Creating executable jar files

A new and cool feature of Java2 is the ability to create executable jar files. A jar file is simply a file containing a collection of java files (typically, java .class files, but any other kind of file could be included as well). To make a jar file executable, you need to specify where the "main" Class is in the jar file, so the "java" command knows what main() method to invoke to get the software going. The procedure is quite simple, and this document provides step-by-step instructions. For more details on jar files, consult http://java.sun.com/docs/books/tutorial/jar/.

1. First, make sure you have installed Java 1.2 or above. This facility is not available in previous versions of Java.

2. Next, create your working java system. In general, you will want to put it into a package. For this example, I created a trivial HelloWorld application that prints out "Hello World" plus the first command line argument, and placed it into the package "psae". Therefore, the HelloWorld files (HelloWorld.class, HelloWorld.java) were located in the directory psae. I tested the system to make sure it worked before going on to the next step.

3. In the directory in which the psae is located, created a file called "mainClass". This file contains a single line specifying where the main Class is to be found in the jar file. Note that I use the package specification. Here is the single line:

   Main-Class: psae.HelloWorld
   

   Note: make sure you type a carriage return after this line; some windows systems need it and will report a "Failed to load Main-Class manifest attribute" error.

4. Next, I create a jar file called psae.jar using the "jar" command in Java2. I use the "m" command line argument to specify the manifest file mainClass, which adds information to the jar file on where the main class will be found. Here is the jar command:

   bertha:~ > jar cmf mainClass psae.jar psae
   

5. Just for fun, and to check what's happened, I print the table of contents for the jar file I just created. Here's the command and its result:

   bertha:~ > jar tf psae.jar
   META-INF/
   META-INF/MANIFEST.MF
   psae/
   psae/HelloWorld.java
   psae/HelloWorld.class
   

6. Having successfully created the jar file, I can now invoke java2 on it with the command line argument:

   bertha:~ > java -jar psae.jar Philip
   Hello World Philip
   

7. Don't believe me? Try it yourself with the actual jar file: psae.jar.