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Servlet - URL Rewriting Servlet - Hidden Form Fields Servlet - Listeners Servlet - Web Security Servlet - Declarative Security

Servlet - Listeners

📌 What You Will Learn

Understand ServletRequestEvent(C)
Understand ServletRequestAttributeEvent(C)
Understand ServletContextListener
See complete working code examples

Listeners is an essential part of the Java Servlet technology. This lesson explains ServletRequestEvent(C), FirstSevlet.java and RequestDemoListener.java with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid.

ServletRequestEvent(C)

This class contains the following methods to return request and context objects.

ServletRequestEvent(C)

ServletRequestEvent is the child class of java.util.EventObject.

EventObject class contains one method getSource()

public Object getSource()

It returns the source which causes the event. In this case web application is the source of event

& hence we will get ServletContext object.

Example02

JCode Cell

1
2	public ServletRequest getServletRequest()
3	public ServletContext getServletContext()
4

FirstSevlet.java

Example03

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstSevlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	out.println("<h1>This is RequestListener Demo Servlet</h1><br>");
11	out.println("<h1>The number of hits for this webapplication is:"+RequestDemoListener.count+"</h1>");
12	}
13	}
14

RequestDemoListener.java

web.xml:

Example04

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	public class RequestDemoListener implements ServletRequestListener
5	{
6	public static int count = 0;
7
8	public void requestInitialized(ServletRequestEvent e)
9	{
10	count++;
11	System.out.println("Request Object created at :"+new java.util.Date());
12	System.out.println("The hit count for this web-application is :"+count);
13	}
14	public void requestDestroyed(ServletRequestEvent e)
15	{
16	System.out.println("Request Object destroyed :"+new java.util.Date());
17	}
18	}
19

RequestDemoListener.java

|-WEB-INF

|-web.xml

|-classes

|-FirstServlet.class

|-RequestDemoListener.class

Note:

We can configure Listener in web.xml by using <listener> tag. <listener> tag is the direct child

tag of <web-app> and hence we can take anywhere within <web-app>

We can configure more than one listener of same type. The order of execution of these listeners

is depends on the order of <listener> tags in web.xml

Web container is responsible for the creation of Listener class object.For this web container

always calls public no-arg constructor. Hence every listener class should compulsory contains

public no-arg constructor.

Web container will create Listener object automatically at the time of either server startup or at

application deployment.

Example05

JCode Cell

1
2	<web-app>
3
4	<listener>
5	<listener-class>RequestDemoListener</listener-class>
6	</listener>
7	<servlet>
8	<servlet-name>FirstSevlet</servlet-name>
9	<servlet-class>FirstSevlet</servlet-class>
10	</servlet>
11
12	<servlet-mapping>
13	<servlet-name>FirstSevlet</servlet-name>
14	<url-pattern>/test</url-pattern>
15	</servlet-mapping>
16
17	</web-app>listener1
18

RequestDemoListener.java

This listener listens the events related to request scoped attributes like attribute

addition,attribute removed and attribute replaced.

This interface defines the following 3 methods

Example06

JCode Cell

1
2	ServletRequestAttributeListener(I):
3

RequestDemoListener.java

This method will be executed automatically by the web container whenever we are adding an

attribute in the request scope.

Example07

JCode Cell

1
2	public void attributeAdded(ServletRequestAttributeEvent e)
3

RequestDemoListener.java

Example08

JCode Cell

1
2	public void attributeRemoved(SRAE e)
3	public void attributeReplaced(SRAE e)
4

ServletRequestAttributeEvent(C)

This class defines the following 2 methods.

ServletRequestAttributeEvent(C)

Returns the name of the attribute which is added or removed or replaced in the request scope.

Example10

JCode Cell

1
2	public String getName()
3

ServletRequestAttributeEvent(C)

returns the value of the attribute which is added or replaced or removed.

In the case of attribute addition and removal this method returns the corresponding attribute

value.But in the case of replacement this method returns old value of the attribute.

Example11

JCode Cell

1
2	public Object getValue()
3

FirstSevlet.java

Example12

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstSevlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	req.setAttribute("durga","scwcd");
11	req.setAttribute("pavan","scjp");
12	req.removeAttribute("pavan");
13	req.setAttribute("durga","scbcd");
14	out.println("<h1>This is ServletRequestAttributeListener Demo </h1>");
15
16	}
17
18	}
19

Output


<h1>This is ServletRequestAttributeListener Demo </h1>

RequestAttributeDemoListener.java

web.xml:

Example13

JCode Cell

1
2	import javax.servlet.*;
3	public class RequestAttributeDemoListener implements ServletRequestAttributeListener
4	{
5	public void attributeAdded(ServletRequestAttributeEvent e)
6	{
7	System.out.println(e.getName()+"... Attribute Added");
8	}
9	public void attributeRemoved(ServletRequestAttributeEvent e)
10	{
11	System.out.println(e.getName()+"...Attribute Removed");
12	}
13	public void attributeReplaced(ServletRequestAttributeEvent e)
14	{
15	System.out.println(e.getName()+"...Attribute Replaced");
16	}
17
18	}
19

RequestAttributeDemoListener.java

|-WEB-INF

|-web.xml

|-classes

|-FirstServlet.class

|-RequestAttributeDemoListener.class

Example14

JCode Cell

1
2	<web-app>
3
4	<listener>
5	<listener-class>RequestAttributeDemoListener</listener-class>
6	</listener>
7
8	<servlet>
9	<servlet-name>FirstSevlet</servlet-name>
10	<servlet-class>FirstSevlet</servlet-class>
11	</servlet>
12
13	<servlet-mapping>
14	<servlet-name>FirstSevlet</servlet-name>
15	<url-pattern>/test</url-pattern>
16	</servlet-mapping>
17	</web-app>listener2
18

ServletContextListener

This listener listens the life cycle events of ServletContext like creation and destruction.

This interface defines the following 2 methods

ServletContextListener

This method will be executed automatically by the web container at the time of context object

creation. i.e at the time of application deployment or server startup.

Example16

JCode Cell

1
2	public void contextInitialized(ServletContextEvent e)
3

ServletContextListener

This method will be executed automatically by the web container at the time of context object

destruction. i.e at the time of application undeployment or server shutdown.

Example17

JCode Cell

1
2	public void contextDestroyed(ServletContextEvent e)
3

ServletContextEvent(C)

It is the child class of java.util.EventObject.

It contains only one method

public ServletContext getServletContext()

Demo program for ServletContextListener to display hitcount of the application where count value

will be preserved across Server restarts.

ServletContextEvent(C)

Increments count value for every request.

Example19

JCode Cell

1
2	RequestDemoListener.java:
3

ServletContextEvent(C)

Example20

JCode Cell

1
2	import javax.servlet.*;
3	public class RequestDemoListener implements ServletRequestListener
4	{
5	public static int count = 0;
6	public void requestInitialized(ServletRequestEvent e)
7	{
8	count++;
9	}
10	public void requestDestroyed(ServletRequestEvent e)
11	{
12	}
13	}
14

ServletContextEvent(C)

saves the count value to abc.txt file at the time of context object destruction(server shutdown)

It reads the count value from abc.txt file and assign to RequestDemoListener.count variable at

context object creation(server startup)

Example21

JCode Cell

1
2	ContextDemoListener.java:
3

ServletContextEvent(C)

3.FirstServlet.java:

To display count value to end user

Example22

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class ContextDemoListener implements ServletContextListener
6	{
7	public void contextInitialized(ServletContextEvent e)
8	{
9	try{
10	String path=e.getServletContext().getRealPath("abc.txt");
11	BufferedReader br = new BufferedReader( new FileReader(path));
12	String s = br.readLine();
13	if(s != null)
14	{
15	int c = Integer.parseInt(s);
16	RequestDemoListener.count = c;
17	}
18	}
19	catch(Exception e1){ }
20	}
21	public void contextDestroyed(ServletContextEvent e)
22	{
23	try{
24	String path=e.getServletContext().getRealPath("abc.txt");
25	PrintWriter pw = new PrintWriter(path);
26	pw.println(RequestDemoListener.count);
27	pw.flush();
28	}
29	catch(Exception e1) {
30	}
31	}
32	}
33

ServletContextEvent(C)

4.web.xml:

For declaring listeners

Example23

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstSevlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	out.println("<h1>The number of hits for this webapplication is:"+RequestDemoListener.count+"</h1>");
11	}
12	}
13

ServletContextEvent(C)

Example24

JCode Cell

1
2	<web-app>
3
4	<listener>
5	<listener-class>RequestDemoListener</listener-class>
6	</listener>
7
8	<listener>
9	<listener-class>ContextDemoListener</listener-class>
10	</listener>
11
12	<servlet>
13	<servlet-name>FirstSevlet</servlet-name>
14	<servlet-class>FirstSevlet</servlet-class>
15	</servlet>
16
17	<servlet-mapping>
18	<servlet-name>FirstSevlet</servlet-name>
19	<url-pattern>/test</url-pattern>
20	</servlet-mapping>
21
22	</web-app>
23

ServletContextEvent(C)

To save count value

listenersc

|-abc.txt

|-WEB-INF

|-web.xml

|-classes

|-FirstServlet.class

|-RequestDemoListener.class

|-ContextDemoListener.class

Example25

JCode Cell

1
2	abc.txt:
3

ServletContextAttributeListener(I)

This listener listens the events related to context scoped attributes like addition,removal and

replacement.

This interface defines the following 3 methods.

ServletContextAttributeListener(I)

Example27

JCode Cell

1
2	public void attributeAdded(ServletContextAttributeEvent e)
3	public void attributeRemoved(SCAE e)
4	public void attributeReplaced(SCAE e)
5

ServletContextAttributeEvent(C)

It is the child class of ServletContextEvent.

It defines the following 2 methods

ServletContextAttributeEvent(C)

Example29

JCode Cell

1
2	public String getName()
3	public Object getValue()
4

HttpSessionListener(I)

This listener listens the life cycle events of HttpSession object like creation and destruction.

This interface defines the following 2 methods

HttpSessionListener(I)

Example31

JCode Cell

1
2	public void sessionCreated(HttpSessionEvent e)
3	public void sessionDestroyed(HttpSessionEvent e)
4

HttpSessionEvent(C)

This class defines only one method getSession()

public HttpSession getSession()

Eg: HS session = req.getSession();

Demo Program to display the number of users currently online

This listener increments count value for every session object creation and decrements count value

for every session object destruction.

Example33

JCode Cell

1
2	SessionCounter.java:
3

Demo Program to display the number of users currently online

Example34

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	public class SessionCounter implements HttpSessionListener
5	{
6	static int count = 0;
7	public void sessionCreated(HttpSessionEvent e)
8	{
9	count++;
10	System.out.println("A new session created with id: "+e.getSession().getId());
11	}
12	public void sessionDestroyed(HttpSessionEvent e)
13	{
14	count--;
15	System.out.println("An existing session destroyed with id: "+e.getSession().getId());
16	}
17	}
18

Demo Program to display the number of users currently online

Example35

JCode Cell

1
2	FirstServlet.java
3

Demo Program to display the number of users currently online

Example36

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstSevlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	HttpSession s = req.getSession();
11	s.setMaxInactiveInterval(120);
12	out.println("<h1>The number of users online is:"+SessionCounter.count+"</h1>");
13	}
14	}
15

Demo Program to display the number of users currently online

Example37

JCode Cell

1
2	web.xml
3

Demo Program to display the number of users currently online

|-WEB-INF

|-web.xml

|-classes

|-FirstServlet.class

|-SessionCounter.class

Example38

JCode Cell

1
2	<web-app>
3
4	<listener>
5	<listener-class>SessionCounter</listener-class>
6	</listener>
7
8	<servlet>
9	<servlet-name>FirstSevlet</servlet-name>
10	<servlet-class>FirstSevlet</servlet-class>
11	</servlet>
12
13	<servlet-mapping>
14	<servlet-name>FirstSevlet</servlet-name>
15	<url-pattern>/test</url-pattern>
16	</servlet-mapping>
17
18	</web-app>listener3
19

Demo Program to display the number of users currently online

This listener listens the events related to session scoped attributes like attribute addition,removal

and replacement.

This interface defines the following 3 methods.

public void attributeAdded(HttpSessionBindingEvent e)

public void attributeRemoved(HSBE e)

public void attributeReplaced(HSBE e)

Note:

There is no event class named with HttpSessionAttributeEvent. For this equivalent event class is

HttpSessionBindingEvent.

Example39

JCode Cell

1
2	HttpSessionAttributeListener(I):
3

HttpSessionBindingEvent

It is the child class of HttpSessionEvent.

This class contains the following 2 methods

public String getName()

public Object getValue()

HttpSessionBindingListener(I)

Whenever a particular type of object adding|removing|replacing in session scope, if we want to

perform certain activities then we should go for HttpSessionBindingListener.

This listener defines the following 2 methods

HttpSessionBindingListener(I)

This method will be executed automatically at the time of attribute addition.

Example42

JCode Cell

1
2	public void valueBound(HttpSessionBindingEvent e)
3

HttpSessionBindingListener(I)

This method will be executed automatically at the time of attribute removal.

Note: In the case of replacement both methods will be executed but valueBound() method first

followed by valueUnbound().

Note:

It is not required to configure HttpSessionBindingListener in web.xml. Whenever we are adding an

attribute,web container will check the corresponding class implements HttpSessionBindingListener

or not. If it implements then the corresponding method will be executed.

If both attribute and binding listeners are configured,then binding listener will be executed first

followed by attribute listener.

Example43

JCode Cell

1
2	public void valueUnbound(HttpSessionBindingEvent e)
3

Demo Program for HttpSessionBindingListener

Dog.java:

Demo Program for HttpSessionBindingListener

Example45

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	public class Dog implements HttpSessionBindingListener
5	{
6	public void valueBound(HttpSessionBindingEvent e)
7	{
8	System.out.println("dog object has added to the session scope");
9	}
10	public void valueUnbound(HttpSessionBindingEvent e)
11	{
12	System.out.println("Dog object has removed from the session scope");
13	}
14	}
15

Output


dog object has added to the session scope
Dog object has removed from the session scope

SessionAttributeListener.java

web.xml:

Example46

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	public class SessionAttributeListener implements HttpSessionAttributeListener
5	{
6	public void attributeAdded(HttpSessionBindingEvent e)
7	{
8	System.out.println("attribute added");
9	}
10	public void attributeRemoved(HttpSessionBindingEvent e)
11	{
12	System.out.println("attribute removed");
13	}
14	public void attributeReplaced(HttpSessionBindingEvent e)
15	{
16	System.out.println("attribute replaced");
17	}
18	}
19

Output


attribute added
attribute removed
attribute replaced

SessionAttributeListener.java

Note:

We have to configure only AttributeListener and we are not required to configure binding listener.

Example47

JCode Cell

1
2	<web-app>
3
4	<listener>
5	<listener-class>SessionAttributeListener</listener-class>
6	</listener>
7
8	<servlet>
9	<servlet-name>FirstSevlet</servlet-name>
10	<servlet-class>FirstSevlet</servlet-class>
11	</servlet>
12
13	<servlet-mapping>
14	<servlet-name>FirstSevlet</servlet-name>
15	<url-pattern>/test</url-pattern>
16	</servlet-mapping>
17
18	</web-app>
19

FirstServlet.java

listener4

|-WEB-INF

|-web.xml

|-classes

|-FirstServlet.class

|-Dog.class

|-SessionAttributeListener.class

Example48

JCode Cell

1
2	import javax.servlet.*;
3	import javax.servlet.http.*;
4	import java.io.*;
5	public class FirstSevlet extends HttpServlet
6	{
7	public void doGet(HttpServletRequest req,HttpServletResponse resp) throws ServletException,IOException
8	{
9	PrintWriter out = resp.getWriter();
10	HttpSession s = req.getSession();
11	s.setAttribute("a1","SCJP");
12	s.setAttribute("a1","SCWCD");
13	s.setAttribute("a2",new Dog());
14	s.setAttribute("a3",new Dog());
15	s.setAttribute("a2",new Dog());
16	s.removeAttribute("a3");
17	out.println("<h1>This is Session Binding demo</h1>");
18	}
19
20	}
21

Output


<h1>This is Session Binding demo</h1>

HttpSessionActivationListener

If functionality is distributed across several JVMs, then that application is called distributed web

application.

The main advantages of distributed web applications are

By Load balancing we can improve performance of the application
By Handling Fail over situations,we can keep our web application robust.(The chance of failure

is very very less).

In the distributed applications session object has to migrate from one JVM to another JVM.

Whenever a session object is migrated from one JVM to another JVM,compulsory the

corresponding session attributes should be migrated across the network. Hence every session

attribute should be compulsory implements Serializable interface.

At the time of session object migration if we want to perform certain activities then we should go

for HttpSessionActivationListener.

This interface defines the following 2 methods.

HttpSessionActivationListener

This method is called on each implementing object bound to the session just before serialization.

Example50

JCode Cell

1
2	public void sessionWillPassivate(HttpSessionEvent e)
3

HttpSessionActivationListener

This method is called on each implementing object bound to the session just after

deserialization.

Summary of all Listeners:

Listener Purpose Corresponding Corresponding Corresponding Is

Methods	Events	Event Methods	required

Servlet

Request To listens life requestInitialized() ServletRequest getServletReque to

Listener	cycle events of	requestDestroyed()	Event	st()
request object.	getServletConte	web.xml
Servlet	i.e., request	ServletRequest xt()	Yes
Request	object creation	AttributeEvent
Attribute	& destruction	getName()	Yes
Listener	ServletContext getValue()
To listens	attributeAdded()	Event	Yes
Servlet	events related	attributeReplaced()	getServletConte
Context	request scoped	attributeRemoved()	ServletContext xt()	Yes
Listener	attributes	AttributeEvent

getName() Yes

Servlet	To listens life	contextInitialized() HttpSession	getValue()
Context	cycle events of	contextDestroyed()	Event
Attribute	context object.	getSession()

Listener i.e., context

object creation

HttpSession & destruction

Listener

To listen events attributeAdded()

related to	attributeReplaced()
context scoped	attributeRemoved()

attributes

To listens life	sessionCreated()
cycle events of	sessionDestroyed()

session object.

i.e., session

object creation

& destruction

HttpSession To listen events attributeAdded()	HttpSession	getName()	Yes
Attribute	related to	attributeReplaced()	AttributeEvent getValue()
Listener	session scoped	attributeRemoved()	HttpSession	not
attributes	BindingEvent	getName()	required
HttpSession	getValue()
Binding	When ever we	HttpSession	not
Listener	are adding or	BindingEvent	getSession() required

removing or

HttpSession replacing a	valueBound()
Activation	particular type	valueUnbound()

Listener of object in

session scope ,

to perform

certain activity

then we should

go for this

Listener

If we want to	sessionWillPassivate HttpSession

perform any () Event

activity just	sessionDidActivate()

before

serialization

and just after

de-serialization

in distributed

web-

applications

@WebListener Annotation in Servlet 3.0V:

This annotation is replacement for listener configuration in web.xml

Eg:

import javax.servlet.annotation.*;

@WebListener

public class RequestDemoListener implements ServletRequestListener

{

...

}

whenever we are using @WebListener annotation then we are not required to configure listener

in web.xml

Example51

JCode Cell

1
2	public void sessionDidActivate(HttpSessionEvent e)
3

📝 Key Takeaways

Key ideas of Servlet - Listeners explained simply
Ready-to-use code examples
Exam-style questions at the end

🧠 Test Your Knowledge

3 Questions

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Servlet

Nearby lessons

Servlet - Listeners

01ServletRequestEvent(C)

02ServletRequestEvent(C)

03FirstSevlet.java

04RequestDemoListener.java

05RequestDemoListener.java

06RequestDemoListener.java

07RequestDemoListener.java

08RequestDemoListener.java

09ServletRequestAttributeEvent(C)

10ServletRequestAttributeEvent(C)

11ServletRequestAttributeEvent(C)

12FirstSevlet.java

13RequestAttributeDemoListener.java

14RequestAttributeDemoListener.java

15ServletContextListener

16ServletContextListener

17ServletContextListener

18ServletContextEvent(C)

19ServletContextEvent(C)

20ServletContextEvent(C)

21ServletContextEvent(C)

22ServletContextEvent(C)

23ServletContextEvent(C)

24ServletContextEvent(C)

25ServletContextEvent(C)

26ServletContextAttributeListener(I)

27ServletContextAttributeListener(I)

28ServletContextAttributeEvent(C)

29ServletContextAttributeEvent(C)

30HttpSessionListener(I)

31HttpSessionListener(I)

32HttpSessionEvent(C)

33Demo Program to display the number of users currently online

34Demo Program to display the number of users currently online

35Demo Program to display the number of users currently online

36Demo Program to display the number of users currently online

37Demo Program to display the number of users currently online

38Demo Program to display the number of users currently online

39Demo Program to display the number of users currently online

40HttpSessionBindingEvent

41HttpSessionBindingListener(I)

42HttpSessionBindingListener(I)

43HttpSessionBindingListener(I)

44Demo Program for HttpSessionBindingListener

45Demo Program for HttpSessionBindingListener

46SessionAttributeListener.java

47SessionAttributeListener.java

48FirstServlet.java

49HttpSessionActivationListener

50HttpSessionActivationListener

51HttpSessionActivationListener

🧠 Test Your Knowledge

ServletRequestEvent(C)

ServletRequestEvent(C)

FirstSevlet.java

RequestDemoListener.java

RequestDemoListener.java

RequestDemoListener.java

RequestDemoListener.java

RequestDemoListener.java

ServletRequestAttributeEvent(C)

ServletRequestAttributeEvent(C)

ServletRequestAttributeEvent(C)

FirstSevlet.java

RequestAttributeDemoListener.java

RequestAttributeDemoListener.java

ServletContextListener

ServletContextListener

ServletContextListener

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextEvent(C)

ServletContextAttributeListener(I)

ServletContextAttributeListener(I)

ServletContextAttributeEvent(C)

ServletContextAttributeEvent(C)

HttpSessionListener(I)

HttpSessionListener(I)

HttpSessionEvent(C)

Demo Program to display the number of users currently online

Demo Program to display the number of users currently online

Demo Program to display the number of users currently online

Demo Program to display the number of users currently online

Demo Program to display the number of users currently online

Demo Program to display the number of users currently online

Demo Program to display the number of users currently online

HttpSessionBindingEvent

HttpSessionBindingListener(I)

HttpSessionBindingListener(I)

HttpSessionBindingListener(I)

Demo Program for HttpSessionBindingListener

Demo Program for HttpSessionBindingListener

SessionAttributeListener.java

SessionAttributeListener.java

FirstServlet.java

HttpSessionActivationListener

HttpSessionActivationListener

HttpSessionActivationListener