jar cf test.jar test.class

Main-Class: test

java -jar test.jar

class test {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

C:\>javac test.java

C:\>java test
Hello World!

C:\>jar cf test.jar test.class

Manifest-Version: 1.0
Main-Class: test
Created-By: 1.6.0_24 (Sun Microsystems Inc.)

C:\>java -jar test.jar
Hello World!

Originally posted by cean at 2011-06-28 21:18:23:
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Setting an Application's Entry Point
If you have an application bundled in a JAR file, you need some way to indicate which class within the JAR file is your application's entry point. You provide this information with the Main-Class header in the manifest, which has the general form:

Main-Class: classname

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

Recall that the entry point is a class having a method with signature public static void main(String[] args).

After you have set the Main-Class header in the manifest, you then run the JAR file using the following form of the java command:

java -jar JAR-name

The main method of the class specified in the Main-Class header is executed.