Home > Tutorials > The Core JDK Tools > Java - Your Application Launcher

Javadoc - the Documentation Generator

➜

Java - Your Application Launcher

➜

JShell - The Java Shell Tool

Previous in the Series: Javadoc - the Documentation Generator

Next in the Series: JShell - The Java Shell Tool

Java - Your Application Launcher

Introducing the Java Tool

java - launch a Java application

Synopsis

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

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...]

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.

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 command.

-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.

[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 Runtime Environment, 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:

class HelloWorld {
    public static void main(String[] args) { }
}

In source-file mode, the java command can launch a class declared in a source file.

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 the 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:

java HelloWorld.java

The example illustrates that the class can be in a named package, and does not need to be in the unnamed package. This use of source-file mode is informally equivalent to using the following two commands where hello.World is the name of the class in the package:

javac -d memory HelloWorld.java
java -cp memory hello.World

In source-file mode, any additional command-line options are processed as follows:

The launcher scans the options specified before the source file for any that are relevant in order to compile the source file.
This includes: --class-path, --module-path, --add-exports, --add-modules, --limit-modules, --patch-module, --upgrade-module-path, and any variant forms of those options. It also includes the --enable-preview option, described in JEP 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 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 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.

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: Control the runtime behavior of the Java HotSpot VM.
Advanced JIT Compiler Options: Control the dynamic just-in-time (JIT) compilation performed by the Java HotSpot VM.
Advanced Serviceability Options: Enable gathering system information and performing extensive debugging.
Advanced Garbage Collection Options: 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%.

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.

Linux and macOS: If the option -agentlib:foo is specified, then the JVM attempts to load the library named libfoo.so in the location specified by the LD_LIBRARY_PATH system variable (on macOS this variable is DYLD_LIBRARY_PATH).
Windows: If the option -agentlib:foo is specified, then the JVM attempts to load the library named foo.dll in the location specified by the PATH system variable.

The following example shows how to load the Java Debug Wire Protocol (JDWP) library and listen for the socket connection on port 8000, suspending the JVM before the main class loads:

-agentlib:jdwp=transport=dt_socket,server=y,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-path classpath, -classpath classpath, or -cp classpath A semicolon (;) separated list of directories, JAR archives, and ZIP archives to search for class files.

Specifying classpath overrides any setting of the CLASSPATH environment variable. If the class path option isn’t used and classpath isn’t set, then the user class path consists of the current directory (.).

As a special convenience, a class path element that contains a base name of an asterisk () is considered equivalent to 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 directory mydir contains a.jar and b.JAR, then the class path element mydir/ is expanded to A.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. The CLASSPATH 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 calling System.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-path modulepath... or -p modulepath A semicolon (;) separated list of directories in which each directory is a directory of modules.

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

--add-modules module[,module...] Specifies the root modules to resolve in addition to the initial module. module also can be ALL-DEFAULT, ALL-SYSTEM, and ALL-MODULE-PATH.

--list-modules Lists the observable modules and then exits.

-d module-name or --describe-module module-name Describes a specified module and then exits.

--dry-run Creates the VM but doesn’t execute the main method. This --dry-run option might be useful for validating the command-line options such as the module system configuration.

--validate-modules Validates all modules and exits. 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.

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 the MyClass application with assertions enabled in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, use the following command:

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

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 the MyClass application with assertions enabled only in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, use the following command:

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

For example, to show the splash.gif file from the images directory when starting your application, use the following option:

-splash:images/splash.gif

-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 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 -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 the java command, then the command line could simply be:

java @argfile

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.

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. 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.

-Xmn size Sets the initial and maximum 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. 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.

-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 1MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The following examples show how to set the size of allocated memory to 6MB 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

-Xmx size 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 letter k or K to indicate kilobytes, m or M to indicate megabytes, and 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 80MB 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.

Linux and macOS:
The JVM catches signals to implement shutdown hooks for unexpected termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.
Applications embedding the JVM frequently need to trap signals such as SIGINT or SIGTERM, 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 for SIGINT, SIGTERM, SIGHUP, and SIGQUIT aren’t changed by the JVM, and signal handlers for these signals aren’t installed.
Windows:
The JVM watches for console control events to implement shutdown hooks for unexpected termination. Specifically, the JVM registers a console control handler that begins shutdown-hook processing and returns TRUE for CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, and CTRL_SHUTDOWN_EVENT.
The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes. The JVM uses CTRL_BREAK_EVENT to perform thread dumps.
If the JVM is run as a service (for example, as a servlet engine for a web server), then it can receive CTRL_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 process CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.

There are two consequences of specifying -Xrs:

Linux and macOS: SIGQUIT thread dumps aren’t available.
Windows: Ctrl + Break thread dumps aren’t available.

User code is responsible for causing shutdown hooks to run, for example, by calling the System.exit() when the JVM is to be terminated.

-Xshare:mode Sets the class data sharing (CDS) mode.

Possible mode 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.

off Instructs not to use CDS.

-XshowSettings Shows all settings and then continues.

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

all Shows all categories of settings. This is the default value.

locale Shows settings related to locale.

properties Shows settings related to system properties.

vm Shows the settings of the JVM.

system Linux: Shows host system or container configuration and continues.

-Xss size Sets the thread stack size (in bytes). Append the letter k or K to indicate KB, m or M to indicate MB, and g or G to indicate GB. The default value depends on the platform:

Linux/x64 (64-bit): 1024 KB
macOS (64-bit): 1024 KB
Windows: The default value depends on virtual memory

The following examples set the thread stack size to 1024 KB in different units:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize.

-Xverify Sets the mode of the bytecode verifier.

--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.

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

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 to permit except that a warning message is issued for each illegal reflective-access operation.
debug: This mode is identical to warn 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 the warn or the debug 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 the jdeps tool with the --jdk-internals option. For performance reasons, the current JDK does not issue warnings for illegal static-access operations.

--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.

--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.

--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 Runtime Options

These java options control the runtime behavior of the Java HotSpot VM.

-XX:ActiveProcessorCount=x Overrides the number of CPUs that the VM will use to calculate the size of thread pools it will use for various operations such as Garbage Collection and ForkJoinPool.

The VM normally determines the number of available processors from the operating system. This flag can be useful for partitioning CPU resources when running multiple Java processes in docker containers. This flag is honored even if UseContainerSupport 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. See Compiler 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. Sets n 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 as jcmd, jstack, jmap, and jinfo.

-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 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

Linux and macOS: The following example shows how to set the error log to /var/log/java/java_error.log: ```shell
XX:ErrorFile=/var/log/java/java_error.log ```
Windows: The following example shows how to set the error log file to C:/log/java/java_error.log: ```shell
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:

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

-XX:+ExtensiveErrorReports Enables the reporting of more extensive error information in the ErrorFile. This option can be turned on in environments where maximal information is desired - even if the resulting logs may be quite large and/or contain information that might be 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.

-XX:FlightRecorderOptions=parameter=value Sets the parameters that control the behavior of JFR.

The following list contains the available JFR parameter=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 for memorysize. Change the memorysize parameter to alter the size of global buffers.

maxchunksize=size Specifies the maximum size (in bytes) of the data chunks in a recording. Append m or M to specify the size in megabytes (MB), and g or G 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 the globalbuffersize and numglobalbuffers parameters based on the size specified. Append m or M to specify the size in megabytes (MB), and g or G to specify the size in gigabytes (GB). By default, the memory size is set to 10 MB.

numglobalbuffers Specifies the number of global buffers used. The default value is based on the memory size specified. Change the memorysize parameter to alter the number of global 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 Linux and macOS: Sets the maximum size (in bytes) for large pages used for the Java heap. The size argument must be a power of 2 (2, 4, 8, 16, and so on). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, and g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for large pages automatically.

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 the java.nio package, direct-buffer allocations. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, and g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations 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 is 200 milliseconds. The specified value doesn’t adapt to your heap size.

-XX:NativeMemoryTracking=mode Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:

off Instructs not to track JVM native memory usage. This is the default behavior if you don’t specify the -XX: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 individual CallSite, 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

-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.

Linux and macOS: The following example shows how the -XX:OnError option can be used to run the gcore 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 the userdump.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 the userdump.exe utility is specified in the PATH environment variable: ```shell
XX:OnError="userdump.exe %p" ```

-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:ParallelGCThreads=n Sets the value of the STW worker threads. Sets the value of n to the number of logical processors. The value of n 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 of n to approximately 5/8 of the logical processors. This works in most cases except for larger SPARC systems where the value of n can be approximately 5/16 of the logical processors.

-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:

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

-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:

java/lang/Object
hello/Main

The classes that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths.

-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:

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

disk={true|false} Specifies whether to write data to disk while recording. By default, this parameter is enabled.

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

duration=time Specifies the duration of the recording. Append s to specify the time in seconds, m for minutes, h for hours, and d for days. For example, specifying 5h 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 the disk parameter is set to true. Append s to specify the time in seconds, m for minutes, h for hours, and d for days. For example, specifying 30s means 30 seconds. By default, the maximum age isn’t limited, and this parameter is set to 0s.

maxsize=size Specifies the maximum size (in bytes) of disk data to keep for the recording. This parameter is valid only when the disk parameter is set to true. The value must not be less than the value for the maxchunksize parameter set with -XX:FlightRecorderOptions. Append m or M to specify the size in megabytes, and g or G to specify the size in gigabytes (GB). By default, the maximum size of disk data isn’t limited, and this parameter is set to 0.

path-to-gc-roots={true|false} Specifies whether to collect the path to garbage 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 the settings parameter is set to profile, 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, the default.jfc file is used, which is located in JRE_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 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:

Linux: 1024 KB
macOS: 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. See Enable 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. The number_of_files must be greater than or equal to 1. The default is 1.

-XX:GCLogFileSize=number Handles large log files. The number can be in the form of numberM or numberK. The default is set to 512K.

-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:+UseMembar Enables issuing of members 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 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.

-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

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:AllocateInstancePrefetchLines=1

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