Servlet Lifecycle

The lifecycle 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 Creating and Initializing a Servlet)
The container invokes the service method, passing request and response objects. Service methods are discussed in Writing Service Methods.

If it needs to remove the servlet, the container finalizes the servlet by calling the servlet’s destroy method. For more information, see Finalizing a Servlet.

Handling Servlet Lifecycle Events

You can monitor and react to events in a servlet’s lifecycle by defining listener objects whose methods get invoked when lifecycle 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 18-1 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 18-1 Servlet Lifecycle Events

Object	Event	Listener Interface and Event Class
Web context	Initialization and destruction	`javax.servlet.ServletContextListener` and `ServletContextEvent`
Web context	Attribute added, removed, or replaced	`javax.servlet.ServletContextAttributeListener` and `ServletContextAttributeEvent`
Session	Creation, invalidation, activation, passivation, and timeout	`javax.servlet.http.HttpSessionListener`, `javax.servlet.http.HttpSessionActivationListener`, and `HttpSessionEvent`
Session	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`
Request	Attribute added, removed, or replaced	`javax.servlet.ServletRequestAttributeListener` and `ServletRequestAttributeEvent`

Use the @WebListener annotation to define a listener to get events for various operations on the particular web application context. Classes annotated with @WebListener must implement one of the following interfaces:

javax.servlet.ServletContextListener
javax.servlet.ServletContextAttributeListener
javax.servlet.ServletRequestListener
javax.servlet.ServletRequestAttributeListener
javax.servlet..http.HttpSessionListener
javax.servlet..http.HttpSessionAttributeListener

For example, the following code snippet defines a listener that implements two of these interfaces:

import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener()
public class SimpleServletListener implements ServletContextListener,
        ServletContextAttributeListener {
    ...

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 following message:

A Servlet Exception Has Occurred

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