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.
If an instance of the servlet does not exist, the web container
Loads the servlet class.
Creates an instance of the servlet class.
Initializes the servlet instance by calling the init method. Initialization is covered in Initializing a Servlet.
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:
Expand your application’s project node.
Expand the project’s Web Pages and WEB-INF nodes.
Double-click web.xml.
Click General at the top of the web.xml editor.
Expand the Web Application Listeners node.
Click Add.
In the Add Listener dialog, click Browse to locate the listener class.
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.