Servlet Attribute Listener, Modifying the condition of a particular instance is known as an event. Servlet API(Application Program interface)2.4 onwards the event handling was introduced. There are many event classes and their corresponding listeners where each class will have own functionality, all the classes and listeners will available in javax.servlet, javax.servlet.http packeges. ServletContextAttributeEvent class let’s programmer know if an attribute in a web application context has been added, removed or replaced. Implementation of ServletContextAttributeListener interface, receive notifications of changes to the attribute list on the ServletContext of a web application.
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On HttpSessionAttribute object

Following are the methods of ServletContextAttributeListener.

Servlet Attribute Listener – Following is the way to write the code by using attribute listener.

MyAttributeListener.java

package attribute;

import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

public class MyAttributeListener implements HttpSessionAttributeListener {

	@Override
	public void attributeAdded(HttpSessionBindingEvent event) {
		String attributeName = event.getName();
		Object attributeValue = event.getValue();
		System.out.println("Attribute added : " + attributeName + " : " + attributeValue);
	}

	@Override
	public void attributeRemoved(HttpSessionBindingEvent event) {
		String attributeName = event.getName();
		Object attributeValue = event.getValue();
		System.out.println("Attribute removed : " + attributeName + " : " + attributeValue);
	}

	@Override
	public void attributeReplaced(HttpSessionBindingEvent event) {
		String attributeName = event.getName();
		Object attributeValue = event.getValue();
		System.out.println("Attribute replaced : " + attributeName + " : " + attributeValue);	
	}
}

web.xml

<web-app >
        <listener>
		<listener-class>attribute.MyAttributeListener</listener-class>
	</listener>
</web-app>

The servlet incorporates the ability to track events in Web applications through listeners. This usefulness permits more effective resource management that depends on the event status status.

The listeners are resemble triggers that can be connected to evenets in application server. With listeners client can track application-level, session-level, life-cycle changes, characteristic changes and so on. The executed interfaces are javax.servlet.Listener interface.

• Servlet Attribute Listener – While working with events tag should be added in web.xml
• Servlet Attribute Listener – Through WAR file source code file needs to be deploy in Tomcat.