Java Network Programming, Second Edition

Chapter 15. The URLConnection Class

URLConnection is an abstract class that represents an active connection to a resource specified by a URL. The URLConnection class has two different but related purposes. First, it provides more control over the interaction with a server than the URL class. With a URLConnection, you can inspect the MIME headers sent by an HTTP server and respond accordingly. You can adjust the MIME header fields used in the client request. You can use a URLConnection to download binary files. Finally, a URLConnection lets you send data back to a web server with POST or PUT and use other HTTP request methods. We will explore all of these techniques in this chapter.

Second, the URLConnection class is part of Java’s protocol handler mechanism, which also includes the URLStreamHandler class. The idea behind protocol handlers is simple: they separate the details of processing a protocol from processing particular data types, providing user interfaces, and doing the other work that a monolithic web browser performs. The base java.net.URLConnection class is abstract; to implement a specific protocol, you write a subclass. These subclasses can be loaded at runtime by your own applications or by the HotJava browser; in the future, it may be possible for Java applications to download protocol handlers over the Net as needed, making them automatically extensible. For example, if your browser runs across a URL with a strange prefix, such as compress:, rather than throwing ...