Java Network Programming (3rd ed) [Electronic resources] نسخه متنی

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\16>java SourceViewer4 finger://rama.poly.edu/
Login       Name               TTY         Idle    When    Where
nadats   Nabeel Datsun         pts/0         55 Fri 16:54  128.238.213.227
marcus   Marcus Tullius       *pts/1         20 Thu 12:12  128.238.10.177
marcus   Marcus Tullius       *pts/5       2:24 Thu 16:42  128.238.10.177
wri      Weber Research Insti  pts/10        55 Fri 13:26  rama.poly.edu
jbjovi   John B. Jovien        pts/9        25d Mon 14:54  128.238.213.229

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/aboutl
<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="/image/library/english/10151_generic_ora_header_wide.gif"
width="515" height="37"  Reilly and Associates">
...