Java Network Programming, 3rd Edition

Chapter 15. URLConnections

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 (especially an HTTP server) than the URL class. With a URLConnection, you can inspect the header sent by the server and respond accordingly. You can set the 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 applications. For example, if the browser runs across a URL with a strange scheme, such as compress, rather than throwing up its hands and issuing an error message, it can download a protocol handler for this unknown protocol and use it to communicate with the server. Writing protocol handlers is the subject ...