Just as the createURLStreamHandler( ) method of the URLStreamHandlerFactory interface was responsible for finding and loading the appropriate protocol handler, so too the createContentHandler( ) method of the ContentHandlerFactory interface is responsible for finding and loading the appropriate ContentHandler given a MIME type:

public abstract ContentHandler createContentHandler(String mimeType)

This method should be called only by the getContent( ) method of a URLConnection object. For instance, Example 17.7 is a ContentHandlerFactory that knows how to find the right handler for the text/tab-separated-values content handler of Example 17.1:

package com.macfaq.net.www.content;

import java.net.*;

public class TabFactory implements ContentHandlerFactory {

  public ContentHandler createContentHandler(String mimeType)) {

    if (mimeType.equals("text/tab-separated-values") {
      return new com.macfaq.net.www.content.text.tab_separated_values(  );
    }
    else {
      return null; // look for the handler in the default locations
    }
  }
}

This factory knows how to find only one kind of content handler, but there’s no limit to how many a factory can know about. For example, this createContentHandler( ) method also suggests handlers for application/x-time, text/plain, video/mpeg, and model/vrml. Notice that when you’re using a ContentHandlerFactory, you don’t necessarily have to stick to standard naming conventions for ContentHandler subclasses:

public ContentHandler createContentHandler(String mimeType)) {

  if (mimeType.equals("text/tab-separated-values") {
      return new com.macfaq.net.www.content.text.tab_separated_values(  );
  }
  else if (mimeType.equals("application/x-time") {
    return new com.macfaq.net.www.content.application.x_time(  );
  }
  else if (mimeType.equals("text/plain") {
    return new sun.net.www.content.text.plain(  );
  }
  if (mimeType.equals("video/mpeg") {
    return new com.macfaq.video.MPEGHandler(  );
  }
  if (mimeType.equals("model/vrml") {
    return new com.macfaq.threed.VRMLModel(  );
  }
  else {
    return null; // look for the handler in the default locations
  }
}
               

A ContentHandlerFactory is installed in an application using the static URLConnection.setContentHandlerFactory( ) method:

public static void setContentHandlerFactory(ContentHandlerFactory fac)
  throws SecurityException, Error

Note that this method is in the URLConnection class, not the ContentHandler class. It may be invoked at most once during any run of an application. It throws an Error if it is called a second time.

Using a ContentHandlerFactory such as the TabFactory in Example 17.5, it’s possible to write a standalone application that can automatically load our tab-separated-values content handler and that runs in Java 1.1 through 1.3 without any major hassles with the class path. Example 17.8 is such a program. However, as with most other setFactory( ) methods, untrusted applets will generally not be allowed to set the content handler factory. Attempting to do so will throw a SecurityException. Consequently, installing new content handlers in applets pretty much requires directly accessing the getContent( ) method of the ContentHandler subclass itself. Ideally, this shouldn’t be necessary, but until Sun provides better support for downloadable content handlers in browsers, this is what we’re stuck with.

import java.io.*;
import java.net.*;
import java.util.*;
import com.macfaq.net.www.content.*;

public class TabLoader {

  public static void main (String[] args) {  
    
    URLConnection.setContentHandlerFactory(new TabFactory(  ));
    
    for (int i = 0; i < args.length; i++) {
      try {
        URL u = new URL(args[i]);
        Object content = u.getContent(  );
        Vector v = (Vector) content;
        for (Enumeration e = v.elements() ; e.hasMoreElements(  ) ;) {
          String[] sa = (String[]) e.nextElement(  );
          for (int j = 0; j < sa.length; j++) {
            System.out.print(sa[j] + "	");
          }
          System.out.println(  );
        } 
      }
      catch (MalformedURLException e) {
        System.err.println(args[i] + " is not a good URL"); 
      }
      catch (Exception e) {
        e.printStackTrace(  );
      }
    }
  }
}

Here’s a typical run. As usual, tabs are indicated by arrows:

% java TabLoader http://metalab.unc.edu/javafaq/addresses.tab
JPE Associates             →341 Lafayette St, Suite 1025  →New York  →NY →10012
O'Reilly & Associates →103 Morris St, Suite A           →Sebastopol  →CA →95472