The @WebListener annotation is used to annotate a listener to get events for various operations on the particular web application context.

Below diagram shows a class diagram od @WebListener annotation which contains a single attribute value:

The classes annotated with @WebListener must implement one of the following interfaces:

javax.servlet.ServletContextListener
javax.servlet.ServletContextAttributeListener
javax.servlet.ServletRequestListener
javax.servlet.ServletRequestAttributeListener
javax.servlet.http.HttpSessionListener
javax.servlet.http.HttpSessionAttributeListener
javax.servlet.http.HttpSessionIdListener

Let's add servlet 4.0.1 dependency to pom.xml:

<!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
            <scope>provided</scope>
        </dependency>

@WebListener Annotation Example

The following example code uses the @WebListener annotation to register a class as a listener for the ServletContextListener’s events:

package net.javaguides.servlet.tutorial.listener;

import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

/**
 * Web application lifecycle listener.
 */
@WebListener
public class SimpleServletListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("Context initialized");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("Context destroyed");
    }
}

The following example code registers a listener which implements two interfaces with description:

package net.javaguides.servlet.tutorial.listener;

import java.util.logging.Level;
import java.util.logging.Logger;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

/**
 * Web application lifecycle listener.
 */
@WebListener()
public class SimpleServletListener implements ServletContextListener, ServletContextAttributeListener {

    static final Logger log = Logger.getLogger(SimpleServletListener.class);

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("Context initialized");
        log.info("Context initialized");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("Context destroyed");
        log.info("Context destroyed");
    }

    @Override
    public void attributeAdded(ServletContextAttributeEvent event) {
        log.log(Level.INFO, "Attribute {0} has been added, with value: {1}",
            new Object[] {
                event.getName(), event.getValue()
            });
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent event) {
        log.log(Level.INFO, "Attribute {0} has been removed", event.getName());
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent event) {
        log.log(Level.INFO, "Attribute {0} has been replaced, with value: {1}",
            new Object[] {
                event.getName(), event.getValue()
            });
    }
}