16.5 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 loading protocol handlers. The easiest way is to install a URLStreamHandlerFactory in the application:

 public abstract interface URLStreamHandlerFactory 

Only applications are allowed to install a new URLStreamHandlerFactory . 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 SecurityException .

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. The method needs to know how to find the protocol handler for a given protocol. This step 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, it should simply return null , which tells Java to follow the default procedure for locating stream handlers; that is, to look for a class named protocol .Handler in one of the packages listed in the java.protocol.handler.pkgs system property or in the sun.net.www.protocol package.

To install the stream handler factory, pass an instance of the class that implements the URLStreamHandlerFactory interface to the static method URL.setURLStreamHandlerFactory( ) at the start of the program. Example 16-9 is a URLStreamHandlerFactory( ) with a createURLStreamHandler( ) method that recognizes the finger, daytime, and chargen protocols and returns the appropriate handler from the last several examples. Since these classes are all named Handler , fully package-qualified names are used.

Example 16-9. A URLStreamHandlerFactory for finger, daytime, and chargen
 package com.macfaq.net.www.protocol; import java.net.*; public class NewFactory implements URLStreamHandlerFactory {    public URLStreamHandler createURLStreamHandler(String protocol) {        if (protocol.equalsIgnoreCase("finger")) {       return new com.macfaq.net.www.protocol.finger.Handler( );     }     else if (protocol.equalsIgnoreCase("chargen")) {       return new com.macfaq.net.www.protocol.chargen.Handler( );     }     else if (protocol.equalsIgnoreCase("daytime")) {       return new com.macfaq.net.www.protocol.daytime.Handler( );     }     else {       return null;     }   } } 

Example 16-9 uses the equalsIgnoreCase() method from java.lang.String to test the identity of the protocol; it shouldn't make a difference whether you ask for finger://rama.poly.edu or FINGER://RAMA.POLY.EDU . If the protocol is recognized, createURLStreamHandler( ) creates an instance of the proper Handler class and returns it; otherwise , the method returns null , which tells the URL class to look for a URLStreamHandler in the standard locations.

Since browsers, HotJava included, generally don't allow you to install your own URLStreamHandlerFactory , this will be of use only in applications. Example 16-10 is a simple character mode program that uses this factory and its associated protocol handlers to print server data on System.out . Notice that it does not import com.macfaq.net.www.protocol.chargen , com.macfaq.net.www.protocol.finger , or com.macfaq.net.www.protocol.daytime . All this program knows is that it has a URL. It does not need to know how that protocol is handled or even how the right URLConnection object is instantiated .

Example 16-10. A SourceViewer program that sets a URLStreamHandlerFactory
 import java.net.*; import java.io.*; import com.macfaq.net.www.protocol.*; public class SourceViewer4 {   public static void main (String[] args) {     URL.setURLStreamHandlerFactory(new NewFactory( ));     if  (args.length > 0) {       try {         //Open the URL for reading         URL u = new URL(args[0]);         InputStream in = new BufferedInputStream(u.openStream( ));                 // chain the InputStream to a Reader         Reader r = new InputStreamReader(in);         int c;         while ((c = r.read( )) != -1) {           System.out.print((char) c);         }        }       catch (MalformedURLException ex) {         System.err.println(args[0] + " is not a parseable URL");       }       catch (IOException ex) {         System.err.println(ex);       }     } //  end if   } // end main }  // end SourceViewer3 

Aside from the one line that sets the URLStreamHandlerFactory , this is almost exactly like the earlier SourceViewer program in Example 7-5 (Chapter 7). For instance, here the program reads from a finger URL:

 D:\JAVA\JNP2\examples>  java SourceViewer4 finger://rama.poly.edu/  Login       Name               TTY         Idle    When    Where nadats   Nabeel Datsun         pts/0         55 Fri 16:54 marcus   Marcus Tullius       *pts/1         20 Thu 12:12 marcus   Marcus Tullius       *pts/5       2:24 Thu 16:42 wri      Weber Research Insti  pts/10        55 Fri 13:26  rama.poly.edu jbjovi   John B. Jovien        pts/9        25d Mon 14:54 

Here it reads from a daytime URL:

 %  java SourceViewer4 daytime://tock.usno.navy.mil/  <html><head><title>The Time at tock.usno.navy.mil</title></head><body> <h1>Fri Oct 29 21:22:49 1999 </h1></body></html> 

However, it still works with all the usual protocol handlers that come bundled with the JDK. For instance here are the first few lines of output when it reads from an http URL:

 %  java SourceViewer4 http://www.oreilly.com/oreilly/about.html  <HTML> <HEAD> <TITLE>About O'Reilly &amp; Associates</TITLE> </HEAD> <BODY LINK="#770000" VLINK="#0000AA" BGCOLOR="#ffffff"> <table border=0 cellspacing=0 cellpadding=0 width=515> <tr> <td> <img src="http://www.oreilly.com/graphics_new/generic_ora_header_wide.gif" width="515" height="37" ALT="O'Reilly and Associates"> ... 

Java Network Programming
Java Network Programming, Third Edition
ISBN: 0596007213
EAN: 2147483647
Year: 2003
Pages: 164

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