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ServletRequestAttributeListener



In this article, we are going to understand how to create a listener class which listens to the event of adding, replace or removing of ServletRequest attribute in the web application. For this, we are going to create a webpage which asks the user to enter his name and password in a form and click the submit button.

On submitting the form, the user request goes to the requested Servlet class where the operations such adding, replace or removing of ServletRequest attribute generates the events and calls the listener class, listening to an such events.

To do this, we are going to create a Listener class by implementing ServletRequestAttributeListener interface and by providing implementation of its methods.

ServletRequestAttributeListener is a part of the java.util.* package.




ServletRequestAttributeListener methods


Let's take a look at the methods of ServletRequestAttributeListener interface.

Methods Description
void attributeAdded(ServletRequestAttributeEvent srae) This method receives notification about an attribute has been to added to the ServletRequest.
void attributeReplaced(ServletRequestAttributeEvent srae) This method receives notification about an attribute has been replaced in the ServletRequest.
void attributeRemoved(ServletRequestAttributeEvent srae) This method receives notification about an attribute has been deleted from the ServletRequest.





Creating a webpage which calls the Servlet


First we execute the webpage containing the form, asking the user to enter his/her name and city and press submit button. On submitting the form, the user request goes to the Servlet class, where the operations of adding, replace and removing the ServletRequest attributes will generate the events which will eventually call the listener class, listening to the such events.

Form1.jsp
<html>

<head>
<title> ServletRequest Demo </title>
</head>


<body>
Please enter your details :

<form action = "MyServlet">
Name : <input type = "text"   name = "username" />
City : <input type = "text"   name = "cityname" /> <br/>


<input type = "submit" value = "submit" />
</form>

</body>
</html>





Creating a listener class to listen to an event of user request




RequestAttributeListener1.java
import java.io.*;
import javax.servlet.*;
import java.util.*;


public class RequestAttributeListener1 implements ServletRequestAttributeListener
{

public void attributeAdded(ServletRequestAttributeEvent srae)
{	    
	System.out.println("A new request attribute is added");
	System.out.println("Name of this attribute " + srae.getName());
	System.out.println("Value of this attribute " + srae.getValue());
	System.out.println();
	System.out.println();
}



public void attributeRemoved(ServletRequestAttributeEvent srae)
{

	System.out.println("A request attribute is removed");
	System.out.println("Name of this attribute " + srae.getName());
	System.out.println("Value of this attribute " + srae.getValue());
	System.out.println();
	System.out.println();

}


public void attributeReplaced(ServletRequestAttributeEvent srae)
{

	System.out.println("A request attribute is replaced");
	System.out.println("Name of this attribute " + srae.getName());
	System.out.println("Value of this attribute " + srae.getValue());
	System.out.println();
	System.out.println();

}

}



Directory Structure of Servlet files




The diagram above depicts how to arrange the Servlet files in a specific directory structure, as per Java Servlet Specification-






Creating the Deployment Descriptor file


As per the Java Servlet specifications, every web application based on Servlet must have a Deployment Descriptor file(an XML file) named web.xml. So, let's create one -


web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
  version="4.0"
  metadata-complete="true">

  <display-name>Welcome tomcat</display-name>
  <description>
     Welcome tomcat
  </description>

  
<listener>
	 <listener-class>RequestAttributeListener1 </listener-class>
</listener>



<servlet>
 	<servlet-name>MyServlet</servlet-name>
	<servlet-class>MyServlet1</servlet-class>
</servlet>



<servlet-mapping>
	<servlet-name>MyServlet</servlet-name>
	<url-pattern>/Serv</url-pattern>
</servlet-mapping>

</web-app>


In deployment descriptor file, <listener> tag has a child tag <listener class> :




<servlet> tag has two child tags <servlet-name> and <servlet-class> :






Note


The child tag <servlet-name> of <servlet> tag is matched with the <servlet-name> child tag of <servlet-mapping>. The <url-pattern> child tag is used to specify the URL to access Servlet and we have named this URL MyServlet.




Setting the classpath


Much of the support for developing the web applications based on the Java Servlet technology does not come with the core Java. Hence, in order to compile the Servlet programs, we have to set the classpath to a jar file named servlet-api.jar.

This jar file provides all the classes that are required for the Servlet programming and it comes within the lib Folder of Tomcat installation folder.

For example, in our case we have installed Tomcat Web Server within the C: Drive, hence the path to our lib folder containing the servlet-api.jar is - C:\apache-tomcat-9.0.2\lib

There are two ways to set the classpath -




Compiling the Servlet class


After setting the classpath, you need to compile the Servlet class and the Listener class by entering the command at the folder where you've stored the Servlet and the Listener class file.

javac -d WEB-INF/classes MyServlet1.java


javac -d WEB-INF/classes RequestAttributeListener1.java





Executing the Servlet


First we execute the webpage containing the form, asking the user to enter the name and city and press the submit button.





As the user submits the form, the user request goes to the Servlet class, where the operations of adding, replace and removing the request attributes will generate the events which will eventually call the listener class(RequestAttributeListener1.java), listening to the such events.

Hence, you will see the Servlet class in execution and displaying the window with the following message, asking the user to check the Tomcat Server Window to check details about the events generated in the web application.






And, eventually the listener class displays the information on the Tomcat Web Server window about the events i.e. adding, replace and removing the request attributes generated in the web application.






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