JAVA: More on Servlets

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Web Design & Development CS506

Lesson 29

More on Servlets

The objective of this handout is to learn about the use and implementation of initialization parameters for a

Servlet. Moreover different ways of redirecting response and forwarding or including requests also

discussed in detail.

Initialization Parameters

Some times at the time of starting up the application we need to provide some initial information e,g, name

of the file where server can store logging information, DSN for database etc. Initial configuration can be

defined for a Servlet by defining some string parameters in web.xml. This allows a Servlet to have initial

parameters from outside. This is similar to providing command line parameters to a standard console based

application.

Example: setting init parameters in web.xml

Let's have a look on the way of defining these parameters in web.xml

<init-param> //defining param 1

<param-name> param1 </param-name>

<param-value> value1 </param-value>

</init-param>

<init-param> //defining param 2

<param-name> param2 </param-name>

<param-value> value2 </param-value>

In the above code, it is shown that for each parameter we need to define separate <initparam> tag that have

two sub tags <param-name> and <param-value>, which contain the name and values of the parameter

respectively.

ServletConfig

Every Servlet has an object called ServletConfig associated with it as shown in the fig. below. It contains

relevant information about the Servlet like initialization parameters defined in web.xml

Reading Initialization Parameters

Now let's see, how we can access init parameters inside the Servlet. The method getInitParameter()of

ServletConfig is usually used to access init parameters. It takes a String as parameter, matches it with

<param-name> tag under all <init-param> tags and returns <param-value> from the web.xml

One way is to override init() method as shown in the code below. The ServletConfig object can then be

used to read initialization parameter.

public void init(ServletConfig config) throws ServletException {

String name = config.getInitParameter("paramName");

}

Another way to read initialization parameters out side the init () method is

Call getServletConfig() to obtain the ServletConfig object

Use getInitParameter() of ServletConfig to read initialization parameters

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public void anyMethod() // defined inside servlet{

ServletConfig config = getServletConfig();

String name = config.getInitParameter("param_name");

}

Example Code: Reading init parameters

MyServlet.java will read the init parameter (log file name) defined inside web.xml. The code is given

below:

import java.io.*;

import java.net.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class MyServlet extends HttpServlet {

// attribute used to store init-parameter value

String fileName;

// overriding init() method

public void init(ServletConfig config) throws ServletException{

super.init(config);

// reading init-parameter "logfilename" stored in web.xmlfileName =

config.getInitParameter("logfilename");

}

/* Both doGet() & doPost() methods are override over here.processRequest() is called from both these

methods. This makespossible for a servlet to handle both POST and GET requestsidentically.

// Handles the HTTP GET request type

protected void doGet(HttpServletRequest request,

HttpServletResponse response)throws ServletException, IOException{

processRequest(request, response);

}

// Handles the HTTP POST request type

protected void doPost(HttpServletRequest request,

HttpServletResponse response)throws ServletException, IOException{

processRequest(request, response);

}

// called from doGet() & doPost()

protected void processRequest(HttpServletRequest request,

HttpServletResponse response)throws ServletException, IOException{

PrintWriter out = response.getWriter();

// writing init-parameter value that is store in fileNameout.println(fileName);

out.close();

}

web.xml

<?xml version="1.0" encoding="UTF-8"?><web-app>

<servlet> <servlet-name> MyServlet </servlet-name> <servlet-class> MyServlet </servlet-class>

<init-param> <param-name> logfilename </param-name> <param-value> logoutput.txt

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</param-value>

</init-param>

</servlet>

<servlet-mapping>

<servlet-name> MyServlet </servlet-name>

<url-pattern> /myservlet </url-pattern>

</servlet-mapping>

</web-app>

Response Redirection

We can redirect the response of the Servlet to another application resource (another Servlet, an HTML

page or a JSP) but the resource (URL) must be available to the calling Servlet, in the same Servlet

context (discussed later).

There are two forms of response redirection that can be possible:

Sending a standard redirect

Sending a redirect to an error page

Sending a standard Redirect

Using response.sendRedirect("myHtml.html") method, a new request is generated which redirects

the user to the specified URL.

If the URL is of another Servlet, that second Servlet will not have access to the original request

object. For example, if the request is redirected from servlet1 to servlet2, then servlet2 would not

be able to access the request object of servlet1.

To have access to the original request object, you must use the request dispatching technique

(discussed later) instead of redirect.

Sending a redirect to an error page

Instead of using response.sendRedirect(), we can use response.sendEorror() to show user an error page.

This methods takes two parameters, first the error number that is a predefined constant of the response class

(listed below) and second the appropriate error message. The steps to redirect the user to an error page are:

An error code is sent as a parameter of response.sendError(int, msg)method

The error page is displayed with the msg passed to method

The error numbers are predefined constants of the HttpServletResponse class. For example:

-SC_NOT_FOUND (404)

-SC_NO_CONTENT (204)

-SC_REQUEST_TIMEOUT (408)

Example Code: Response Redirection

The example given below demonstrates a typical sign on example in which a user is asked to provide

This example consists of login.html, welcome.html, register.html and MyServlet.java files. Let's examine

these one after another.

login.html

This page contains two text fields; one for entering username and another for password. The data from this

page is submitted to MyServlet.java.

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<html>

<body>

<h2> Please provide login details</h2>

<FROM METHOD="POST"

ACTION="http://localhost:8084/redirectionex/myservlet"NAME="myForm" >

<BR> User Id:

<INPUT TYPE="text" name="userid"/>

<BR> Password:

<INPUT TYPE="password" name="pwd"/>

</form>

</body>

</html>

welcome.html

The user is directed to this page only if user provides correct login / password. This page only displays a

successfully logged-in message to the user.

<html>

<body>

<h2> You have successfully logged in </h2> </body>

</body>

</html>

register.html

The user is redirected to this page in case of providing incorrect login/password information. The user can

enter user id, address and phone number here to register.

Note: The code given below will only show fields to the user. It does not register user as no such

functionality is added into this small example.

<html>

<body>

<h2>Your login is incorrect. Please register yourself</h2>

<BR> Name:

<BR> Address:

<BR> Phone No:

</form>

</body>

</html>

MyServle.java

MyServlet.java accepts requests from login.html and redirects the user to welcome.html or register.html

based on the verification of username & password provided. Username & password are compared with fix

values in this example, however you can verify these from database or from a text file etc.

import java.io.*;

import java.net.*;

import javax.servlet.*;

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import javax.servlet.http.*;

public class MyServlet extends HttpServlet {

// Handles the HTTP GET request type

protected void doGet(HttpServletRequest request, HttpServletResponse response)throws

ServletException, IOException{

processRequest(request, response);

}

// Handles the HTTP POST request type

protected void doPost(HttpServletRequest request, HttpServletResponse response)throws

ServletException, IOException{

processRequest(request, response);

}

protected void processRequest(HttpServletRequest request, HttpServletResponse response)throws

ServletException, IOException{

String id = request.getParameter("userid");String pwd = request.getParameter("pwd");

// comparing id & password with fix values

if(id.equals("ali") && pwd.equals("vu")) {

// redirectign user to welcome.html

response.sendRedirect("welcome.html");

else {// redirecting user to register.htmlresponse.sendRedirect("register.html");

/* if you want to display an error message to theuser, you can use the following method

response.sendError(response.SC_PROXY_AUTHENTICATION_REQUIRED,"Send

Error Demo" );*/

}// end else

}

ServletContext

ServletContext belongs to one web application. Therefore it can be used for sharing resources among

servlets in the same web application.

As initialization parameters, for a single servlet are stored in ServletConfig, ServetContext can store

initialization parameters for the entire web application. These parameters are also called context attributes

and exist for the lifetime of the application. The following figure illustrates the sharing of context attributes

among all the servlets of a web application.

Note:

There is a single ServletContextper web application

Different Sevlets will get the same ServletContext object, when calling getServletContext()during

different sessions

Request Dispatcher

RequestDispatcher provides a way to forward or include data from another source. The method

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getRequestDispatcher(String path) of ServletContext returns a RequestDispatcher object associated with

the resource at the given path passed as a parameter.

Two important methods of RequestDispatcher are:

forward(ServletRequest req, ServletResponse resp)

include(ServletRequest req, ServletResponse resp)

RequestDispatcher: forward

Characteristics of forward methods are:

It allows a Servlet to forward the request to another resource (Servlet, JSP or HTML file) in the

same Servlet context.

Forwarding remains transparent to the client unlike res.sendRedirect(String location). You can not

see the changes in the URL.

Request Object is available to the called resource. In other words, it remains in scope.

Before forwarding request to another source, headers or status codes can be set, but

output content cannot be added.

To clarify the concepts, lets take the help from following figure. User initates the request to servlet1.

servlet1 forwards the request to servlet2 by calling forward(request,response). Finally a response is

returned back to the user by servlet2.

RequestDispatcher: include

It allows a Servlet to include the results of another resource in its response. The two major differences from

forward are:

Data can be written to the response before an include

The first Servlet which receive the request, is the one which finishes the response

It will be more cleared from the following figure. User sends a HTTPRequest to Servlet1. Serlet2 is called

by Servlet1 by using include(request, response) method. The response generated by Servlet2 sends back to

Servlet1. Servlet1 can also add its own response content and finally send it back to user.

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References:

Java A Lab Course by Umair Javed

Core Servlets and JSP by Marty Hall

Java API documentation

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