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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 a request and response object. 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 listener objects you

Define the listener class
Specify the listener class in the Web application deployment descriptor

Defining The Listener Class

You define a listener class as an implementation of a listener interface. Servlet Life Cycle Events 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 3-2 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, and timeout	javax.servlet.http. HttpSessionListener and HttpSessionEvent
	Attribute added, removed, or replaced	javax.servlet.http. HttpSessionAttributeListener and HttpSessionBindingEvent

The listeners.ContextListener class creates and removes the database helper and counter objects used in the Duke's Bookstore application. The methods retrieve the Web context object from ServletContextEvent and then store (and remove) the objects as servlet context attributes.

import database.BookDB;
import javax.servlet.*;
import util.Counter;

public final class ContextListener
  implements ServletContextListener {
  private ServletContext context = null;
  public void contextInitialized(ServletContextEvent event) {
    context = event.getServletContext();
    try {
      BookDB bookDB = new BookDB();
      context.setAttribute("bookDB", bookDB);
    } catch (Exception ex) {
      System.out.println(
        "Couldn't create database: " 
        + ex.getMessage());
    }
    Counter counter = new Counter();
    context.setAttribute("hitCounter", counter);
    context.log("Created hitCounter"
      + counter.getCounter());
    counter = new Counter();
    context.setAttribute("orderCounter", counter);
    context.log("Created orderCounter"
      + counter.getCounter());
  }

  public void contextDestroyed(ServletContextEvent event) {
    context = event.getServletContext();
    BookDB bookDB = context.getAttribute(
      "bookDB");
    bookDB.remove();
    context.removeAttribute("bookDB");
    context.removeAttribute("hitCounter");
    context.removeAttribute("orderCounter");
  }
} 

Creating a Listener

To create a listener in the IDE:

Mount the Web module as a filesystem.
Expand the module and WEB-INF nodes.
Right-click the classes node and choose NewJSP & ServletListenersXXXListener, where XXX is the type of the listener.
Type a name for the listener.
Click Finish.

Specifying Event Listener Classes

To specify an event listener in Web application deployment descriptor using the IDE:

Mount the Web module as a filesystem.
Expand the WEB-INF node.
Select the web.xml file.
Select the Deployment tab in the property editor.
Select the Listeners property and open the property editor.
Click Add.
Type the listener class.
Click OK twice.

For an example listener definition, see The Example Servlets.

Handling Errors

Any number of exceptions can occur when a servlet is executed. The Web container will generate a default page containing the message A Servlet Exception Has Occurred when an exception occurs, but you can also specify that the container should return a specific error page for a given exception. To specify such a page, you specify an Error Pages property for the Web application deployment descriptor (see Error Mappings).

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