Java Network Programming, Second Edition

The URLStreamHandlerFactory Interface

The last section showed you how to install new protocol handlers that you wrote into HotJava, an application that someone else wrote. However, if you write your own application, you can implement your own scheme for finding and load ing protocol handlers. The easiest way to do this is to install a URLStream HandlerFactory in your application:

public abstract interface URLStreamHandlerFactory

Note

Only applications are allowed to install a new URL StreamHandlerFactory. Applets that run in the applet viewer or a web browser must use the URLStreamHandlerFactory that is provided. An attempt to set a different one will fail, either because another factory is already installed or because of a Security Exception.

The URLStreamHandlerFactory interface declares a single method, createURLStreamHandler( ):

public abstract URLStreamHandler createURLStreamHandler(String protocol)

This method loads the appropriate protocol handler for the specified protocol. To use this method, write a class that implements the URLStreamHandlerFactory interface and include a createURLStreamHandler( ) method in that class. This method needs to know how to find the protocol handler for a given protocol. This is no more complicated than knowing the names and packages of the custom protocols you’ve implemented.

The createURLStreamHandler( ) method does not need to know the names of all the installed protocol handlers. If it doesn’t recognize a protocol, then it should simply return ...