java - the Java application launcher

Synopsis
Description
Options
See Also

SYNOPSIS

    java [ options ] class [ argument ... ]
    java [ options ] -jar file.jar [ argument ... ]
    javaw [ options ] class [ argument ... ]
    javaw [ options ] -jar file.jar [ argument ... ]

options: Command-line options.
class: Name of the class to be invoked.
file.jar: Name of the jar file to be invoked. Used only with -jar.
argument: Argument passed to the main function.

DESCRIPTION

The java tool launches a Java application. It does this by starting a Java runtime environment, loading a specified class, and invoking 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 must look like the following:

public static void main(String args[])

By default, the first non-option argument is the name of the class to be invoked. A fully-qualified class name should be used. If the -jar option is specified, the first non-option argument is the name of a JAR archive containing class and resource f iles for the application, with the startup class indicated by the Main-Class manifest header.

The Java runtime searches for the startup class, and other classes used, in three sets of locations: the bootstrap class path, the installed extensions, and the user class path.

Non-option arguments after the class name or JAR file name are passed to the main function.

The javaw command is identical to java, except that with javaw there is 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 for some reason.

OPTIONS

The launcher has a set of standard options that are supported on the current runtime environment and will be supported in future releases. In addition, the default Java HotSpot VMs provide a set of non-standard options that are subject to change in future releases.

Standard Options

-client

Select the Java HotSpot Client VM. A 64-bit capable jdk currently ignores this option and instead uses the Java HotSpot Server VM.

For default VM selection, see Server-Class Machine Detection

-server

Select the Java HotSpot Server VM. On a 64-bit capable jdk only the Java HotSpot Server VM is supported so the -server option is implicit. This is subject to change in a future release.

For default VM selection, see Server-Class Machine Detection

-agentlib:libname[=options]

Load native agent library libname, e.g.

-agentlib:hprof

-agentlib:jdwp=help

-agentlib:hprof=help

For more information, see JVMTI Agent Command Line Options.

-agentpath:pathname[=options]

Load a native agent library by full pathname. For more information, see JVMTI Agent Command Line Options.

-classpath classpath

-cp classpath

Specify a list of directories, JAR archives, and ZIP archives to search for class files. Class path entries are separated by semicolons (;). Specifying -classpath or -cp overrides any setting of the CLASSPATH environment variable.

If -classpath and -cp are not used and CLASSPATH is not set, the user class path consists of the current directory (.).

As a special convenience, a class path element containing a basename of * is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR (a java program cannot tell the difference between the two invocations).

For example, if directory foo contains a.jar and b.JAR, then the class path element foo/* is expanded to a 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 classpath entry consisting simply of * expands to a list of all the jar files in the current directory. The CLASSPATH environment variable, where defined, will be similarly expanded. Any classpath wildcard expansion occurs before the Java virtual machine is started -- no Java program will ever see unexpanded wildcards except by querying the environment. For example; by invoking System.getenv("CLASSPATH").

For more information on class paths, see Setting the Class Path.

-Dproperty=value

Set a system property value. If value is a string that contains spaces, you must enclose the string in double quotes:

        java -Dfoo="some string" SomeClass

-disableassertions[:<package name>"..." | :<class name> ]

-da[:<package name>"..." | :<class name> ]

Disable assertions. This is the default.

With no arguments, disableassertions or -da disables assertions. With one argument ending in "...", the switch disables assertions in the specified package and any subpackages. If the argument is simply "...", the switch disables assertions in the unnamed package in the current working directory. With one argument not ending in "...", the switch disables assertions in the specified class.

To run a program with assertions enabled in package com.wombat.fruitbat but disabled in class com.wombat.fruitbat.Brickbat, the following command could be used:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat <Main Class>

The -disableassertions and -da switches apply to all class loaders and to system classes (which do not have a class loader). There is one exception to this rule: in their no-argument form, the switches do not apply to system. This makes it easy to turn on asserts in all classes except for system classes. A separate switch is provided to enable asserts in all system classes; see -disablesystemassertions below.

-enableassertions[:<package name>"..." | :<class name> ]

-ea[:<package name>"..." | :<class name> ]

Enable assertions. Assertions are disabled by default.

With no arguments, enableassertions or -ea enables assertions. With one argument ending in "...", the switch enables assertions in the specified package and any subpackages. If the argument is simply "...", the switch enables assertions in the unnamed package in the current working directory. With one argument not ending in "...", the switch enables assertions in the specified class.

If a single command line contains multiple instances of these switches, they are processed in order before loading any classes. So, for example, to run a program with assertions enabled only in package com.wombat.fruitbat (and any subpackages), the following command could be used:

java -ea:com.wombat.fruitbat... <Main Class>

The -enableassertions and -ea switches apply to all class loaders and to system classes (which do not have a class loader). There is one exception to this rule: in their no-argument form, the switches do not apply to system. This makes it easy to turn on asserts in all classes except for system classes. A separate switch is provided to enable asserts in all system classes; see -enablesystemassertions below.

-enablesystemassertions

-esa

Enable asserts in all system classes (sets the default assertion status for system classes to true).

-disablesystemassertions

-dsa

Disables asserts in all system classes.

-help or -?

Display usage information and exit.

-jar

Execute a program encapsulated in a JAR file. The first argument is the name of a JAR file instead of a startup class name. In order for this option to work, the manifest of the JAR file must contain a line of the form Main-Class: classname. Here, classname identifies the class having the public static void main(String[] args) method that serves as your application's starting point. See the Jar tool reference page and the Jar trail of the Java Tutorial for information about working with Jar files and Jar-file manifests.

When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.

-javaagent:jarpath[=options]

Load a Java programming language agent, see java.lang.instrument.

-jre-restrict-search

Include user-private JREs in the version search.

-no-jre-restrict-search

Exclude user-private JREs in the version search.

-showversion

Display version information and continue. (See also -version.)

-splash:imagepath

Show splash screen with image specified by imagepath.

-verbose

-verbose:class

Display information about each class loaded.

-verbose:gc

Report on each garbage collection event.

-verbose:jni

Report information about use of native methods and other Java Native Interface activity.

-version

Display version information and exit.

-version:release

Specifies that the version specified by release is required by the class or jar file specified on the command line. If the version of the java command invoked does not meet this specification and an appropriate implementation is found on the system, the appropriate implementation will be used.

release not only can specify an exact version, but can also specify a list of versions called a version string. A version string is an ordered list of version ranges separated by spaces. A version range is either a version-id, a version-id followed by a star (*), a version-id followed by a plus sign (+) , or two version-ranges combined using an ampersand (&). The star means prefix match, the plus sign means this version or greater, and the ampersand means the logical adding of the two version-ranges. For example:

-version:"1.6.0_13 1.6* & 1.6.0_10+"

The meaning of the above is that the class or jar file requires either version 1.6.0_13, or a version with 1.6 as a version-id prefix and that is not less than 1.6.0_10. The exact syntax and definition of version strings may be found in Appendix A of the Java Network Launching Protocol & API Specification (JSR-56).

For jar files, the usual preference is to specify version requirements in the jar file manifest rather than on the command line.

See the following NOTES section for important policy information on the use of this option.

Non-Standard Options

-X

Display information about non-standard options and exit.

-Xint

Operate in interpreted-only mode. Compilation to native code is disabled, and all bytecodes are executed by the interpreter. The performance benefits offered by the Java HotSpot Client VM's adaptive compiler will not be present in this mode.

-Xbatch

Disable background compilation. Normally the VM will compile 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.

-Xbootclasspath:bootclasspath

Specify a semicolon-separated list of directories, JAR archives, and ZIP archives to search for boot class files. These are used in place of the boot class files included in the Java 2 SDK. Note: Applications that use this option for the purpose of overriding a class in rt.jar should not be deployed as doing so would contravene the Java 2 Runtime Environment binary code license.

-Xbootclasspath/a:path

Specify a semicolon-separated path of directires, JAR archives, and ZIP archives to append to the default bootstrap class path.

-Xbootclasspath/p:path

Specify a semicolon-separated path of directires, JAR archives, and ZIP archives to prepend in front of the default bootstrap class path. Note: Applications that use this option for the purpose of overriding a class in rt.jar should not be deployed as doing so would contravene the Java 2 Runtime Environment binary code license.

-Xcheck:jni

Perform additional checks for Java Native Interface (JNI) functions. Specifically, the Java Virtual Machine validates the parameters passed to the JNI function as well as the runtime environment data before processing the JNI request. Any invalid data encountered indicates a problem in the native code, and the Java Virtual Machine will terminate with a fatal error in such cases. Expect a performance degradation when this option is used.

-Xfuture

Perform strict class-file format checks. For purposes of backwards compatibility, the default format checks performed by the Java 2 SDK's virtual machine are no stricter than the checks performed by 1.1.x versions of the JDK software. The -Xfuture flag turns on stricter class-file format checks that enforce closer conformance to the class-file format specification. Developers are encouraged to use this flag when developing new code because the stricter checks will become the default in future releases of the Java application launcher.

-Xnoclassgc

Disable class garbage collection. Use of this option will prevent memory recovery from loaded classes thus increasing overall memory usage. This could cause OutOfMemoryError to be thrown in some applications.

-Xincgc

Enable the incremental garbage collector. The incremental garbage collector, which is off by default, will reduce the occasional long garbage-collection pauses during program execution. The incremental garbage collector will at times execute concurrently with the program and during such times will reduce the processor capacity available to the program.

-Xloggc:file

Report on each garbage collection event, as with -verbose:gc, but log this data to file. In addition to the information -verbose:gc gives, each reported event will be preceeded by the time (in seconds) since the first garbage-collection event.
Always use a local file system for storage of this file to avoid stalling the JVM due to network latency. The file may be truncated in the case of a full file system and logging will continue on the truncated file. This option overrides -verbose:gc if both are given on the command line.

-Xmnsize or -XX:NewSize

Sets the size of the young generation (nursery).

-Xmsn

Specify the initial size, in bytes, of the memory allocation pool. This value must be a multiple of 1024 greater than 1MB. Append the letter k or K to indicate kilobytes, or m or M to indicate megabytes. The default value is chosen at runtime based on system configuration.
For more information, see HotSpot Ergonomics

Examples:

       -Xms6291456
       -Xms6144k
       -Xms6m

-Xmxn

Specify the maximum size, in bytes, of the memory allocation pool. This value must be a multiple of 1024 greater than 2MB. Append the letter k or K to indicate kilobytes, or m or M to indicate megabytes. The default value is chosen at runtime based on system configuration.
For server deployments, -Xms and -Xmx are often set to the same value.
For more information, see HotSpot Ergonomics

Examples:

       -Xmx83886080
       -Xmx81920k
       -Xmx80m

-Xprof

Profiles the running program, and sends profiling data to standard output. This option is provided as a utility that is useful in program development and is not intended to be used in production systems.

-Xrs

Reduces usage of operating-system signals by the Java virtual machine (JVM). This option is available beginning with J2SE 1.3.1.

In J2SE 1.3.0, the Shutdown Hooks facility was added to allow orderly shutdown of a Java application. The intent was to allow user cleanup code (such as closing database connections) to run at shutdown, even if the JVM terminates abruptly.

The JVM watches for console control events to implement shutdown hooks for abnormal JVM termination. Specifically, the JVM registers a console control handler which 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 pre-1.2 feature of dumping thread stacks for debugging purposes. Sun's JVM uses CTRL_BREAK_EVENT to perform thread dumps.

If the JVM is run as a service (for example, the servlet engine for a web server), it can receive CTRL_LOGOFF_EVENT but should not initiate shutdown since the operating system will not actually terminate the process. To avoid possible interference such as this, the -Xrs command-line option has been added beginning with J2SE 1.3.1. When the -Xrs option is used on Sun's JVM, the JVM does not install a console control handler, implying that it does not watch for or process CTRL_C_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.

There are two consequences of specifying -Xrs:

Ctrl-Break thread dumps are not available.
User code is responsible for causing shutdown hooks to run, for example by calling System.exit() when the JVM is to be terminated.

-Xssn

Set thread stack size.

-XX:AllocationPrefetchStyle=n

Sets the style of prefetch used during allocation. default=2.

-XX:+AggressiveOpts

Enables aggressive optimization.

-XX:+|-DisableAttachMechanism

This option specifies whether tools (such as jmap and jconsole) are allowed to attach to the JVM. By default, this feature is disabled. That is, attaching is enabled. Example usage:

      java -XX:+DisableAttachMechanism

-XXLargePageSizeInBytes=n

This option specifies the maximum size for large pages.

-XX:MaxGCPauseMillis=n

Sets a target for the maximum GC pause time.
This is a soft goal, and the JVM will make its best effort to achieve it. There is no maximum value set by default.

-XX:NewSize

Sets the size of the young generation (nursery). Sames as -Xmnsize.

-XX:ParallelGCThreads=n

Sets the number of GC threads in the parallel collectors.

-XX:PredictedClassLoadCount=n

This option requires that the UnlockExperimentalVMOptions flag be set first. Use the PredictedClassLoadCount flag if your application loads a lot of classes, and especially if class.forName() is used heavily. The recommended value is the number of classes loaded as shown in the output from -verbose:class.
Example usage:

      java -XX:+UnlockExperimentalVMOptions -XX:PredictedClassLoadCount=60013

-XX:+PrintCompilation

Prints verbose output from the HotSpot dynamic runtime compiler.

-XX:+PrintGCDetails -XX:+PrintGCTimeStamps

Prints garbage collection output along with time stamps.

-XX:SoftRefLRUPolicyMSPerMB=0

This flag enables aggressive processing of software references. Use this flag if HotSpot GC is impacted by the software reference count.

-XX:TLABSize=n

Thread local allocation buffers (TLAB) are enabled by default in HotSpot. HotSpot automatically sizes TLABs based on allocation patterns. The -XX:TLABSize option allows fine-tuning the size of TLABs.

-XX:+UnlockCommercialFeatures

Use this flag to actively unlock the use of commercial features. Commercial features are the products "Oracle Java SE Advanced", or "Oracle Java SE Suite", as defined at the Oracle Java SE Products web page.
If this flag is not specified, the default is to run the Java virtual machine without the commercial features being available. Once they are enabled, it is not possible to disable their use at runtime.

-XX:+|-UseCompressedOops

Enables compressed references in 64-bit JVMs.
This option is true by default.

-XX:+UseConcMarkSweepGC or -XX:+UseG1GC

These flags enable either the Concurrent Mark Sweep (CMS) or the G1 garbage collectors.

-XX:+UseLargePages

Use this flag to enable large page support.

-XX:+UseParallelOldGC

Enables the parallel garbage collectors, which are optimized for throughput and average response time.

NOTES: The -version:release command line option places no restrictions on the complexity of the release specification. However, only a restricted subset of the possible release specifications represent sound policy and only these are fully supported. These policies are:

Any version, represented by not using this option.
Any version greater than an arbitrarily precise version-id. For example:
```
"1.6.0_10+"
```
Would utilize any version greater than 1.6.0_10. This is useful for a case where an interface was introduced (or a bug fixed) in the release specified.
A version greater than an arbitrarily precise version-id, bounded by the upper bound of that release family. For example:
```
"1.6.0_10+ & 1.6*"
```
"Or" expressions of items 2. or 3. above. For example:
```
"1.6.0_10+ & 1.6* 1.7+"
```
This is similar to item 2. This is useful when a change was introduced in a release (1.7) but also made available in updates to previous releases.

Performance Tuning Examples

The following examples show how to use experimental tuning flags to optimize either throughput or faster response time.

Tuning for Higher Throughput

        java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g  -Xms26g -Xmx26g

Tuning for Lower Response Time

        java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStamps

EXIT STATUS

The following exit values are generally returned by the launcher, typically when the launcher is called with the wrong arguments, serious errors, or exceptions thrown from the Java Virtual Machine. However, a Java application may choose to return any value using the API call System.exit(exitValue).

0: Successful completion.
>0: An error occurred.