Sunday, March 18, 2012

SERVELETS & COOKIES


SERVELETS & COOKIES

Reading Servlet Parameters
The ServletRequest class includes methods that allow you to read the names and values of parameters that are included in a client request. We will develop a servlet that illustrates their use. The example contains two files: PostParameters.htm defines a Web page, and PostParametersServlet.java defines a servlet. The HTML source code for PostParameters.htm is shown in the following listing. It defines a table that contains two labels and two text fields. One of the labels is Employee and the other is Phone. The form also includes a submit button. Notice that the action parameter of the form tag specifies a URL. The URL identifies the servlet to process the HTTP POST request.

<html>
<body>
<center>
<form name="Form1"
method="post"
action="http://localhost:8080/servlet/PostParametersServlet">
<table>
<tr>
<td><B>Employee</td>
<td><input type=textbox name="e" size="25" value=""></td>
</tr>
<tr>
<td><B>Phone</td>
<td><input type=textbox name="p" size="25" value=""></td>
</tr>
</table>
<input type=submit value="Submit">
</body>
</html>

The source code for PostParametersServlet.java is shown in the following listing.
The service( ) method is overridden to process client requests. The getParameterNames( ) method returns an enumeration of the parameter names. These are processed in a loop. You can see that the parameter name and value are output to the client. The parameter value is obtained via the getParameter( ) method.

import java.io.*;
import java.util.*;
import javax.servlet.*;
public class PostParametersServlet
extends GenericServlet {
public void service(ServletRequest request,
ServletResponse response)
throws ServletException, IOException {
// Get print writer
PrintWriter pw = response.getWriter();
// Get enumeration of parameter names
Enumeration e = request.getParameterNames();
// Display parameter names and values
while(e.hasMoreElements()) {
String pname = (String)e.nextElement();
pw.print(pname + " = ");
String pvalue = request.getParameter(pname);
pw.println(pvalue);
}
pw.close();
}
}



The example contains two files: ColorGet.htm defines a Web page, and
ColorGetServlet.java defines a servlet. The HTML source code for ColorGet.htm is
shown in the following listing. It defines a form that contains a select element and a
submit button. Notice that the action parameter of the form tag specifies a URL. The URL identifies a servlet to process the HTTP GET request.

<body>
<center>
<form name="Form1"
action="http://localhost:8080/servlet/ColorGetServlet">
<B>Color:</B>
<select name="color" size="1">
<option value="Red">Red</option>
<option value="Green">Green</option>
<option value="Blue">Blue</option>
</select>
<br><br>
<input type=submit value="Submit">
</form>
</body>
</html>

method is overridden to process any HTTP GET requests that are sent to this servlet. It uses the getParameter( ) method of HttpServletRequest to obtain the selection that was made by the user. A response is then formulated.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ColorGetServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
String color = request.getParameter("color");
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>The selected color is: ");
pw.println(color);
pw.close();



The HTML source code for ColorPost.htm is shown in the following listing. It is identical to ColorGet.htm except that the method parameter for the form tag explicitly specifies that the POST method should be used, and the action parameter for the form tag specifies a different servlet.

<html>
<body>
<center>
<form name="Form1"
method="post"
action="http://localhost:8080/servlet/ColorPostServlet">
<B>Color:</B>
<select name="color" size="1">
<option value="Red">Red</option>
<option value="Green">Green</option>
<option value="Blue">Blue</option>
</select>
<br><br>
<input type=submit value="Submit">
</form>
</body>
</html>

The source code for ColorPostServlet.java is shown in the following listing. The
doPost( ) method is overridden to process any HTTP POST requests that are sent to this servlet. It uses the getParameter( ) method of HttpServletRequest to obtain the selection that was made by the user. A response is then formulated.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ColorPostServlet extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
String color = request.getParameter("color");
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>The selected color is: ");
pw.println(color);
pw.close();
}
}

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