Servlet - Context Event and Context Listener

ServletContextEvent and ServletContextListener are used to monitor and handle important activities occurring inside a web application such as application startup, context initialization, and application shutdown. They allow developers to execute custom logic automatically whenever a specific event occurs in the servlet container.

ServletContextEvent represents changes or actions occurring in the ServletContext.
ServletContextListener is a special class that automatically responds to these context events.
They are commonly used for database initialization, logging, resource management, and application-level tracking.

Event Categories

Servlet Context events are mainly divided into two categories:

1. Context Lifecycle Events

These events occur when the web application is started or stopped.

contextInitialized -> triggered when application starts
contextDestroyed -> triggered when application stops

2. Context Attribute Events

These events occur when attributes are added, removed, or modified in ServletContext.

attributeAdded
attributeRemoved
attributeReplaced

Constructor of ServletContextEvent Class

ServletContextEvent contains only one constructor. The web container automatically creates an instance of this class when the ServletContext is initialized during application startup.

Syntax:

ServletContextEvent(ServletContext sc)

Method of ServletContextEvent

ServletContextEvent class has only one method:

ServletContext getServletContext()

This method is used to access the ServletContext instance that is associated with the current event. It helps the listener to get application-level data and perform operations based on context changes.

Syntax:

ServletContext ctx = sce.getServletContext();

Methods of ServletContextListener Interface

Method	Action Performed
void contextInitialized(ServletContextEvent e)	Called when application is initialized
void contextDestroyed(ServletContextEvent e)	Called when application is destroyed

Working of Servlet Context Event and Listener

The client sends a request to the web application.
The servlet container generates a context-related event (startup/shutdown).
The registered listener detects the event automatically.
The listener executes the corresponding method.
The servlet continues processing normally.

Common Servlet Context Listener Interfaces

Listener Interface	Description
ServletContextListener	Handles application startup and shutdown
ServletContextAttributeListener	Handles context attribute changes
ServletRequestListener	Handles request lifecycle events
HttpSessionListener	Handles session lifecycle events

How To Configure Context Listener

1. Create Listener Class

Create a class that implements ServletContextListener.

public class MyListenerGfg implements ServletContextListener {
// methods
}

2. Configure Listener in web.xml

In this step, the listener class is registered in web.xml so the container can automatically detect and execute it during application lifecycle events.

<listener>
<listener-class>MyListenerGfg</listener-class>
</listener>

Steps to Implements a ServletContextEvent and ServletContextListener

In this example, we create a page view counter application. The page view count is stored in a database table named counter, and updated using ServletContext and Listener.

Step 1: Create Database Table

First, create a table in MySQL to store page views by executing these query and This table stores total page visits.

CREATE TABLE counter (
pageview INT
);
INSERT INTO counter VALUES (0);

Step 2: Create Dynamic Web Project

Open Eclipse IDE
Create a project: ServletFilterchain
Select: Dynamic Web Project
Click Finish

Step 3: Create index.html

This HTML page contains a link that calls the servlet when clicked.

<a href="CounterGfg">Total Page views GeeksforGeeks</a>

Step 4: Configure web.xml

The listener is registered to handle application events, and servlet mapping is defined for URL access.

Step 5: Create Listener Class (MyListenerGfg.java)

This class contains a method contextInitialized() which runs when the server starts. It connects to the database and loads the page view count into application memory.

Java

import java.sql.*;
import javax.servlet.*;

// Class
public class MyListenerGfg implements ServletContextListener {
    // Class data members
    ServletContext ctx;
    Connection con;
    Statement s;
    PreparedStatement ps;
    ResultSet rs;
    int count;

    // Method 1
    public void contextInitialized(ServletContextEvent sce) {

        // Try block to check for exceptions 
        try {

            // Loading drivers using forName() method 
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/geeksforgeeks", "root", "root");

            s = con.createStatement();

            // Fetching pageviews value from table counter
            rs = s.executeQuery("select pageview from counter");

            // Iterating using next() method 
            while (rs.next()) {
                count = rs.getInt(1);
            }

            ctx = sce.getServletContext();
            ctx.setAttribute("pcount", count);
        } 

        // Catch block to handle exceptions 
        catch (Exception e) {

            // Display exception with line number
            // using printStackTrace() method 
            e.printStackTrace();
        }
    }

    // Method 2
    public void contextDestroyed(ServletContextEvent sce) {

        try {

            ctx = sce.getServletContext();
            count = (Integer)ctx.getAttribute("pcount");
            ps = con.prepareStatement("update counter set pcount='" + count + "'");
            ps.executeUpdate();
        } 
        catch (Exception e) {
            
            e.printStackTrace();
        }
    }
}

Step 6: Create Servlet (CounterGfg.java)

Retrieves the current page view count from the application (ServletContext).