Java Examples in a Nutshell, 3rd Edition

An HTTP Client

Example 5-6 shows a program, HttpClient, that downloads the contents of a URL from a web server and writes it to a file. It behaves like the GetURL program from Example 5-1. Despite the similarity in behavior, however, the implementation of these two programs is entirely different. Whereas GetURL relies on the URL class and its protocol handlers to handle protocol details, HttpClient connects directly to a web server and communicates with it using the HTTP protocol. As a consequence, HttpClient is restricted to downloading URLs that use the http: or https: protocol. It can’t handle ftp: or other network protocols.

At its heart, HttpClient is much like the Connect program of Example 5-4: it connects to a server, sends some text, and reads the response. This example is more complex, however, for three main reasons. First, it supports the https: protocol in addition to plain HTTP, demonstrating how to do networking with SSL secure sockets (a feature new in Java 1.4). Second, it sends an HTTP request to the server, which is more complex than the single line sent by Connect. Third, it doesn’t just read and print the server’s response; instead, it parses it to separate the HTTP response headers from the document content that follows.

A feature to note in Example 5-6 is its use of the java.net.URI class to parse the HTTP URL it is given. URI is new in Java 1.4. It has more powerful URL parsing features than the URL class we’ve already seen, but has none of the networking ...