Home › Blogs › James's blog

Wrangling the Class Path with Download Extensions

Fri, 09/11/2009 - 06:27 — James

Setting the class path can be quite bothersome when you have many separate JAR files to deal with. An easy way to manage obese class paths is with download extensions. Through a list of JAR files in the MANIFEST.MF file within a given JAR file, you can automatically include an arbitrary number of JAR files on the class path.

Let's imagine we have the following two classes in our project:

package com.earldouglas.greeter;

public class DefaultGreeter {

    public String getGreeting() {
        return "Hello World!";
    }
}

package com.earldouglas.greeter;

public class DefaultGreeterRunner {

    public static void main(String[] arguments) {
        System.out.println(new DefaultGreeter().getGreeting());
    }
}

Now let's imagine that DefaultGreeter is packaged into greeter.jar, while DefaultGreeterRunner is packaged into greeter-runner.jar.

We can run DefaultGreeterRunner from the command line with:

java -classpath greeter.jar;greeter-runner.jar com.earldouglas.greeter.DefaultGreeterRunner

By using download extensions, however, we can trim the command to use a single JAR file in the class path:

java -classpath greeter-runner.jar com.earldouglas.greeter.DefaultGreeterRunner

This is accomplished simply by modifying the MANIFEST.MF file within the META-INF folder in greeter-runner.jar, and adding the dependent JAR files to the Class-Path header.

MANIFEST.MF:

Manifest-Version: 1.0
Created-By: 1.5.0_21 (Sun Microsystems Inc.)
Class-Path: greeter.jar

Remember that each line in the manifest must be no more than 72 characters long, and to span a long line, you simply prepend each continuation line with two spaces.

See the Download Extensions Java Tutorial for more detailed information.