Java Servlet Programming

Last Modified Times

By now, we’re sure you’ve learned that servlets handle GET requests with the doGet() method. And that’s almost true. The full truth is that not every request really needs to invoke doGet(). For example, a web browser that repeatedly accesses PrimeSearcher should need to call doGet() only after the searcher thread has found a new prime. Until that time, any call to doGet() just generates the same page the user has already seen, a page probably stored in the browser’s cache. What’s really needed is a way for a servlet to report when its output has changed. That’s where the getLastModified() method comes in.

Most web servers, when they return a document, include as part of their response a Last-Modified header. An example Last-Modified header value might be:

Tue, 06-May-98 15:41:02 GMT

This header tells the client the time the page was last changed. That information alone is only marginally interesting, but it proves useful when a browser reloads a page.

Most web browsers, when they reload a page, include in their request an If-Modified-Since header. Its structure is identical to the Last-Modified header:

Tue, 06-May-98 15:41:02 GMT

This header tells the server the Last-Modified time of the page when it was last downloaded by the browser. The server can read this header and determine if the file has changed since the given time. If the file has changed, the server must send the newer content. If the file hasn’t changed, the server can reply with a simple, short ...