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Trail: JAR Files
Lesson: Using JAR-related APIs Introduced in 1.2

## The JarRunner Class

> The JarRunner application is launched with a command of this form:

> >     java JarRunner url [arguments]

> In the previous section, we've seen how `JarClassLoader` is able to identify and load the main class of a JAR-bundled application from a given URL. To complete the JarRunner application, therefore, we need to be able to take a URL and any arguments from the command line, and pass them to an instance of `JarClassLoader`. These tasks belong to the `JarRunner` class, the entry point of the JarRunner application.

> It begins by creating a `java.net.URL` object from the URL specified on the command line:

> >     public static void main(String[] args) {
> >         if (args.length < 1) {
> >     	usage();
> >         }
> >         URL url = null;
> >         try {
> >             url = new URL(args[0]);
> >         } catch (MalformedURLException e) {
> >             fatal("Invalid URL: " + args[0]);
> >         }
> >         ...

> If `args.length < 1`, that means no URL was specified on the command line, so a usage message is printed. If the first command-line argument is a good URL, a new `URL` object is created to represent it.

> Next, JarRunner creates a new instance of `JarClassLoader`, passing to the constructor the URL that was specified on the command-line:

> >     JarClassLoader cl = new JarClassLoader(url);

> As we saw in the previous section, it's through `JarClassLoader` that JarRunner taps into the new JAR-handling APIs of the 1.2 platform.

> The URL that's passed to the `JarClassLoader` constructor is the URL of the JAR-bundled application that you want to run. JarRunner next calls the class loader's `getMainClassName` method to identify the entry-point class for the application:

> >     String name = null;
> >     try {
> >         name = cl.getMainClassName();
> >     } catch (IOException e) {
> >         System.err.println("I/O error while loading JAR file:");
> >         e.printStackTrace();
> >         System.exit(1);
> >     }
> >     if (name == null) {
> >         fatal("Specified jar file does not contain a 'Main-Class'" +
> >               " manifest attribute");
> >     }

> The key statement is highlighted in bold. The other statements are for error handling.

> Once `JarRunner` has identified the application's entry-point class, only two steps remain: passing any arguments to the application and actually launching the application. `JarRunner` performs these steps with this code:

> >     // Get arguments for the application
> >     String[] newArgs = new String[args.length - 1];
> >     System.arraycopy(args, 1, newArgs, 0, newArgs.length);
> >     // Invoke application's main class
> >     try {
> >         cl.invokeClass(name, newArgs);
> >     } catch (ClassNotFoundException e) {
> >         fatal("Class not found: " + name);
> >     } catch (NoSuchMethodException e) {
> >         fatal("Class does not define a 'main' method: " + name);
> >     } catch (InvocationTargetException e) {
> >         e.getTargetException().printStackTrace();
> >         System.exit(1);
> >     }

> Recall that the first command-line argument was the URL of the JAR-bundled application. Any arguments to be passed to that application are therefore in element `1` and beyond in the `args` array. `JarRunner` takes those elements, and creates a new array called `newArgs` to pass to the application (bold line above). `JarRunner` then passes the entry-point's class name and the new argument list to the `invokeClass` method of `JarClassLoader`. As we saw in the previous section, `invokeClass` will load the application's entry-point class, pass it any arguments, and launch the application.

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