The JarRunner Class

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 JAR-handling APIs.
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.