jre - The Java Runtime Interpreter (Win32)

jre interprets (executes) Java bytecodes.

SYNOPSIS

jre [ options ] classname <args>
jrew [ options ] classname <args>

DESCRIPTION

The jre command executes Java class files. The jrew command is identical to jre, except that with jrew there is no associated console window. Use jrew when you don't want a command prompt window to appear.

The classname argument is the name of the class file to be executed. Any arguments to be passed to the class must be placed after the classname on the command line. On Windows platforms, the jre tool ignores the CLASSPATH environment variable. The -cp option should be used to specify an application's class path.

Using a JIT

This release supports execution with a Just In Time bytecode compiler, or JIT. 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.

This release does not use a JIT by default. To use a JIT, pass the name of the JIT via the JAVA_COMPILER environment variable or the java.compiler property. If JAVA_COMPILER and java.compiler have conflicting values, java.compiler is used. On Windows, a JIT is in a file named jitname.dll, where jitname is the name of the JIT. The launcher first searches for the JIT file in its own directory, then in each directory in the PATH environment variable.

The Windows version of the JDK software includes a Symantec JIT named "symcjit". (Symantec JIT copyright © 1996-1998 by Symantec Corporation. All rights reserved.) You can enable this JIT via the JAVA_COMPILER environment variable:

    C:\> set JAVA_COMPILER=symcjit
    C:\> jre MyClass 
To disable the JIT, unset JAVA_COMPILER:
    C:\> set JAVA_COMPILER= 
To enable the JIT from the command line, use the -D option:
    C:\> jre -Djava.compiler=symcjit MyClass 

JAR Files and Extensions

In this release, the -new option must be used when launching a program that is contained in a JAR file or when launching a program that makes use of extensions. The need to use the -new option will be removed in a future release.

JAR files
A program can be encapsulated in a JAR archive. The archive can contain all the class files and resources that define the program. In this release, program files in a JAR archive can be passed to the launcher as a unit, using the -new and -jar options in conjunction:

% jre -new -jar foo.jar

Extensions
In this release, programs that make use of extensions must be launched by using the -new option:

% jre -new MyClass

OPTIONS

The Java Runtime Interpreter has a set of standard options that are supported on the current Java virtual machine (VM) and will be supported on futures VMs. In addition, there is a set of non-standard options that are supported on the current VM, but are subject to change in future VMs. The non-standard options begin with -X.



Standard Options

-classpath path(s); -cp path(s)
Specifies the path or paths that jre uses to look up classes. Overrides the default classpath set in the registry. If more than one path is specified, they must be separated by semicolons. Each path should end with the directory containing the class file(s) to be executed. However, if a file to be executed is a zip or jar file, the path to that file must end with the file's name. Here is an example of an argument for -classpath consisting of two paths:
   C:\xyz\classes;C:\usr\local\java\classes\MyClasses.jar

-help
Print a usage message.



-v,  -verbose,  -verbose:class
Causes jre to print a message to stdout each time a class file is loaded.

-verbose:gc
Causes the garbage collector to print out messages whenever it frees memory.

-verbose:jni
Prints JNI-related messages including information about which native methods have been linked and warnings about excessive creation of local references.

-DpropertyName=newValue
Defines a property value. propertyName is the name of the property whose value you want to change and newValue is the value to change it to. For example, this command line
% jre -Dawt.button.color=green ...
sets the value of the property awt.button.color to "green". jre accepts any number of -D options on the command line.

-jar   jarfile
Note: In this release, the -jar option must be used in conjunction with the -new flag:  jre -new -jar ...

Execute a Java program encapsulated in jarfile. Instead of referring to the command line, jre gets the initial class from the jarfile manifest header Main-Class. For example, if the program entry point is COM.MyCompany.MyPackage.MyClass.main(), then this entry must appear in the manifest:

Main-Class: COM.MyCompany.MyPackage.MyClass

-usepolicy[:policyfile]
Note: In this release, the -usepolicy option must be used in conjunction with the -new flag:  jre -new -usepolicy[:policyfile] ...

Use a security policy. The policy is defined in policyfile, if specified, or in a installation-dependent default if policyfile is not specified.

Without -usepolicy, a Security Manager is not installed, and the program runs without security restrictions.

-X
Prints help on non-standard options and exits.

Non-Standard Options

-Xmxx
Sets the maximum size of the memory allocation pool (the garbage collected heap) to x. The default is 16 megabytes of memory. x must be greater than or equal to 1000 bytes.

By default, x is measured in bytes. You can specify x in either kilobytes or megabytes by appending the letter "k" for kilobytes or the letter "m" for megabytes.

-Xmsx
Sets the startup size of the memory allocation pool (the garbage collected heap) to x. The default is 1 megabyte of memory. x must be > 1000 bytes.

By default, x is measured in bytes. You can specify x in either kilobytes or megabytes by appending the letter "k" for kilobytes or the letter "m" for megabytes.

-Xnoasyncgc
Turns off asynchronous garbage collection. When activated no garbage collection takes place unless it is explicitly called or the program runs out of memory. Normally garbage collection runs as an asynchronous thread in parallel with other threads.

-Xnoclassgc
Turns off garbage collection of Java classes. By default, the Java interpreter reclaims space for unused Java classes during garbage collection.

-Xssx
Each Java thread has two stacks: one for Java code and one for C code. The -Xss option sets the maximum stack size that can be used by C code in a thread to x. Every thread that is spawned during the execution of the program passed to jre has x as its C stack size. The default units for x are bytes. The value of x must be greater than or equal to 1000 bytes.

You can modify the meaning of x by appending either the letter "k" for kilobytes or the letter "m" for megabytes. The default stack size is 128 kilobytes ("-Xss128k").

-Xossx
Each Java thread has two stacks: one for Java code and one for C code. The -Xoss option sets the maximum stack size that can be used by Java code in a thread to x. Every thread that is spawned during the execution of the program passed to jre has x as its Java stack size. The default units for x are bytes. The value of x must be greater than or equal to 1000 bytes.

You can modify the meaning of x by appending either the letter "k" for kilobytes or the letter "m" for megabytes. The default stack size is 400 kilobytes ("-Xoss400k").

-Xverify
Performs byte-code verification on the class file.

-Xverifyremote
Runs the verifier on all code that is loaded into the system via a classloader. verifyremote is the default for the interpreter.

-Xnoverify
Turns verification off.

SEE ALSO

CLASSPATH