continues. Possiblecategory 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.

-Xsssize

Sets the thread stack size (in bytes). Append the letterk orK to indicate KB,m orM to indicate MB, andg orG to indicate GB. The default value depends on the platform:

Linux/x64 (64-bit): 1024 KB
macOS (64-bit): 1024 KB
Oracle Solaris/x64 (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.

-Xverify

Sets the mode of the bytecode verifier.

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

Updatesmodule to read thetarget-module, regardless of the module declaration.target-module can be all unnamed to read all unnamed modules.

--add-exportsmodule/package=target-module(,target-module)*

Updatesmodule to exportpackage totarget-module, regardless of module declaration. Thetarget-module can be all unnamed to export to all unnamed modules.

--add-opensmodule/package=target-module(,target-module)*

Updatesmodule to openpackage totarget-module, regardless of module declaration.

--illegal-access=parameter

When present at run time,--illegal-access= takes a keywordparameter to specify a mode of operation:

Note:

This option will be removed in a future release.

permit: This mode opens each package in each module in the run-time image to code in all unnamed modules (such as code on the class path), if that package existed in JDK 8. This enables both static access (for example, by compiled bytecode), and deep reflective access through the platform's various reflection APIs. The first reflective-access operation to any such package causes a warning to be issued. However, no warnings are issued after the first occurrence. This single warning describes how to enable further warnings. This mode is the default for the current JDK but will change in a future release.
warn: This mode is identical topermit except that a warning message is issued for each illegal reflective-access operation.
debug: This mode is identical towarn except that both a warning message and a stack trace are issued for each illegal reflective-access operation.
deny: This mode disables all illegal-access operations except for those enabled by other command-line options, such as--add-opens. This mode will become the default in a future release.

The default mode,--illegal-access=permit, is intended to make you aware of code on the class path that reflectively accesses any JDK-internal APIs at least once. To learn about all such accesses, you can use thewarn or thedebug modes. For each library or framework on the class path that requires illegal access, you have two options:

If the component's maintainers have already released a fixed version that no longer uses JDK-internal APIs then you can consider upgrading to that version.
If the component still needs to be fixed, then you can contact its maintainers and ask them to replace their use of JDK-internal APIs with the proper exported APIs.

If you must continue to use a component that requires illegal access, then you can eliminate the warning messages by using one or more--add-opens options to open only those internal packages to which access is required.

To verify that your application is ready for a future version of the JDK, run it with--illegal-access=deny along with any necessary--add-opens options. Any remaining illegal-access errors will most likely be due to static references from compiled code to JDK-internal APIs. You can identify those by running thejdeps tool with the--jdk-internals option. For performance reasons, the current JDK does not issue warnings for illegal static-access operations.

--limit-modulesmodule[,module...]

Specifies the limit of the universe of observable modules.

--patch-modulemodule=file(;file)*

Overrides or augments a module with classes and resources in JAR files or directories.

--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.

--sourceversion

Sets the version of the source in source-file mode.

Extra Options for macOS

The following extra options are macOS-specific.

-XstartOnFirstThread: Runs themain() 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 Runtime Options for Java

Thesejava 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 partitioning CPU resources when running multiple Java processes in docker containers. This flag is honored even ifUseContainerSupport is not enabled. See-XX:-UseContainerSupport for a description of enabling and disabling container support.

--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.

Alternative memory devices that have the same semantics as DRAM, including the semantics of atomic operations, can be used instead 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 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.

Cases where it may be beneficial to disable Compact Strings include the following:

When it’s known that an application overwhelmingly will be allocating multibyte character Strings
In the unexpected event where a performance regression is observed in migrating from Java SE 8 to Java SE 9 or later and an analysis shows that Compact Strings introduces the regression

In both of these scenarios, disabling Compact Strings makes sense.

-XX:CompilerDirectivesFile=file

Adds directives from a file to the directives stack when a program starts. SeeCompiler Directives and the Command Line.

-XX:CompilerDirectivesPrint

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

-XX:ConcGCThreads=n

Sets the number of parallel marking threads. Setsn to approximately 1/4 of the number of parallel garbage collection threads (ParallelGCThreads).

-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 asjcmd,jstack,jmap, andjinfo.

Note:

The tools such asjcmd,jinfo,jmap, andjstack shipped with the JDK aren’t supported when using the tools from one JDK version to troubleshoot a different JDK version.

-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 namedhs_err_pidpid.log wherepid is the identifier of the process that caused 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

Oracle Solaris, 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 toC:/log/java/java_error.log:
```
-XX:ErrorFile=C:/log/java/java_error.log
```

If the file can’t be created in the specified directory (due to insufficient space, a permission problem, or another issue), then the file is created in the temporary directory for the operating system:

Oracle Solaris, Linux, and macOS: The temporary directory is/tmp.
Windows: The temporary directory is specified by the value of theTMP environment variable. If that environment variable isn’t defined, then the value of theTEMP environment variable is used.

-XX:+ExtensiveErrorReports

Enables the reporting of more extensive error information in theErrorFile. 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:+FailOverToOldVerifier

Enables automatic failover to the old verifier when the new type checker fails. By default, this option is disabled and it’s ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode.

-XX:+FlightRecorder

Enables the use of Java Flight Recorder (JFR) during the runtime of the application.

Note:

The-XX:+FlightRecorder option is no longer required to use JFR. This was a change made in JDK 8u40.

-XX:FlightRecorderOptions=parameter=value

Sets the parameters that control the behavior of JFR.

The following list contains the available JFRparameter=value entries:

allow_threadbuffers_to_disk={true|false}: Specifies whether thread buffers are written directly to disk if the buffer thread is blocked. By default, this parameter is disabled.
globalbuffersize=size: Specifies the total amount of primary memory used for data retention. The default value is based on the value specified formemorysize. Change thememorysize parameter to alter the size of global buffers.
maxchunksize=size: Specifies the maximum size (in bytes) of the data chunks in a recording. Appendm orM to specify the size in megabytes (MB), andg orG to specify the size in gigabytes (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 theglobalbuffersize andnumglobalbuffers parameters based on the size specified. Appendm orM to specify the size in megabytes (MB), andg orG 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 thememorysize parameter to alter the number of global buffers.
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 reduce performance.
threadbuffersize=size: Specifies the per-thread local buffer size (in bytes). By default, the local buffer size is set to 8 kilobytes. Overriding this parameter could reduce performance and is not recommended.

You can specify values for multiple parameters by separating them with a comma.

-XX:InitiatingHeapOccupancyPercent=n

Sets the Java heap occupancy threshold that triggers a marking cycle. The default occupancy is 45 percent of the entire Java heap.

-XX:LargePageSizeInBytes=size

Oracle Solaris: Sets the maximum size (in bytes) for large pages used for the Java heap. Thesize argument must be a power of 2 (2, 4, 8, 16, and so on). Append the letterk orK to indicate kilobytes,m orM to indicate megabytes, andg orG to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for large pages automatically. SeeLarge Pages.

The following example describes how to set the large page size to 4 megabytes (MB):

-XX:LargePageSizeInBytes=4m

-XX:MaxDirectMemorySize=size

Sets the maximum total size (in bytes) of thejava.nio package, direct-buffer allocations. Append the letterk orK to indicate kilobytes,m orM to indicate megabytes, andg orG to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations automatically.

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 option 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:MaxGCPauseMillis=ms

Sets a target value for the desired maximum pause time. The default value is200 milliseconds. The specified value doesn’t adapt to your heap size.

-XX:NativeMemoryTracking=mode

Specifies the mode for tracking JVM native memory usage. Possiblemode 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:NativeMemoryTracking 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 individualCallSite, individual virtual memory region and its committed regions.

-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.

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 commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.

Oracle Solaris, Linux, and macOS: The following example shows how the-XX:OnError option can be used to run thegcore command to create the core image, and the debugger is started to attach to the process in case of an irrecoverable error (the%p designates the current process):
```
-XX:OnError="gcore %p;dbx - %p"
```
Windows: The following example shows how the-XX:OnError option can be used to run theuserdump.exe utility to obtain a crash dump in case of an irrecoverable error (the%p designates the current process). This example assumes that the path to theuserdump.exe utility is specified in thePATH environment variable:
```
-XX:OnError="userdump.exe %p"
```

-XX:OnOutOfMemoryError=string

Sets a custom command or a series of semicolon-separated commands to run when anOutOfMemoryError 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:ParallelGCThreads=n

Sets the value of the STW worker threads. Sets the value ofn to the number of logical processors. The value ofn is the same as the number of logical processors up to a value of 8. If there are more than 8 logical processors, then this option sets the value ofn to approximately 5/8 of the logical processors. This works in most cases except for larger SPARC systems where the value ofn can be approximately 5/16 of the logical processors.

-XX:+PerfDataSaveToFile

If enabled, savesjstat binary data when the Java application exits. This binary data is saved in a file namedhsperfdata_pid, wherepid is the process identifier of the Java application that you ran. Use thejstat command to display the performance data contained in this file as follows:

jstat -class file:///path/hsperfdata_pid

jstat -gc file:///path/hsperfdata_pid

-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:+RelaxAccessControlCheck

Decreases the amount of access control checks in the verifier. By default, this option is disabled, and it’s ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode.

-XX:SharedArchiveFile=path

Specifies the path and name of the class data sharing (CDS) archive file

SeeApplication Class Data Sharing.

-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 classesjava.lang.Object andhello.Main, create a text file that contains the following two lines:

java/lang/Objecthello/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 bootstrap class paths.

SeeApplication Class Data Sharing.

-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 theJFR.start diagnostic command that starts a recording during runtime. You can set the followingparameter=value entries when starting a JFR recording:

delay=time

Specifies the delay between the Java application launch time and the start of the recording. Appends to specify the time in seconds,m for minutes,h for hours, andd for days. For example, specifying10m means 10 minutes. By default, there’s no delay, and this parameter is set to 0.

disk={true|false}

Specifies whether to write temporary data to the disk repository. By default, this parameter is set tofalse. To enable it, set the parameter totrue.

dumponexit={true|false}

Specifies if the running recording is dumped when the JVM shuts down. If enabled and afilename is not entered, the recording is written to a file in the directory where the process was started. The file name is a system-generated name that contains the process ID, recording ID, and current timestamp, similar tohotspot-pid-47496-id-1-2018_01_25_19_10_41.jfr. By default, this parameter is disabled.

duration=time

Specifies the duration of the recording. Appends to specify the time in seconds,m for minutes,h for hours, andd for days. For example, specifying5h means 5 hours. By default, the duration isn’t limited, and this parameter 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 thedisk parameter is set totrue. Appends to specify the time in seconds,m for minutes,h for hours, andd for days. For example, specifying30s means 30 seconds. By default, the maximum age isn’t limited, and this parameter is set to0s.

maxsize=size

Specifies the maximum size (in bytes) of disk data to keep for the recording. This parameter is valid only when thedisk parameter is set totrue. The value must not be less than the value for themaxchunksize parameter set with-XX:FlightRecorderOptions. Appendm orM to specify the size in megabytes, andg orG to specify the size in gigabytes. By default, the maximum size of disk data isn’t limited, and this parameter is set to0.

path-to-gc-roots={true|false}

Specifies whether to collect the path to garbage collection (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 thesettings parameter is set toprofile, the stack trace from where the potential leaking object was allocated is included in the information collected.

settings=path

Specifies the path and name of the event settings file (of type JFC). By default, thedefault.jfc file is used, which is located inJRE_HOME/lib/jfr. This default settings 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.

-XX:ThreadStackSize=size

Sets the Java thread stack size (in kilobytes). Use of a scaling suffix, such ask, 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:

Linux: 1024 KB
macOS: 1024 KB
Oracle Solaris: 1024 KB
Windows: The default value depends on the 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:-UseBiasedLocking

Disables the use of biased locking. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, but applications with certain patterns of locking may see slowdowns.

By default, this option is enabled.

-XX:-UseCompressedOops

Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB. When this option is 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 of less than 32 GB. This option works only for 64-bit JVMs.

It’s also possible to use compressed pointers when Java heap sizes are 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 docker containers. It uses this information to allocate system resources. 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 diagnose issues related to this support.

Use-Xlog:os+container=trace for maximum logging of container information. SeeEnable Logging with the JVM Unified Logging Framework for a description of using Unified Logging.

XX:+UseGCLogRotation

Handles large log files. This option must be used with-Xloggc:filename.

-XX:NumberOfGClogFiles=number_of_files

Handles large log files. Thenumber_of_files must be greater than or equal to1. The default is1.

-XX:GCLogFileSize=number

Handles large log files. Thenumber can be in the form ofnumberM ornumberK. The default is set to512K.

-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.

SeeLarge Pages.

-XX:+UseLargePages

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

SeeLarge Pages.

-XX:+UseMembar

Enables issuing of membars on thread-state transitions. This option is disabled by default on all platforms except ARM servers, where it’s enabled. (It’s recommended that you don’t disable this option on ARM servers.)

-XX:+UsePerfData

Enables theperfdata feature. This option is enabled by default to allow JVM monitoring and performance testing. Disabling it suppresses the creation of thehsperfdata_userid directories. To disable theperfdata feature, specify-XX:-UsePerfData.

-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

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.

Advanced JIT Compiler Options for java

Thesejava 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:AllocateInstancePrefetchLines=1