Java Network Programming, Second Edition

The URLStreamHandler Class

The abstract URLStreamHandler class is a superclass for classes that handle specific protocols—for example, HTTP. You rarely call the methods of the URLStreamHandler class directly; they are called by other methods in the URL and URLConnection classes. By overriding the URLStreamHandler methods in your own subclass, you teach the URL class how to handle new protocols. Therefore, we’ll focus on overriding the methods of URLStreamHandler rather than on calling the methods.

The Constructor

You do not create URLStreamHandler objects directly. Instead, when a URL is constructed with a protocol that hasn’t been seen before, Java asks the application’s URLStreamHandlerFactory to create the appropriate URLStreamHandler subclass for the protocol. If that fails, Java guesses at the fully package-qualified name of the URLStreamHandler class and uses Class.forName( ) to attempt to construct such an object. This means concrete subclasses should have a noargs constructor. The single constructor for URLStreamHandler doesn’t take any arguments:

public URLStreamHandler(  )

Because URLStreamHandler is an abstract class, this constructor is never called directly; it is only called from the constructors of subclasses.

Methods for Parsing URLs

The first responsibility of a URLStreamHandler is to split a string representation of a URL into its component parts and use those parts to set the various fields of the URL object. The parseURL( ) method splits the URL into parts, possibly ...