Servlets

Servlets

•       A Java program which resides and executes on a server to provide functionality to the server or processing of data on the server.

Lifecycle:

•       The life cycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps.

1.     If an instance of the servlet does not exist, the Web container

a.     Loads the servlet class.

b.     Creates an instance of the servlet class.

c.      Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet.

2.     Invokes the service method, passing a request and response object. Service methods are discussed in the section Writing Service Methods.

3.       If the container needs to remove the servlet, it finalizes the servlet by calling the servlet's destroy method. Finalization is discussed in Finalizing a Servlet.

Example:

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class HelloWorld extends HttpServlet

{

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException

{

PrintWriter pw = response.getWriter(); pw.println("Hello World");

}

}

Servlet API (refer java complete ref)

Two packages contain the classes and interfaces that are required to build servlets. These are javax.servlet and javax.servlet.http. They constitute the Servlet API. Keep in mind that these packages are not part of the Java core packages. Instead, they are standard extensions. Therefore, they are not included in the Java Software Development Kit. You must download Tomcat to obtain their functionality.

The Servlet API has been in a process of ongoing development and enhancement. The current servlet specification is version is 2.3 and that is the one used in this book. However, because changes happen fast in the world of Java, you will want to check for any additions or alterations. This chapter discusses the core of the Servlet API, which will be available to most readers.

The javax.servlet package contains a number of interfaces and classes that establish

the framework in which servlets operate. The following table summarizes the core interfaces that are provided in this package. The most significant of these is Servlet. All servlets must implement this interface or extend a class that implements the interface. The ServletRequest and ServletResponse interfaces are also very important.

The following table summarizes the core classes that are provided in the javax.servlet package.