Navigating with Servlets

The first thing to understand is how a servlet begins and ends. Similar to an applet, the life cycle of the servlet begins with an automatic call to its init method. The init method is called once by the server for initialization and not again for each user request. Because the server creates a single class instance that handles every request of the servlet, performance is greatly improved, eliminating the need to create overhead that would otherwise be necessary if each request required the server to create a new object.

Next, the service method is called, performing the work of the servlet, and passing ServletRequest and ServletResponse objects as needed. These objects collect or pass information such as the values of the named attributes, theIP address of the agent, or the port number on which the request was received.

Lastly, like an applet, it is time to remove a previously loaded servlet instance, the servlet's destroy method is called. This gives the servlet a chance to close database connections, save information to a log file, or perform other cleanup tasks before it is shut down. If you have special cleanup tasks you'd like your servlet to perform before being removed from memory, the destroy is the place to write those instructions.

All three methods, init, service, and destroy, can be overridden.

If you need a servlet to load with customized initialization behavior, you can override the init method using either of the following two formats:

No argument format:

public void init()throws ServletException{

Takes ServletConfig object:

public void init(ServletConfig config)
throws ServletException {
super.init(config);
//Initialization code . .

The latter is used when the servlet needs to be initialized with server information such as:

1. Password files
2. A hit count number
3. Serialized cookie information
4. Data from previous requests

When using the ServletConfig format, create a call to the super.init so that the super class registers the information where the servlet can find it later.

Which format you use depends on what information needs to be known at initialization time. If no information is needed when the servlet is first invoked, then the no argument format may be used.

The Heart of the Servlet

The javax.servlet package and the javax.servlet.http package provide the classes and interfaces to define servlets. HTML servlet classes extend the javax.servlet.http.HttpServlet abstract class, which provides a framework for handling HTTP protocol. Because HttpServlet is abstract your servlet must extend it and override at least one of the following methods:

1. doGetget information such as a document, the results of a database query, or strings passed from the browser.
2. doPost posts information such as data to be stored in a database, user login and password information, or other strings passed from the browser.
3. doPutplaces documents directly on the server.
doDelete deletes information or documents from the server.
4. getServletInfo returns descriptive information about the servlet, possibly its purpose, author, or version number.

These methods are the heart of the servlet, where instructions and the purpose of the servlet are carried out. You will likely only need to use, or override, a few of the methods. RedirectServlet overrides doGet and doPost, but does not need any of the other methods.

public class Example extends HttpServlet {
public void doGet ( HttpServletRequest req,
HttpServletResponse res)
When a client calls a servlet by typing the URL in the browser, submitting a form, or clicking a button on a menu, the servlet's service method checks the HTTP request type, such as POST or GET. This in turn calls doGet, doPOST, doPUT, or doDelete as needed.

You can override the service method without implementing doGet and doPost, but it's generally better to call both doGet and doPost. See the JDC RedirectServlet.

The doPost and doGet Methods

The doPost or doGet methods instruct the server about what it must do, whether printing information back to the client's browser, writing to a database, or simply redirecting the client to a requested URL. Within these methods you will use Java programming syntax to give specific instructions for the servlet to interact between the client and the server.

Servlets to Process Forms

A form is a powerful web site tool, enabling clients to take polls, enter personal information for online shopping, or subscribe to an e-newsletter. In other words, forms turn static web pages into dynamic pages. But a form cannot give instructions to a server. Without an application between the form and the server, the form is useless.

Servlets process form information in three steps:

1. Retrieve or request the data
2. Store or passing the data
3. Respond to the request

RedirectServlet Servlet

As with most servlets, the JDC RedirectServlet imports the following packages:

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

Servlets extend the abstract HttpServlet class, whichextends the GenericServlet base class.

public class RedirectServlet extends HttpServlet {
So the servlet collects the parameter NAME and corresponding value as a String pair, the String object must be declared:

private String paramName;

To initialized the servlet with the NAME and value String pair, the init method is overridden, and the ServletConfig object is passed as a parameter. Because errors can occur, such as a bad URL, the throws ServletException is included.

public void init(ServletConfig config)
throws ServletException {

When overriding the init method, call super.init(config). After the call to super.init(config), the servlet can invoke its own getInitParameter method as shown.The getInitParameter method returns a string containing the value of the named initialization parameter, or null if the requested parameter does not exist. Init parameters have a single string value.

super.init(config);
paramName =
config.getInitParameter("paramName");
}

A servlet must override doGet or doPost, depending on whether data is sent by POST or GET in the HTML form. The drawback to overriding only one of these methods is that if production changes the HTML form to call POST instead of GET, the servlet won't work. Generally, it is better to override both methods, as shown below.

RedirectServlet overrides the doGet method and takes two arguments:

HttpServletRequest, with the variable req

HttpServletRequest has useful methods such as getParameter that takes the value of a NAME attribute as a string or null if the parameter does not exist.

HttpServletResponse with the variable res has methods that let you specify outgoing information, such as getWriter, which returns a print writer for writing formatted text responses to the browser, or in this case sendRedirect, which sends a temporary redirect response to the client using the specified redirect location URL.

public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.sendRedirect(
request.getParameter(paramName));

The doGet method calls the getParameter method through the request object, passing in the paramName object and its value as a string pair, in this case url and the actual URL. A special sendRedirect method then passes the string to the browser and redirects the user to the desired destination.

The RedirectServlet also forces doPost to call doGet, making it possible to use GET or POST in the HTML form without breaking the servlet.


}
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException{
doGet(request, response);
}
}

As long as the value is not null, the code moves onto the res object and its sendRedirect method, which redirects the client to the URL that was passed to the method.This makes for smooth navigation when a client's browsers cannot support JavaScript, or the user has JavaScript toggled off.

Web site design issues frequently pose challenges because of browser incompatibility, user preferences, and non-browser problems, such as sending information to a database or email address. Servlets serve functionality in these situations, acting as a messenger between HTML pages and the server, or as back-up for other technologies.

RedirectServlet is a short yet reliable servlet that works as back-up to JavaScript, and it doesn't need to deal with browser compatibility, or other client related issues because it works on the server side.