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java - the Java application launcher - JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

java - the Java application launcher

• Synopsis
• Description
• Java and OldJava
• The JIT
• Options
• See Also

SYNOPSIS

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

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.

JAVA AND OLDJAVA

These are the differences between java and oldjava

• The oldjava launcher does not support the extension mechanism. Extension packages can still be used as class and resource archives.

• The oldjava launcher uses the bootstrap class loader for all classes, while the java launcher uses the bootstrap class loader only for bootstrap classes. This allows programming techniques that are incompatible with the base class loader.

• With java, the -classpath and -cp options specify a search path for user classes, and cannot be used to specify locations for bootstrap classes. With oldjava the -classpath and -cp options specify locations for all classes, including botstrap classes.

  Note that java and oldjava do not differ in their use of the CLASSPATH environment variable. Unless overridden by -classpath or -cp, CLASSPATH always specifies the location of user classes, but says nothing about the bootstrap classes.

• The oldjava launcher supports all the 1.2 security features. With oldjava, however, classes loaded from the class path will not have a protection domain and will therefore have all permissions which is true only for bootstrap classes with the java launcher.

This command using oldjava

oldjava -classpath <path> ...

is equivalent to this command using java

java -Xbootclasspath:<path> -Djava.ext.dirs= ...

where in both cases <path> contains the file rt.jar. (See the JDK File Structure document for a description of rt.jar and its location in the JDK software.)

THE JIT

The default behavior of the launcher is to execute bytecodes using a Just-in-Time compiler, or JIT. The default compiler is located at jre\bin\symcjit.dll. When a class is loaded, the JIT translates the class bytecodes into native machine code. Using a JIT causes a slight delay after each class load, but can improve overall program performance. In some cases, execution time will improve by a factor of ten.

If the JIT is disabled, bytecodes are executed directly by an interpreter. There are two ways to disable the JIT:

• Set the environment variable JAVA_COMPILER to NONE, using the set command.

  C:\> set JAVA_COMPILER=NONE
  

  On Windows NT, you can also use the System Control Panel to set JAVA_COMPILER.

• Use the -D command-line option to set java.compiler to NONE.

  C:\> java -Djava.compiler=NONE myapp
  

  Using the command-line option overrides the setting of the JAVA_COMPILER environment variable.

You can also use JAVA_COMPILER or java.compiler to the specify that an alternative JIT should be used:

C:\> set JAVA_COMPILER=foo

or

C:\> java -Djava.compiler=foo myapp

The .dll filename extension is added to "foo", so that, in this example, the virtual machine will search for a JIT compiler named foo.dll. The search for the alternative compiler is made in the jre\bin directory and on the system's PATH. If no such compiler is found, the virtual machine will default to using the interpreter.

OPTIONS

The launcher has a set of standard options that are supported on the current runtime environment and will be supported in future releases. An additional set of non-standard options are specific to the current virtual machine implementation and are subject to change in the future. Non-standard options begin with -X

Standard 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.

Used with java or javaw, -classpath or -cp only specify the class path for user classes. Used with oldjava or oldjavaw, -classpath or -cp specify the class path for both user classes and bootstrap classes.

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.

-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.

The oldjava and oldjavaw tools do not support the -jar option.

-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.

-?

-help

Display usage information and exit.

-X

Display information about non-standard options and exit.

Non-Standard Options

-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 JDK 1.2 software.

-Xdebug

Start with the debugger enabled. The Java interpereter prints out a password for jdb's use. Refer to jdb description for more details and an example.

-Xnoclassgc

Disable class garbage collection

-Xmsn

Specify the initial size of the memory allocation pool. This value must greather than 1000. To multiply the value by 1000, append the letter k. To multiply the value by 1 million, append the letter m. The default value is 1m

-Xmxn

Specify the maximum size of the memory allocation pool. This value must greather than 1000. To multiply the value by 1000, append the letter k. To multiply the value by 1 million, append the letter m. The default value is 64m.

-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

Reduce the use of operating system signals.

-Xcheck:jni

Perform additional check for Java Native Interface functions.

-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.

SEE ALSO

• javac - the Java programming language compiler
• jdb - Java Application Debugger
• javah - C Header and Stub File Generator
• jar - JAR Archive Tool
• The Java Extensions Framework
• Security Features.