Another advantage servlets gain by being developed in an object-oriented language such as
Java is that they can be extended and polymorphed into new objects that better suit your needs.
A good example of this is an online catalog. You might want to display the same catalog search
tool at the top of every dynamic page throughout your Web site. You definitely don't want to
add this code to every one of your servlets. So, you implement a base servlet that builds and
initializes the search tool and then extend it to display transaction-specific responses.
Servlets run on the server side, inheriting the security provided by the Web server. Servlets can
also take advantage of the Java Security Manager.
The Java Servlet Architecture
Two packages make up the servlet architecture: javax.servlet and javax.servlet.http . The
javax.servlet package contains the generic interfaces and classes that are implemented and
extended by all servlets. The java.servlet.http package contains the classes that are
extended when creating HTTP-specific servlets. An example of this is a simple servlet that
responds using HTML.
At the heart of this architecture is the interface javax.servlet.Servlet . It provides the frame-
work for all servlets. The Servlet interface defines five methods. The three most important are
the init() method, which initializes a servlet; the service() method, which receives and
responds to client requests; and the destroy() method, which performs cleanup. All servlets
must implement this interface, either directly or through inheritance. It is a very clean object-
oriented approach that makes the interface easy to extend. Figure 2.3 shows an object model
that gives a high-level view of the servlet framework.
GenericServlet and HttpServlet
The two main classes are the GenericServlet and HttpServlet classes. The HttpServlet
class is extended from GenericServlet . When you are developing your own servlets, you will
most likely be extending one of these two classes. Java servlets do not have a main() method,
which is why all servlets must implement the javax.servlet.Servlet interface. Every time a
server receives a request that points to a servlet it calls that servlet's service() method.
If you decide to extend the GenericServlet class, you must implement the service() method.
The GenericServlet.service() method has been defined as an abstract method to force you
to follow this framework. The service() method prototype is defined as follows:
public abstract void service(ServletRequest req,
ServletResponse res) throws ServletException, IOException;s