Learning Java

Writing a Protocol Handler

A URL object uses a protocol handler to establish a connection with a server and perform whatever protocol is necessary to retrieve data. For example, an HTTP protocol handler knows how to talk to an HTTP server and retrieve a document; an FTP protocol handler knows how to talk to an FTP server and retrieve a file. All types of URLs use protocol handlers to access their objects. Even the lowly “file” type URLs use a special “file” protocol handler that retrieves files from the local filesystem. The data a protocol handler retrieves is then fed to an appropriate content handler for interpretation.

While we refer to a protocol handler as a single entity, it really has two parts: a java.net.URLStreamHandler and a java.net.URLConnection . These are both abstract classes that we will subclass to create our protocol handler. (Note that these are abstract classes, not interfaces. Although they contain abstract methods we are required to implement, they also contain many utility methods we can use or override.) The URL looks up an appropriate URLStreamHandler, based on the protocol component of the URL. The URLStreamHandler then finishes parsing the URL and creates a URLConnection when it’s time to communicate with the server. The URLConnection represents a single connection with a server, and implements the communication protocol itself.

Locating Protocol Handlers

Protocol handlers are organized in a package hierarchy similar to content handlers. But unlike ...