java - the Java application launcher

SYNOPSIS

    java [ options ] class [ argument ... ]
    java [ 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 declaration must look like the following: 
    public static void main(String args[])
The method must be declared public and static, it must not return any value, and it must accept a String array as a parameter. 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 files 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.

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 current implementations of the virtual machines support a set of non-standard options options that are subject to change in future releases.

Standard Options

-hotspot

Select the Java HotSpot Client VM. This is the default.

-server

Select the Java HotSpot Server VM.

-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 colons (:). 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 (.).

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

-Dproperty=value

Set a system property value.

-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 s 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.

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

-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 rent 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 lt;Main Class>

The -disableassertions and -da switches apply to all ss 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.

-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.

-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.

On Solaris 8, JAR files that can be run with the "java -jar" option can have their execute permissions set so they can be run without using "java -jar".

-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.

-showversion

Display version information and continue.

-?

-help

Display usage information and exit.

-X

Display information about non-standard options and exit.

Non-Standard Options

-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 VMs' 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.

-Xdebug

Start with the debugger enabled. Refer to jdb description for more details and an example.

-Xbootclasspath:bootclasspath

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

-Xbootclasspath/p:path

Specify a colon-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.

-Xincgc

Enable the incremental garbage collector. The incremental garbage collector, which is off by default, will eliminate occasional garbage-collection pauses during program execution. However, it can lead to a roughly 10% decrease in overall GC performance.

-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.

-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 2MB. Examples: 
       -Xms6291456
       -Xms6144k
       -Xms6m

-Xmxn

Specify the maximum size, in bytes, of the memory allocation pool. This value must 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 64MB. Examples: 
       -Xmx83886080
       -Xmx81920k
       -Xmx80m

-Xssn

Set thread stack size.

-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 be used in production systems.

-Xrunhprof[:help][:<suboption>=<value>,...]
Enables cpu, heap, or monitor profiling. This option is typically followed by a list of comma-separated "<suboption>=<value>" pairs . Run the command java -Xrunhprof:help to obtain a list of suboptions and their default values.

-Xrs

Reduces use of operating-system signals by the Java virtual machine (JVM).

In a previous release, 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.

Sun's JVM catches signals to implement shutdown hooks for abnormal JVM termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.

The JVM uses a similar mechanism to implement the pre-1.2 feature of dumping thread stacks for debugging purposes. Sun's JVM uses SIGQUIT to perform thread dumps.

Applications embedding the JVM frequently need to trap signals like SIGINT or SIGTERM, which can lead to interference with the JVM's own signal handlers. The -Xrs command-line option is available to address this issue. When -Xrs is used on Sun's JVM, the signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT are not changed by the JVM, and signal handlers for these signals are not installed.

There are two consequences of specifying -Xrs:

  • SIGQUIT 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.

SEE ALSO
Java Software