Java Servlets

You are no doubt familiar with accessing simple, static HTML pages using a browser that sends a request to a web server, which, in turn, sends back a web page that’s stored at the server, as illustrated in next Figure. In that role, the web server is simply being used as a virtual librarian that returns a document based on a request.

That model of serving up static web pages doesn’t provide for dynamically generated content, though. For example, suppose that the web client wants the server to return a list of HTML documents based on some query criteria. In that case, some means of generating HTML on the fly and returning it to the client is needed, as illustrated in next Figure.

Servlets are one of the technologies developed to enhance servers. A Servlet is a Java component implementing the javax.servlet.Servlet interface. It is invoked as a result of a client request for that particular Servlet. The Servlet model is fairly generic and not necessarily bound to the Web and HTTP, but all of the Servlets that you’ll encounter will fall into that category. The web server receives a request for a given Servlet in the form of an HTTP query. The web server, in turn, invokes the Servlet and passes back the results to the requesting client. The Servlet can be passed parameters from the requesting web client. The Servlet is free to perform whatever computations it cares to, and returns results to the client in the form of HTML.

The Servlet itself is managed and invoked by the Java EE Servlet container. When the web server receives the request for the Servlet, it notifies the Servlet container, which will load the Servlet as necessary, and invoke the appropriate javax.servlet.Servlet interface service method to satisfy the request.

Java Servlets are portable, and as you will see in later chapters, the Servlet containers provide support for session management that allows you to write complex web-based applications. Servlets can also incorporate JavaBean components (which share little more than a name with Enterprise JavaBeans) that provide an additional degree of application compartmentalization.