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Tomcat session listener example

As part of Servlet 2.3 specification, we can now make use of session creation and destruction events. Our listener object will be called every time a session is created or destroyed by the server.

You can use two interfaces as listener for your Session:

  • HttpSessionListener triggers the listener when a new session is created or destroyed
  • HttpSessionBindingListener triggers the listener when an Object is bound/unbound from the Session
HttpSessionListener example:

package com.sample;

import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSessionEvent;

public class SessionCounter implements HttpSessionListener {

private static int activeSessions = 0;

public void sessionCreated(HttpSessionEvent se) {
activeSessions++;
}

public void sessionDestroyed(HttpSessionEvent se) {

if(activeSessions > 0)
activeSessions--;
}



public static int getActiveSessions() {
return activeSessions;
}

}

As you can see, all you have to do is implementing two methods: sessionCreated() method
will be called by the server every time a session is created and sessionDestroyed() method will be called every time a session is invalidated or destroyed.

In order to activate the listener, you have to configure it in your web.xml

<listener>
<listener-class>
com.sample.SessionCounter
</listener-class>
</listener>

HttpSessionBindingListener example :

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

public class JDBCQueryBean implements HttpSessionBindingListener
{
  public void JDBCQueryBean() {   }

  private Connection conn = null;

  private void runQuery() {
    StringBuffer sb = new StringBuffer();
    Statement stmt = null;
    ResultSet rset = null;
    try {
      if (conn == null) {
        DriverManager.registerDriver(new .OracleDriver());
        conn = DriverManager.getConnection("jdbc:oracle:oci8:@",
                                           "scott", "tiger");

      }
   
      stmt = conn.createStatement();
      rset = stmt.executeQuery ("SELECT * from ....");
  
    } catch (SQLException e) {
       e.printStackTrace();
  }
    finally {
      try {
        if (rset != null) rset.close();
        if (stmt != null) stmt.close();
      }
      catch (SQLException ignored) {}
    }
  }

  public void valueBound(HttpSessionBindingEvent event) {
    // do nothing -- the session-scoped bean is already bound
  }

  public synchronized void valueUnbound(HttpSessionBindingEvent event) {
    try {
      if (conn != null) conn.close();
    }
    catch (SQLException ignored) {}
  }
}

This is a sample code for JDBCQueryBean, a JavaBean that implements the HttpSessionBindingListener interface. The Connection object is bound into the Session and the listener takes care to close the connection as soon as the Session expires.

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