The Java EE 6 Tutorial, Volume I

Servlet Life Cycle

The life cycle of a servlet is controlled by the container in which the servlet has been deployed. When a request is mapped to a servlet, the container performs the following steps.

  1. If an instance of the servlet does not exist, the web container

    1. Loads the servlet class.

    2. Creates an instance of the servlet class.

    3. Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet.

  2. Invokes the service method, passing request and response objects. Service methods are discussed in Writing Service Methods.

If the container needs to remove the servlet, it finalizes the servlet by calling the servlet’s destroy method. Finalization is discussed in Finalizing a Servlet.

Handling Servlet Life-Cycle Events

You can monitor and react to events in a servlet’s life cycle by defining listener objects whose methods get invoked when life-cycle events occur. To use these listener objects you must define and specify the listener class.

Defining the Listener Class

You define a listener class as an implementation of a listener interface. Table 10–1 lists the events that can be monitored and the corresponding interface that must be implemented. When a listener method is invoked, it is passed an event that contains information appropriate to the event. For example, the methods in the HttpSessionListener interface are passed an HttpSessionEvent, which contains an HttpSession.

Table 10–1 Servlet Life-Cycle Events

Object 

Event 

Listener Interface and Event Class 

Web context (see Accessing the Web Context)

Initialization and destruction 

javax.servlet.ServletContextListener and

ServletContextEvent

Attribute added, removed, or replaced 

javax.servlet.ServletContextAttributeListener and

ServletContextAttributeEvent

Session (See Maintaining Client State)

Creation, invalidation, activation, passivation, and timeout 

javax.servlet.http.HttpSessionListener, javax.servlet.http.HttpSessionActivationListener, and

HttpSessionEvent

Attribute added, removed, or replaced 

javax.servlet.http.HttpSessionAttributeListener and

HttpSessionBindingEvent

Request 

A servlet request has started being processed by web components 

javax.servlet.ServletRequestListener and

ServletRequestEvent

Attribute added, removed, or replaced 

javax.servlet.ServletRequestAttributeListener and

ServletRequestAttributeEvent

Specifying Event Listener Classes

You specify an event listener class using the listener element of the deployment descriptor.

    You can specify an event listener using the deployment descriptor editor of NetBeans IDE by doing the following:

  1. Expand your application’s project node.

  2. Expand the project’s Web Pages and WEB-INF nodes.

  3. Double-click web.xml.

  4. Click General at the top of the web.xml editor.

  5. Expand the Web Application Listeners node.

  6. Click Add.

  7. In the Add Listener dialog, click Browse to locate the listener class.

  8. Click OK.

Handling Servlet Errors

Any number of exceptions can occur when a servlet executes. When an exception occurs, the web container generates a default page containing the message


A Servlet Exception Has Occurred

But you can also specify that the container should return a specific error page for a given exception.