Using JAR-related APIs

The Java platform contains several classes for use with JAR files. Some of these APIs are:

• The java.util.jar package
• The java.net.JarURLConnection class
• The java.net.URLClassLoader class

To give you an idea of the possibilities that are opened up by these new APIs, this lesson guides you through the inner workings of a sample application called JarRunner.

An Example - The JarRunner Application

JarRunner enables you to run an application that's bundled in a JAR file by specifying the JAR file's URL on the command line. For example, if an application called TargetApp were bundled in a JAR file at http://www.example.com/TargetApp.jar, you could run the application using this command:

java JarRunner http://www.example.com/TargetApp.jar

In order for JarRunner to work, it must be able to perform the following tasks, all of which are accomplished by using the new APIs:

• Access the remote JAR file and establish a communications link with it.
• Inspect the JAR file's manifest to see which of the classes in the archive is the main class.
• Load the classes in the JAR file.

The JarRunner application consists of two classes, JarRunner and JarClassLoader. JarRunner delegates most of the JAR-handling tasks to the JarClassLoader class. JarClassLoader extends the java.net.URLClassLoader class. You can browse the source code for the JarRunner and JarClassLoader classes before proceeding with the lesson:

• JarRunner.java
• JarClassLoader.java

This lesson has two parts:

The JarClassLoader Class

This section shows you how JarClassLoader uses some of the new APIs to perform tasks required for the JarRunner application to work.

The JarRunner Class

This section summarizes the JarRunner class that comprises the JarRunner application.