JAVA: Dispatching Requests

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Web Design & Development CS506

Lesson 30

Dispatching Requests

In this handout we will start with request dispatching techniques and give some examples related to that.

Further more some methods of HttpResponse and HttpRequestwill also be discussed. Finally, this handout

will be concluded by discussing the importance of session tracking. Before starting, let's take a look at the

summery of the previous lecture.

Recap

In the previous lecture we had some discussion about Response Redirection and Request Dispatcher. We

said that Response Redirection was used to redirect response of the Servlet to another application resource.

This resource might be another Servlet or any JSP page.

Two forms of Response redirection were discussed. These were:

. Sending a standard request:

Using response.sendRedirect("path of resource") method, a new request is generated which

redirects the user to the given URL. If the URL is of another servlet, that second servlet will not be

able to access the original request object.

. Redirection to an error page:

An error code is passed as a parameter along with message to response.sendError(int, msg)

method. This method redirects the user to the particular error page in case of occurrence of

specified error.

Similarly request dispatching provides us the facility to forward the request processing to another servlet,

or to include the output of another resource (servlet, JSP or HTML etc) in the response. Unlike Response

Redirection, request object of calling resource is available to called resource. The two ways of Request

Dispatching are:

. Forward:

Forwards the responsibility of request processing to another resource.

. Include:

Allows a servlet to include the results of another resource in its response. So unlike forward, the

first servlet to receive the request is the one which finishes the response.

Example Code: Request Dispatching Include

Lets start with the example of include. We will see how a Servlet includes the output of another resource in

its response. The following example includes a calling Servlet MyServlet and Servlet IncludeServlet,

who's output will be included in the calling Servlet.

The code of MyServlet.java servlet is given below.

MyServlet.java

import java.io.*;

import java.net.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class MyServlet extends HttpServlet {

/* this method is being called by both doGet() and doPost().We

usually follow this practice, when we are not sure about the

type of incoming request to the servlet. So the actual

processing is being done in the processRequest().

protected void processRequest(HttpServletRequest request, HttpServletResponse response)

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throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println("<html>");

out.println("<body>");

out.println("<h1>Start of include request </h1>");

out.flush();

// getting the object of ServletContext, that will be used to

// obtain the object of RequestDispacther

ServletContext context = getServletContext();

// getting the object of RequestDispatcher by passing the path

// of included resource as a parameter

RequestDispatcher rd = context.getRequestDispatcher("/includeservlet");

// calling include method of RequestDispatcher by passing

// request and response objects as parameters. This will execute

//the second servlet and include its output in the first servlet

rd.include(request, response);

/* the statements below will be executed after including the

output of the /includeservlet */

out.println("<h1>End of

out.println("</body>");

out.println("</html>");

// closing PrintWriter stream

out.close();

}

// This method only calls processRequest()

protected void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

// This method only calls processRequest()

protected void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

} // end MyServlet

Include Servlet

Now lets take a look at the code of IncludeServlet.java

import java.io.*;

import java.net.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class IncludeServlet extends HttpServlet {

// this method is being called by both doGet() and doPost()

protected void processRequest(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

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// Obtaining the object of PrintWriter, this will return the

// same PrintWriter object we have in MyServlet

PrintWriter out = response.getWriter();

// Including a HTML tag using PrintWriter

out.println("<h1> <marquee>I am included </marquee></h1>");

}

protected void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

protected void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

}

} // end IncludeServlet

In the processRequest(), firstly we get the PrintWriter stream from the HttpServletResponse object. Then

we include an HTML tag to the output of the calling servlet. One thing that must be considered is that

PrintWriter stream is not closed in the end, because it is the same stream that is being used in the calling

servlet and this stream may also be used in the calling servlet again. So, if it is closed over here, it can not

be used again in the calling servlet.

web.xml

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

<web-app>

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

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

</servlet>

<servlet-name>IncludeServlet</servlet-name>

<servlet-class>IncludeServlet</servlet-class>

</servlet>

<servlet-mapping>

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

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

</servlet-mapping>

<servlet-mapping>

<servlet-name>IncludeServlet</servlet-name>

<url-pattern>/includeservlet</url-pattern>

</servlet-mapping>

</web-app>

Code Example: Request Dispatcher - forward

As discussed earlier, we can forward the request processing to another resource using forward method of

request dispatcher. In this example, the user enters his/her name and salary on the index.html and submits

the form to FirstServlet, which calculates the tax on salary and forwards the request to another servlet for

further processing i.e. SecondServlet.

index.html

<html>

<body>

<h2> Enter your name</h2>

<br/>

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<h2> Salary</h2>

</form>

</html>

FirstServlet.java

import java.io.*;

import java.net.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class FirstServlet extends HttpServlet {

// this method is being called by both doGet() and doPost()

protected void processRequest(HttpServletRequest request, HttpServletResponse response)

sthrows ServletException, IOException {

// getting value of salary text filed of the HTML form

String salary = request.getParameter("salary");

// converting it to the integer.

int sal = Integer.parseInt(salary);

// calculating 15% tax

int tax = (int)(sal * 0.15);

// converting tax into string

String taxValue = tax + "";

// request object can store values in key-value form, later it

// can be retrieved by using getAttribute() method

request.setAttribute("tax", taxValue);

// getting object of servletContext

ServletContext sContext = getServletContext();

// getting object of request dispatcher

RequestDispatcher rd = sContext.getRequestDispatcher("/secondservlet");

// calling forward method of request dispatcher

rd.forward(request, response);

}

// This method is calling processRequest()

protected void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

// This method is calling processRequest()

protected void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

Note:

It the case of Forward, it is illegal to make the reference of PrintWriter stream in the calling

Servlet. Only the called resource can use PrintWriter stream to generate response

SecondServlet.java

import java.io.*;

import java.net.*;

import javax.servlet.*;

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import javax.servlet.http.*;

public class SecondServlet extends HttpServlet {

// this method is being called by both doGet() and doPost()

protected void processRequest(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

// obtaining values of name and salary text fields of index.html

String name = request.getParameter("name");

String salary = request.getParameter("salary");

/* getting attribute value that has been set by the calling

servlet i.e. FirstServlet */

String tax = (String)request.getAttribute("tax");

// generating HTML tags using PrintWriter

out.println("<html>");

out.println("<head>");

out.println("<title>SecondServlet</title>");

out.println("</head>");

out.println("<body>");

out.println("<h1> Welcome " + name+ "</h1>");

out.println("<h3> Salary " + salary+ "</h3>");

out.println("<h3> Tax " + tax+ "</h3>");

out.println("</body>");

out.println("</html>");

out.close();

}

// This method is calling processRequest()

protected void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

// This method is calling processRequest()

protected void doPost(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

processRequest(request, response);

}

web.xml

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

<web-app>

<servlet-name>FirstServlet</servlet-name>

<servlet-class>FirstServlet</servlet-class>

</servlet>

<servlet-name>SecondServlet</servlet-name>

<servlet-class>SecondServlet</servlet-class>

</servlet>

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<servlet-mapping>

<servlet-name>FirstServlet</servlet-name>

<url-pattern>/firstservlet</url-pattern>

</servlet-mapping>

<servlet-mapping>

<servlet-name>SecondServlet</servlet-name>

<url-pattern>/secondservlet</url-pattern>

</servlet-mapping>

</web-app>

HttpServletRequest Methods

Let's discuss some methods of HttpServletRequestclass

setAttribute(String, Object)

We can put any object to the context using setAttribute() method in the key-value pair form.. These

attributes are also set or reset between requests. These are often used in conjunction with Request

Dispatcher. This has also been illustrated in the above example. These attributes are available every where

in the same web application so that any other Servlet or JSP resource can access them by using

getAttribute() method.

getAttribute(String)

The objects set by the setAttribute() method can be accessed using getAttribute() method. Passing the key

in the form of string as a parameter to this method will return the object associated with that particular key

in the context. Cast the object into its appropriate type.

getMethod()

This method returns the name of HTTP method which was used to send the request. The two possible

returning values could be, get or post.

getRequestURL()

It can be used to track the source of Request. It returns the part of the request's URL with out query string.

getProtocol()

It returns the name and version of the protocol used.

getHeaderNames()

It returns the enumeration of all available header names that are contained in the request.

getHearderName()

It takes a String parameter that represents the header name and returns that appropriate header. Null value

is returned if there is no header exists with the specified name.

HttpServletResponse Methods

Let's discuss some methods of HttpServletResponse class

setContentType()

Almost every Servlet uses this header. It is used before getting the PrintWriter Stream. It is used to set the

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Content Type that the PrintWriter is going to use. Usually we set "text/html", when we want to send text

output or generate HTML tags on the client's browser.

setContentLength(

)

This method is used to set the content length. It takes length as an integer parameter.

addCookie()

This method is used to add a value to the Set-Cookie header. It takes a Cookie object as a parameter and

adds it to the Cookie-header. We will talk more about Cookies in the session tracking part.

sendRedirect()

This method redirects the user to the specific URL. This method also accepts the relative URL. It takes

URL string as parameter and redirects the user to that resource.

SESSION TRACKING

Many applications require a series of requests from the same client to be associated with one another. For

example, any online shopping application saves the state of a user's shopping cart across multiple requests.

Web-based applications are responsible for maintaining such state, because HTTP protocol is stateless. To

support applications that need to maintain state, Java Servlet technology provides an API for managing

sessions and allows several mechanisms for implementing sessions.

Before looking inside the session tracking mechanism lets see the limitation of HTTP protocol to get the

real picture of problems that can happen with out maintaining the session.

Continuity problem- user's point of view

Suppose a user logs on to the online bookshop, selects so

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Continuity problem- Server's point of view

The server has a very different point of view. It considers each request independent from other even if the

requests are made by the same client.

References

Java A Lab Course by Umair Javed

Core Servlet and JSP by Marty Hall

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