Tutorials : Creating Content and Protocol Handlers in Java, Part 2 :

Creating Content and Protocol Handlers in Java, Part 2

by Anghel Leonard

Part 1 of this series showed you how to work with an existing content handler and how to create a new one. It also discussed how to define a new MIME type for a new kind of file (the .xy file) and how to transfer it from a simple HTTP server to a Java client. In principle, the Internet browser (Internet Explorer 6.0) and the Java client that you have developed for the new content handler use the same idea—that is, both of them use the HTTP specification protocol for connecting and sending a request to the HTTP server HTTPServer.java (the source code for this minimal HTTP server can be viewed in Part 1). When they get the sever response, they process it using their appropriate content handlers.

The question is: how does the Java client (or the browser) know that it will "talk" with an HTTP server and how does it know what to "say"? The answer is that the Java client has no idea what kind of server it will "talk" to. The Java client just parses the provided URL and extracts the protocol name (http, ftp, etc.). After that, it associates the protocol name with the corresponding protocol implementation. These tasks are accomplished by the protocol handler.

In this article, you will learn how to work with an existing Java protocol handler and how to create a new one. Of course, when you're writing your own protocol handler, it will not be recognized by Internet servers, or any other kind of servers, because they can't possibly recognize it. For testing purposes, these examples use the HTTPServer.java you developed in Part 1. This server will recognize the protocol handler defined in Part 2 and the content handler defined in Part 1.

Working with an Existing Protocol Handler

1. The first task is to parse the specified URL (conforming to URL specifications) and extract the protocol name.
2. The second task is based on the protocol name—the browser is choosing "the right words" for connecting and communicating with the requested server. The final task is to put the received resource into a readable form. These first two tasks are accomplished by the protocol handler.
3. The final task is accomplished by the content handler. The browser has a protocol handler for every kind of protocol, so the HTTP protocol handler know how to "talk" with an HTTP server, a FTP protocol handler know how to "talk" with an FTP server, etc.

The default protocol handlers' class names, along with their associated package names, reflect the protocol names themselves. For example, the http protocol's proper protocol handler is in the sun.net.www.protocol.http package and its name is Handler.class. The sun.net.www.protocol path is fixed and the rest depends on the protocol name. The table below contains the correspondence between some protocol names and their protocol handlers (J2SE 1.5):

Protocol name	path	class
doc	sun.net.www.protocol.doc	Handler.class
file	sun.net.www.protocol.file	Handler.class
ftp	sun.net.www.protocol.ftp	Handler.class
gopher	sun.net.www.protocol.gopher	Handler.class
http	sun.net.www.protocol.http	Handler.class
jar	sun.net.www.protocol.jar	Handler.class
mailto	sun.net.www.protocol.mailto	Handler.class
netdoc	sun.net.www.protocol.netdoc	Handler.class
systemresource	sun.net.www.protocol.systemresorce	Handler.class
verbatim	sun.net.www.protocol.verbatim	Handler.class
Xy (your protocol handler)	sun.net.www.protocol.xy	Handler.class

Note: Usually, the sun.net.www.protocol... path is in the <JDK_HOME>/jre/lib/rt.jar.

Listing 1 demonstrates how to create a simple Java client that uses the protocol handler specific to the HTTP protocol for connecting and communicating with an HTTP server.

As you can see, the default HTTP protocol handler eliminates the need to write any HTTP commands because Java knows how to implement it in the sun.net.www.protocol.http package.

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