Identifing Listeners and Filters

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Identifying Listeners

& Filters


Objectives

In this session, you will learn to:

Identify listeners

Identify usage of listeners in Web application

Identify filters

Explore lifecycle of filters

Implement filters in a Web applications


Identifying Listeners

How events are handled and who performs the

action on the occurrence of an event?


A listener is an implemented interface, which responds to a particular event.

For example, pressing a mouse button or selecting an item from a list box, each one is an event, and the listeners are all the programs that execute on the occurrence of these events.

The servlet specification includes the capability to track and handle events in your Web applications through event listeners.

Identifying Listeners (Contd.)

Button Press Event


The following figure depicts the events and listeners for a real life application:


Event and Listener Interaction


Different levels of servlet events are:

Servlet context-level (application-level) event

It involves resources or state held at the level of the application servlet context object.

Session-level event

It involves resources or state associated with the series of requests from a single user session; that is, associated with the HTTP session object.

Each of these two levels has two event categories:

Lifecycle changes

It involves the change in the resources or state that invokes the creation or destruction of an object.

Attribute changes

It involves the change in the properties or attributes associated with a resource or state.



The following table lists the Event categories and the Event Listeners:


Event Category Listeners

Servlet

context

Lifecycle changes javax.servlet.ServletContextListener

Attribute changes javax.servlet.ServletContextAttribut

eListener

Session

Lifecycle changes javax.servlet.http.HttpSessionListen

er

Attribute changes javax.servlet.http.HttpSessionAttrib

uteListener


The following table lists the methods of the ServletContextListenerinterface and ServletContextEvent class:

The servlet container creates a ServletContextEvent object that is input for calls to ServletContextListener methods.


Class/Interface Methods Description

ServletContextListener

void contextInitialized

(ServletContextEvent s)Notifies the listener that the servlet context has been created and the application is ready to process requests.

void contextDestroyed

(ServletContextEvent s)Notifies the listener that the application is about to be shut down.

ServletContextEvent ServletContext

getServletContext()Retrieves the servlet context object that was created or is about to be destroyed, from which you can obtain information as desired.


The following table lists the methods of the ServletContextAttributeListener interface and the ServletContextAttributeEvent class:

The servlet container creates a ServletContextAttributeEventobject that is input for calls to ServletContextAttributeListenermethods.



ServletContext

AttributeListener

void attributeAdded

(ServletContextAttribute

Event s)

Notifies the listener that an attribute was added to the servlet context.

void attributeRemoved


Event s)

Notifies the listener that an attribute was removed from the servlet context.

void attributeReplaced


Event s)

Notifies the listener that an attribute was replaced in the servlet context.

ServletContext

AttributeEvent

String getName() Returns the name of the attribute that was added, removed, or replaced.

Object getValue() Returns the value of the attribute that was added, removed, or replaced.


The following table lists the methods of the HttpSessionListenerinterface and HttpSessionEvent class:

The servlet container creates a HttpSessionEvent object that is input for calls to HttpSessionListener methods.



HttpSessionListener

Void sessionCreated

(HttpSessionEvent h)Notifies the listener that a session was created.

void sessionDestroyed

(HttpSessionEvent h)Notifies the listener that a session was destroyed.

HttpSessionEvent HttpSession getSession() Retrieves the session object that was created or destroyed, from which you can obtain information as desired.


The following table lists the methods of the HttpSessionAttributeListener interface and HttpSessionBindingEvent class:

The servlet container creates a HttpSessionBindingEvent object that is input for calls to HttpSessionAttributeListenermethods.



ServletContext

AttributeListener

void attributeAdded

(HttpSessionBindingEvent s)

Notifies the listener that an attribute was added to the session.

void attributeRemoved


Notifies the listener that an attribute was removed from the servlet context.

void attributeReplaced


Notifies the listener that an attribute was replaced in the servlet context.

HttpSession

BindingEvent

String getName() Returns the name of the attribute that was added, removed, or replaced.

Object getValue() Returns the value of the attribute that was added, removed, or replaced.

HttpSession getSession() Retrieves the session object that had the attribute change.


It is used to handle events related to the start and shutdown of an application.

Implementing a ServletContextListener:

1. Import the ServletContextEvent and ServletContextListenerinterfaces from the javax.servlet package.

2. Create a class that implements the ServletContextListener interface, as shown in the following code:

3. Define the contextInitializedmethod, as shown in the following code:

@Override

public void contextInitialized(ServletContextEvent arg) {

System.out.println("ServletContextListener started");

}

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

public class MyContextListener implements

ServletContextListener{

. . . .

}

Implementing ServletContextListener


4. Define the contextDestroyedmethod, as shown in the following code:

5. Map the declared listener in the web.xml file, as shown in the following code:

You can annotate the listener class declaration with @WebListener, to avoid the mapping.

<listener>

<listener-class>

com.example.listener.MyContextListener

</listener-class>

</listener>

@Override

public void contextDestroyed(ServletContextEvent arg) {

System.out.println("ServletContextListener destroyed");

}

Implementing ServletContextListener (Contd.)


package com.example.listener;

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContextListener;

public class MyContextListener implements

ServletContextListener{

@Override

public void contextInitialized(ServletContextEvent arg) {

System.out.println("ServletContextListener started");

}

@Override

public void contextDestroyed(ServletContextEvent arg) {

System.out.println("ServletContextListener destroyed");

}

}

On completion of the previous steps, the listener class appears as shown in the following code:

Implementing ServletContextListener (Contd.)


Activity: Context Lifecycle Listener

Consider a scenario where you have been asked to create a Web application that displays a message at the time of initialization and at the time of destruction of an application. For this, you need to implement ServletContextListener interface in your application.

.


Implement HttpSessionListener

The HttpSessionListener listener is used to handle the events related to a session.

Implementing HttpSessionListener:

1. Import the HttpSessionEvent and HttpSessionListenerinterfaces from the javax.servlet.http package.

2. Create a class that implements the HttpSessionListener interface, as shown in the following code:

3. Define the sessionCreatedmethod. Following is an example to implement it:

In the preceding code snippet, the sessionCount variable gets incremented whenever a new session is created.

public void sessionCreated(HttpSessionEvent event) {

synchronized (this) {

sessionCount++;

}

import javax.servlet.http.HttpSessionEvent;

import javax.servlet.http.HttpSessionListener;

public class MySessionListener implements HttpSessionListener

{. . . .}


Session Lifecycle Listener (Contd.)

4. Define the sessionDestroyedmethod, as shown in the following code:

5. Map the listener class in the web.xml file, as shown in the following code:

<listener>

<description>sessionListener</description>

<listener-class>

com.example.listener.MySessionListener

</listener-class>

</listener>

public void sessionDestroyed(HttpSessionEvent event) {


sessionCount--;

}


Session Lifecycle Listener (Contd.)

package com.example.listener;

import javax.servlet.http.HttpSessionEvent;

import javax.servlet.http.HttpSessionListener;

public class MySessionListener implements HttpSessionListener

{

public static int sessionCount;

public void sessionCreated(HttpSessionEvent event) {


sessionCount++;

}

public void sessionDestroyed(HttpSessionEvent event) {


sessionCount--;

}

}

On completion of the previous steps, the listener class appears as shown in the following code:


Activity: Implementing HttpSessionListener

Consider a scenario where you have been asked to create a Web application that will keep track of number of sessions currently available. For this, you need to implement the HttpSessionListener.


Listeners can be used to:

Handle events raised by the Web application.

Manage the database connection.

Track HTTP session use.

Perform one-time application setup and shutdown task.

Usage of Listeners


Identifying Filters

What does filters means in servlet

technology?


Filters:

Are Java classes that implements the javax.servlet.Filter interface.

Are objects that intercept the requests and response that flow between a client and a servlet.

Modify the headers and content of a request coming from a Web client and forward it to the target servlet.

Intercept and manipulate the headers and contents of the response that the servlet sends back.

The following figure depicts how filters works:

Identifying Filters (Contd.)


In a Web application, servlet filters can be used to:

Identify the type of request coming from the Web client, such as HTTP and File Transfer Protocol (FTP) and invoke the servlet that needs to process the request.

Validate a client using servlet filters before the client accesses the servlet.

Retrieve the user information from the request parameters to authenticate the user.

Identify the information about the MIME types and other header contents of the request.

Transform the MIME types into compatible types corresponding to the servlet.

Facilitate a servlet to communicate with the external resources.

Intercept responses and compress it before sending the response to the client.

Identifying Filters (Contd.)


The following figure displays how the servlet container calls filter.

Exploring the Execution of Filters

Servlet Invocation With Filters


The order in which filters are executed depends on the order in which they are configured in web.xml.

The first filter in web.xml is the first one to be invoked during the request.

The last filter in web.xml is the first one to be invoked during the response.

The order of filters’ execution in response is reverse of that in request and vice versa.

Exploring the Execution of Filters (Contd.)


A servlet filter comprises of the following methods:

Exploring Methods of Filters

Method Description

public void

init(FilterConfig)The Web container calls this method to initialize a filter that is being placed into service. It uses FilterConfig to provide init parameters and ServletContext object to Filter. The init method enables you to get the parameters defined in the web.xml file.

public void doFilter

(ServletRequest,

ServletResponse, FilterChain)

The Web container calls this method each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain. You can add the desired functionality to a filter using this method.

public void destroy() The Web container calls this method to notify to a filter that it is being taken out of service. This method is called only once in the lifetime of filter.


Implementing Filters in a Web Application

Create a filter

Map a filter

To implement servlet filter in a Web application, you need to perform following steps:


Implementing Filters in a Web Application (Contd.)

Implementing filters in a Web application:

1. Create a Java class that implements the Filter interface.

2. Import the Filter, FilterChain, and FilterConfig interfaces from the javax.servlet package.

3. Annotate the Java class as filter. For this you need to import the @WebFilter annotation from the javax.servlet.annotationpackage, as shown in the following code:package com.example;

. . . .

@WebFilter("/AuthFilter")

public class AuthFilter implements Filter {

. . . . . }



4. Define the init()method in the filter class, as shown in the following code:

5. Define the doFilter()method, as shown in the following code:

public void doFilter(ServletRequest req, ServletResponse res,

FilterChain chain)throws IOException, ServletException {

String uri = req.getRequestURI();

HttpSession session = req.getSession(false);

if(session == null && !(uri.endsWith("html")||

uri.endsWith("LoginServlet"))){

res.sendRedirect("login.html");

}else{

chain.doFilter(request, response);

}

public void init(FilterConfig f) throws ServletException {

this.context = f.getServletContext();

}



In the preceding code snippet, the following execution takes place:

The URI of the request is fetched from the request header.

The object of the HttpSession class is retrieved using the getSessionmethod.

If the session is null and the request is made to any other Web page except Login page or LoginServlet, the res object redirects the user to login.html page.

If the session is not null, the requested Web page will be fetched.



On completion of the previous steps, the listener class appears as shown in the following code: package com.example;

import java.util.*;

import javax.servlet.*;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.HttpServletResponse;

@WebFilter("/AuthFilter")

public class AuthFilter implements Filter {

public void init(FilterConfig f) throws ServletException {

this.context = f.getServletContext();}

public void doFilter(ServletRequest req, ServletResponse res,

FilterChain chain)throws IOException, ServletException {

String uri = req.getRequestURI();

HttpSession session = req.getSession(false);

if(session == null && !(uri.endsWith("html")||

uri.endsWith("LoginServlet"))){

res.sendRedirect("login.html");

}else{

chain.doFilter(request, response);}

}

}



Map a filter:

You need to map the filter class in the web.xml file.

To map the AuthFilter class, you can use the following code snippet:

You can map multiple filters by adding <filter> and <filter-mapping> tag for each filter class.

<filter>

<filter-name>AuthFilter</filter-name>

<filter-class>com.example.AuthFilter</filter-class>

</filter>

<filter-mapping>

<filter-name>AuthFilter</filter-name>

<url-pattern>/*</url-pattern>

</filter-mapping>


Activity: Implementing Filters in a Web Application

Consider a scenario where for the maintenance purpose you need to take your website offline for some times. For this you decide to implement filter on the home page of your Web application so that whenever any client try to access your application, filter will bypass your request to a Web page that displays a "Website is temporarily closed for maintenance. We will get back soon!!! “.


Activity: Implementing Filters in a Web Application

Consider a scenario where you need to create a small Web application that fulfills the following requirements:

Provides a form to accept user name and password, as shown in the following figure.

Displays the welcome message with user name, if the user name is admin and password is pass@123.

Displays “Access Denied” for users having user name James, Shelly, and Tom.

The Expected User Interface


Summary

In this you learned that:

A listener is an implemented interface, which responds to some event.

An event is a change in the state of an object like pressing mouse button.

The two level of events are:

Servlet context-level event

Session-level event

Servlet context-level event is an event related to application.

Session-level event is related to a session.

Based on the changes, the two types of event are:

Lifecycle changes

Attribute changes

Context Lifecycle Listener is an interface used to handle application level events.

The Filter class implements the javax.servlet.Filter interface.


Summary (Contd.)

A filter can be used to:

Validate a client using servlet filters before the client accesses the servlet.

Retrieve the user information from the request parameters to authenticate the user.

Identify the information about the MIME types and other header contents of the request.

Transform the MIME types into compatible types corresponding to the servlet.

Facilitate a servlet to communicate with the external resources.

The order in which filters are executed depends on the order in which they are configured in web.xml.

The first filter in web.xml is the first one to be invoked during the request.

The last filter in web.xml is the first one to be invoked during the response.

The order of filters’ execution in response is reverse of that in request and vice versa.

Technology

Identifing Listeners and Filters