ServletContextListener is one of the many Servlet Listener we have. This is the fifth article in the series of Java Web Application, you might want to check out earlier four articles too.

In this tutorial, we will look into servlet listener, benefits of servlet listeners, some common tasks that we can do with listeners, servlet API listener interfaces and Event objects. In the end we will create a simple web project to show example of commonly used Listener implementation for ServletContext, Session and ServletRequest.

  • Why do we have Servlet Listener?

We know that using ServletContext, we can create an attribute with application scope that all other servlets can access but we can initialize ServletContext init parameters as String only in deployment descriptor (web.xml). What if our application is database oriented and we want to set an attribute in ServletContext for Database Connection. If you application has a single entry point (user login), then you can do it in the first servlet request but if we have multiple entry points then doing it everywhere will result in a lot of code redundancy. Also if database is down or not configured properly, we won’t know until first client request comes to server. To handle these scenario, servlet API provides Listener interfaces that we can implement and configure to listen to an event and do certain operations.

Event is occurrence of something, in web application world an event can be initialization of application, destroying an application, request from client, creating/destroying a session, attribute modification in session etc.

Servlet API provides different types of Listener interfaces that we can implement and configure in web.xml to process something when a particular event occurs. For example, in above scenario we can create a Listener for the application startup event to read context init parameters and create a database connection and set it to context attribute for use by other resources.

  • Servlet Listener Interfaces and Event Objects

Servlet API provides different kind of listeners for different types of Events. Listener interfaces declare methods to work with a group of similar events, for example we have ServletContext Listener to listen to startup and shutdown event of context. Every method in listener interface takes Event object as input. Event object works as a wrapper to provide specific object to the listeners.

Servlet API provides following event objects.

  1. javax.servlet.AsyncEvent – Event that gets fired when the asynchronous operation initiated on a ServletRequest (via a call to ServletRequest#startAsync or ServletRequest#startAsync(ServletRequest, ServletResponse)) has completed, timed out, or produced an error.
  2. javax.servlet.http.HttpSessionBindingEvent – Events of this type are either sent to an object that implements HttpSessionBindingListener when it is bound or unbound from a session, or to a HttpSessionAttributeListener that has been configured in the web.xml when any attribute is bound, unbound or replaced in a session.
    The session binds the object by a call to HttpSession.setAttribute and unbinds the object by a call to HttpSession.removeAttribute.
    We can use this event for cleanup activities when object is removed from session.
  3. javax.servlet.http.HttpSessionEvent – This is the class representing event notifications for changes to sessions within a web application.
  4. javax.servlet.ServletContextAttributeEvent – Event class for notifications about changes to the attributes of the ServletContext of a web application.
  5. javax.servlet.ServletContextEvent – This is the event class for notifications about changes to the servlet context of a web application.
  6. javax.servlet.ServletRequestEvent – Events of this kind indicate lifecycle events for a ServletRequest. The source of the event is the ServletContext of this web application.
  7. javax.servlet.ServletRequestAttributeEvent – This is the event class for notifications of changes to the attributes of the servlet request in an application.

Servlet API provides following Listener interfaces.

  1. javax.servlet.AsyncListener – Listener that will be notified in the event that an asynchronous operation initiated on a ServletRequest to which the listener had been added has completed, timed out, or resulted in an error.
  2. javax.servlet.ServletContextListener – Interface for receiving notification events about ServletContext lifecycle changes.
  3. javax.servlet.ServletContextAttributeListener – Interface for receiving notification events about ServletContext attribute changes.
  4. javax.servlet.ServletRequestListener – Interface for receiving notification events about requests coming into and going out of scope of a web application.
  5. javax.servlet.ServletRequestAttributeListener – Interface for receiving notification events about ServletRequest attribute changes.
  6. javax.servlet.http.HttpSessionListener – Interface for receiving notification events about HttpSession lifecycle changes.
  7. javax.servlet.http.HttpSessionBindingListener – Causes an object to be notified when it is bound to or unbound from a session.
  8. javax.servlet.http.HttpSessionAttributeListener – Interface for receiving notification events about HttpSession attribute changes.
  9. javax.servlet.http.HttpSessionActivationListener – Objects that are bound to a session may listen to container events notifying them that sessions will be passivated and that session will be activated. A container that migrates session between VMs or persists sessions is required to notify all attributes bound to sessions implementing HttpSessionActivationListener.
  • Servlet Listener Configuration

We can use @WebListener annotation to declare a class as Listener, however the class should implement one or more of the Listener interfaces.

We can define listener in web.xml as:

  • Servlet Listener Example

Let’s create a simple web application to see servlet listener in action. We will create dynamic web project in Eclipse ServletListenerExample those project structure will look like below image.

Servlet-Listener-Example

 

web.xml: In deployment descriptor, I will define some context init params and listener configuration.

DBConnectionManager: This is the class for database connectivity, for simplicity I am not providing code for actual database connection. We will set this object as attribute to servlet context.

MyServlet: A simple servlet class where I will work with session, attributes etc.

Now we will implement listener classes, I am providing sample listener classes for commonly used listeners – ServletContextListener, ServletContextAttributeListener, ServletRequestListener and HttpSessionListener.

  • ServletContextListener

We will read servlet context init parameters to create the DBConnectionManager object and set it as attribute to the ServletContext object.

  • ServletContextAttributeListener

A simple implementation to log the event when attribute is added, removed or replaced in servlet context.

  • HttpSessionListener

A simple implementation to log the event when session is created or destroyed.

  • ServletRequestListener

A simple implementation of ServletRequestListener interface to log the ServletRequest IP address when request is initialized and destroyed.

Now when we will deploy our application and access MyServlet in browser with URL https://localhost:8080/ServletListenerExample/MyServlet, we will see following logs in the server log file.

Notice the sequence of logs and it’s in the order of execution. The last log will appear when you will shutdown the application or shutdown the container.

Thats all for listener in servlet, we will look into cookies and some common servlet examples next.

You can download the project from below link and play around with it to learn more.

By admin

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