Notes on Runnable Jars

You can easily package an application's entire set of classes and resources into a Java Archive (JAR). In fact, that is one goal of having jar files. Another is to let users easily execute the application stored in the archive. Why then are jar files second-class citizens in the Java universe—functioning only as archives—when they can be first class, right alongside native executables?

To execute a jar file, you can use the java command's -jar option. For example, say you have a runnable jar file called myjar.jar. Because the file is runnable, you can execute it like this: java -jar myjar.jar.

Alternatively, the Java Runtime Environment (JRE), when installed on an OS like Microsoft Windows, associates jar files with the JVM so you can double-click on them to run the application. These JARs must be runnable.

The question is: How do you make a JAR runnable?

The manifest file and the Main-Class entry

Inside most JARs, a file called MANIFEST.MF is stored in a directory called META-INF. Inside that file, a special entry called Main-Class tells the java -jar command which class to execute.

The problem is that you must properly add this special entry to the manifest file yourself—it must go in a certain place and must have a certain format. However, some of us don't like editing configuration files.

Let the API do it for you

Since Java 1.2, a package called java.util.jar has let you work with jar files. (Note: It builds on the java.util.zip package.) Specifically, the jar package lets you easily manipulate that special manifest file via the Manifest class.

For more details, refer to the link:

http://www.javaworld.com/javaworld/javatips/jw-javatip127.html