0 The Java Servlet API

Now that we  have a basic understanding of  HTTP , we can move on and talk about
the Servlet API that you’ll be using to create HTTP servlets, or any kind of servlets,
for that matter. Servlets use classes and interfaces from two packages: javax.
servlet and javax.servlet.http.

The javax.servlet package contains classes to support generic, protocol-independent servlets. These classes are extended by the classes in the javax.servlet.http package to add HTTPspecific functionality. The top-level package name is javax instead of the familiar java, to indicate that the Servlet API is a standard extension.

Every servlet must implement the javax.servlet.Servlet interface. Most servlets
implement it by extending one of two special classes: javax. servlet.

GenericServlet or javax.servlet.http.HttpServlet. A protocol-independent
servlet should subclass GenericServlet, while an HTTP servlet should
subclass HttpServlet, which is itself a subclass of GenericServlet with added
HTTP-specific functionality.

Unlike a regular Java program, and just like an applet, a servlet does not have a
main() method. Instead, certain methods of a servlet are invoked by the server in
the process of handling requests. Each time the server dispatches a request to a
servlet, it invokes the servlet’s service() method.

A generic servlet should override its service() method to handle requests as
appropriate for the servlet. The service() method accepts two parameters: a
request object and a response object. The request object tells the servlet about the
request, while the response object is used to return a response. Figure 2-1 shows
how a generic servlet handles requests.

 

 In contrast, an HTTP servlet usually does not override the service() method.
Instead, it overrides doGet() to handle GET requests and doPost() to handle
POST requests. An HTTP servlet can override either or both of these methods,
depending on the type of requests it needs to handle. The service() method of
HttpServlet handles the setup and dispatching to all the doXXX() methods,
which is why it usually should not be overridden. Figure 2-2 shows how an HTTP
servlet handles GET and POST requests.

An HTTP servlet can override the doPut() and doDelete() methods to handle
PUT and DELETE requests, respectively. However, HTTP servlets generally don’t
touch doHead(), doTrace(), or doOptions(). For these, the default implementations
are almost always sufficient.

The remainder in the javax.servlet and javax.servlet.http packages are
largely support classes. For example, the ServletRequest and ServletResponse
classes in javax.servlet provide access to generic server requests and
responses, while HttpServletRequest and HttpServletResponse in javax.
servlet.http provide access to HTTP requests and responses. The javax.
servlet.http package also contains an HttpSession class that provides built-in
session tracking functionality and a Cookie class that allows you to quickly set up
and process HTTP cookies.