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Trail: JAR Files

## Lesson: Using JAR-related APIs Introduced in 1.2

> Version 1.2 of the JavaTM platform contains several new classes that greatly expand the number of ways that you can use JAR files. Some of these new 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.xxx.yyy/TargetApp.jar`, you could run the application using this command:

> >     java JarRunner http://www.xxx.yyy/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 new `java.net.URLClassLoader` class of the 1.2 platform. 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.

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