This excerpt is taken from the url http://java.sun.com/docs/books/tutorial/jar/basics/run.html

JAR Files as Applications - 1.2 platform only

In version 1.2 of the JDK software, you can run JAR-packaged applications with the Java interpreter. The basic command is:

java -jar jar-file

The -jar flag tells the interpreter that the application is packaged in the JAR file format.

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Note: The -jar option is not available for interpreters prior to version 1.2 of the Java Development Kit.

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Before this command will work, however, the runtime environment needs to know which class within the JAR file is the application's entry point.

To indicate which class is the application's entry point, you must add a Main-Class header to the JAR file's manifest. The header takes the form:

Main-Class: classname

The header's value, classname, is the name of the class that's the application's entry point.

To create a JAR file having a manifest with the appropriate Main-Class header, you can use the Jar tool's m flag as described in the Modifying a Manifest section. You would first prepare a text file consisting of single line with the Main-Class header and value. For example, if your application was the single-class HelloWorld application, the entry point would of course be the HelloWorld class, and your text file would have this line:

Main-Class: HelloWorld

Assuming your text file was in a file called mainClass, you could merge it into a JAR file's manifest with a command such as this:

jar cmf mainClass app.jar HelloWorld.class

With your JAR file prepared in this way, you can run the HelloWorld application from the command line:

java -jar app.jar