06 Lifecycle Listeners

8/3/2019 06 Lifecycle Listeners

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© 2009 Marty Hall

The Web ApplicationLife-Cycle Events

Originals of Slides and Source Code for Examples:

http://courses.coreservlets.com/Course-Materials/msajsp.html

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© 2009 Marty Hall

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Agenda

• Reason for listeners

• Monitoring creation and destruction

of the servlet context• e ec ng c anges n serv e con ex

attributes

destruction

• in session attributes

• Combinin activities

5

Life-Cycle Events Framework

• Motivation 1: Respond to events in theoverall application life cycle. – Notice when certain attributes are removed from the

.

• Motivation 2: Perform tasks more general

than an one servlet or JSP a e hasresponsibility for. – Insert application-wide data into the servlet context

e ore any serv e s or pages are accesse

– Keep track of certain session attributes, regardless of which servlet or JSP a e added the attributes

• New with servlets 2.3. – Note: this section has not been updated to 2.4 or 2.5!6



Available Listeners

• Servlet context listeners. – These listeners are notified when the servlet context (i.e.,

the Web application) is initialized and destroyed..

– These listeners are notified when attributes are added to,removed from, or replaced in the servlet context.

• Session listeners. – These listeners are notified when session objects are

create , nva ate , or t me out.

• Session attribute listeners. – ,removed from, or replaced in any session.

7

General Implementation

1. Implement the appropriate interface. – Use Serv etContextL stener, Serv etContextAttr uteL stener,

HttpSessionListener, or HttpSessionAttributeListener.

2. Override the methods needed to respond to theevents of interest.

– Provide empty bodies for the other methods in the interface.

. . – Six objects that you are likely to use in event-handling methods:

• The servlet context

• The name o the servlet context attribute thatchanged

• The value of the servlet context attribute thatchanged

• The session object8



General Implementation

4. Use these objects. – T s process s app cat on spec c, ut t ere are some common

themes. For example, with the servlet context, you are most likelyto read initialization parameters (getInitParameter), store data for

,(getAttribute).

5. Declare the listener. – You do this with the listener and listener-class

elements of the general Web application deployment descriptor (web.xml ) or of a tag library descriptor file.

. rov e any nee e n a za on parame ers. – Servlet context listeners commonly read context initialization

parameters to use as the basis of data that is made available to allservlets and JSP pages. You use the context-param web.xml element to provide the names and values of these initialization

parameters.9

Monitoring Creation and Destruction

• The ServletContextListener class respondsto the initialization and destruction of theservlet context. –

the Web application itself.

• ServletContextListener is most commonlused to – Set up application-wide resources like database

connec on poo s

– Read the initial values of application-wide data that will be used b multi le servlets and JSP a es.

10



Specific Implementation Strategy:

1. Implement the ServletContextListener interface.

2. Override contextInitialized and contextDestroyed.

– contextInitialized is triggered when the Web application is firstloaded and the servlet context is created. Most common tasks:

• Creating application-wide data (e.g., by readingcontext init params)

• . – contextDestroyed is triggered when the Web application is being

shut down and the servlet context is about to be destroyed. Mostcommon task:

• Releasing resources (e.g. closing connections).

3. Obtain a reference to the servlet context. – The contextInitialized and contextDestroyed methods each take a

ServletContextEvent as an argument.

– The ServletContextEvent class has a getServletContext method11

Specific Implementation Strategy:

4. Use the servlet context. – Read initialization parameters: getInitParameter

– Store data:setAttribute

– .

5. Declare the listener.<listener><listener-class>package.Listener</listener-class>

</listener>

. .<context-param>

<param-name>name</param-name>

- -

</context-param>

12



Example: Web Site for Company

public class InitialCompanyNameListener

im lements ServletContextListener

private static final String DEFAULT_NAME =

"MISSING-COMPANY-NAME";

public void contextInitialized(ServletContextEvent event) {

ServletContext context = event.getServletContext();

setInitialAttribute(context,

" ",

DEFAULT_NAME);

setInitialAttribute(context,

"formerCompanyName",

"");

}

pu c vo contextDestroye Serv etContextEvent event

13

Servlet Context Listener

private void setInitialAttribute(ServletContext context,

Strin initParamName

String defaultValue) {

String initialValue =

context.getInitParameter(initParamName);

if (initialValue != null) {

context.setAttribute(initParamName, initialValue);

} else {

. ,

}

}

…

}

14



Servlet Context Listener:.

<?xml version="1.0" encoding="ISO-8859-1"?>

…

<web-app>

<context-param>

<param-name>companyName</param-name>

<param-value>not-dot-com.com</param-value>

</context-param>

<context- aram>

<param-name>formerCompanyName</param-name>

<param-value>hot-dot-com.com</param-value>-

…

15

Servlet Context Listener:.

…

<listener>

<listener-class>

</listener-class>

</listener>

</web-app>

16



Servlet Context Listener: Using

<%

String companyName =

InitialCompanyNameListener.getCompanyName(application);

String formerCompanyName =

InitialCompanyNameListener.getFormerCompanyName(application);… %>

< >< = company ame >< >

…

<TABLE BORDER=5 ALIGN="CENTER">

<TR><TH CLASS="TITLE">

<%= formerCompanyDescription %>

…

<UL>

<LI><A HREF=" roducts. s "><%= com an Name %> roducts</A>

<LI><A HREF="services.jsp"><%= companyName %> services</A>

<LI><A HREF="history.jsp"><%= companyName %> history</A>

<LI><A HREF="invest.jsp">investing in <%= companyName %></A><LI><A HREF="contact.jsp">contacting <%= companyName %></A>

</UL>

…

17

Servlet Context Listener:

18



Detecting Changes in Servlet

• ServletContextListener – Lets you set up initial values of resources and store

references to them in the servlet context.

these resources change? – For exam le what if the value of resource B de ends on

the value of resource A? If resource A changes, you needto automatically update the value of resource B.

– attribute listeners.

19

Implementation Strategy: Servlet

1. Implement ServletContextAttributeListener

2. Override attributeAdded, attributeReplaced, andattributeRemoved.

– attributeAdded is tri ered when a new attribute name is firstadded to the servlet context.

– attributeReplaced is triggered when a new value is assigned to an

existing name. attributeAdded is not triggered in this case. The oldvalue is obtained via event.getValue and the new value is obtainedvia context.getAttribute.

– attributeRemoved is triggered when a servlet context attribute isremoved altogether.

3. Obtain references to the attribute name, attributevalue and servlet context.

– Call the following methods of the event object: getName,getValue, and getServletContext

20



Implementation Strategy: Servlet

4. Use the objects. – You normally compare attribute name to a stored name

to see if it is the one you are monitoring. The attributevalue is used in an a lication-s ecific manner. The servlet context is usually used to read previously storedattributes (getAttribute), store new or changed attributes

, .

5. Declare the listener. – Use the listener and listener-class elements

to list the fully qualified name of the listener class,

<listener><listener-class>

somePackage.SomeListener

</listener-class>

</listener>21

Example: Updating Former

public class ChangedCompanyNameListener

public void attributeAdded

(ServletContextAttributeEvent event) {}

public void attributeRemoved(ServletContextAttributeEvent event) {}

22




(ServletContextAttributeEvent event) {

if (event.getName().equals("companyName")) {

String oldName = (String)event.getValue();

ServletContext context =

event.getServletContext();

context.setAttribute("formerCompanyName",

oldName);

}

}

23


<?xml version="1.0" encoding="ISO-8859-1"?>

…

<web-app>

>

<listener-class>

moreservlets.listeners.ChangedCompanyNameListener

- >

</listener>

24




<servlet>

-

ChangeCompanyName

</servlet-name>

<servlet-class>

moreservlets.ChangeCompanyName

</servlet-class>

</servlet>

<servlet>

<servlet-name>Redirector</servlet-name>

<servlet-class>.

</servlet-class>

</servlet>25


<servlet-mapping>

<serv et-name>

ChangeCompanyName

</servlet-name>

<url-pattern>

/admin/ChangeCompanyName

-

</servlet-mapping>

<servlet-mapping>

<servlet-name>Redirector</servlet-name>

- -

</servlet-mapping>

26




-

<web-resource-collection>

<web-resource-name>Admin</web-resource-name>

ur -pa ern a m n ur -pa ern

</web-resource-collection>

<auth-constraint>

<ro e-name>ceo< ro e-name>

</auth-constraint>

</security-constraint>

27


-

<auth-method>FORM</auth-method>

<form-login-config>

<form-login-page>

/admin/login.jsp

</form-login-page><form-error-page>

/admin/login-error.jsp

</form-error- a e>

</form-login-config>

</login-config>

28




29


30




31

Packaging Listeners with Tag

• Tag libraries provide encapsulation of logic – But what if tag depends on a listener?

• E.g., a tag that uses servlet context attributes put there bya ServletContextListener

– You'd like to include the listener with the tag library• Make it easy to reuse tag library in multiple Web apps

• ener c approac – Put tag library and listener classes in a JAR file

– - –

– Put TLD file in WEB-INF

– Requires you to modify WEB-INF/web.xml to declarelisteners

• Listener not purely associated with tag library32




• New JSP 1.2 Behavior – en e app ca on s oa e , e server au oma ca y

looks for .tld files in WEB-INF and its subdirectories• Not counting classes and lib directories

• – Move listener element from web.xml to the TLD file

– Put TLD file in the WEB-INF hierarch

• Tomcat 4.0 problem – Tomcat 4.0.x automatically detects TLD files only when

.alias for the location of the TLD files. Works on other

servers and Tomcat 4.1 and later – this is an.

– Defeats the purpose• If you have to modify web.xml, why not just declare

listeners?33


<%@ taglib uri="/company-name-taglib.tld"

prefix="msajsp" %>

Learn more about <B><msajsp:companyName/></B>

<msajsp:formerCompanyName fullDescription="true"/>

<UL>

= .

<LI><A HREF="services.jsp"><msajsp:companyName/> services</A>

<LI><A HREF="history.jsp"><msajsp:companyName/> history</A>

<LI><A HREF="invest.jsp">investing in<msajsp:companyName/></A>

<LI><A HREF="contact.jsp">

contacting

</UL>

34




<taglib>…

<listener>

<listener-class>

moreservlets.listeners.InitialCompanyNameListener</listener-class>

<listener>

<listener-class>

moreservlets.listeners.ChangedCompanyNameListener

</listener-class>

</listener>

<tag>

<tag-class>moreservlets.tags.CompanyNameTag

</tag-class>

<body-content>empty</body-content>

<description>The current company name

</description>

</tag> … </taglib>35

Packaging Listeners with Tag. .

…

<we -app>

…

<taglib>

<taglib-uri>

/company-name-taglib.tld

-

<taglib-location>

/WEB-INF/company-name-taglib.tld

</taglib-location>

</taglib>

…

</web-app>

36



Recognizing Session Creation

• The previous listeners monitor Web-application-wide data.

– What if you want to monitor user-specific data?

• memory drains on your server –

• The answer to both is the same – Use session listeners!

37

Using HttpSessionListener

1. Implement the HttpSessionListener interface.

2. Override sessionCreated and sessionDestroyed. – sessionCreated is triggered when a new session is created.

– sessionDestroyed is triggered when a a session is destroyed. Thisdestruction could be due to an explicit call to the invalidatemethod or because the elapsed time since the last client access

exceeds the session timeout. – Multithreaded access is possible. Synchronize if necessary.

3. Obtain a reference to the session and possibly to

– Each of the two HttpSessionListener methods takes anHttpSessionEvent as an argument. The HttpSessionEvent class hasa etSession method that rovides access to the session ob ect. You almost always want this reference; you occasionally alsowant a reference to the servlet context. If so, first obtain thesession object and then call getServletContext on it.38



Using HttpSessionListener

• Use the objects. –

setAttribute. Do this in sessionCreated if you want to guarantee that

all sessions have a certain attribute. – Wait! What about getAttribute? Nope. In sessionCreated, there is

nothing in the session yet, so getAttribute is pointless. In addition,all attributes are removed before sessionDestroyed is called, socalling getAttribute is also pointless there. If you want to clean up

,attributeRemoved method of HttpSessionAttributeListener. So,sessionDestroyed is mostly reserved for listeners that are simplykeeping track of the number of sessions in use.

• Declare the listener. – In web.xml or the TLD file, use listener and listener-

class to list fully qualified name of listener class, as below.

<listener-class>package.SomeListener</listener-class></listener>

39

Example: Tracking Active

public class SessionCounter

private int totalSessionCount = 0;

private int currentSessionCount = 0;

=

private ServletContext context = null;

totalSessionCount++;

currentSessionCount++;

if currentSessionCount > maxSessionCount

maxSessionCount = currentSessionCount;

}

if (context == null) {

storeInServletContext(event);

}

}40




currentSessionCount--;

}

…

// Register self in the servlet context so that

// counts.

rivate void storeInServletContext Htt SessionEvent

event) {

HttpSession session = event.getSession();

context = session.getServletContext();

context.setAttribute("sessionCounter", this);

}

}41

Problem: How to Test Session

• Disable cookies, then use frame-based page that

<FRAMESET ROWS="*,*,*,*" COLS="*,*,*,*">

<FRAME SRC="test. s "><FRAME SRC="test. s ">

<FRAME SRC="test.jsp"><FRAME SRC="test.jsp">


<FRAME SRC="test.jsp"><FRAME SRC="test.jsp"><FRAME SRC="test.jsp"><FRAME SRC="test.jsp">



< = es . sp >< = es . sp >

<NOFRAMES><BODY>

This example requires a

- .

</BODY></NOFRAMES>

</FRAMESET>42




43

Watching for Changes

• HttpSessionListener uses: – Lets you detect when a session is created or destroyed.

• HttpSessionListener deficiencies: – nce sess on attr utes are remove e ore sess on

destruction, this listener does not let you clean up

attributes that are in destroyed sessions.• Solution:

– HttpSessionAttributeListener interface.

44



Using

1. Implement HttpSessionAttributeListener.

2. Override attributeAdded, attributeReplaced, and

attributeRemoved. – attributeAdded is tri ered when a new attribute name is first

added to a session.

– attributeReplaced is triggered when a new value is assigned to anexisting name. attributeAdded is not triggered in this case. The oldvalue is obtained via event.getValue and the new value is obtainedvia session.getAttribute.

– attributeRemoved is triggered when a session attribute is removedaltogether. This removal can be due to an explicit programmer callto removeAttribute, but is more commonly due to the system

removing all attributes of sessions that are about to be deletedecause t e r t meout exp re .

45

Using

3. Obtain references to the attribute name, attribute, , .

– The HttpSessionAttributeListener methods take anHttpSessionBindingEvent as args. HttpSessionBindingEvent hasthree useful methods: getName (name of attribute that waschanged), getValue (value of changed attribute—new value for attributeAdded and previous value for attributeReplaced andattributeRemoved), and getSession (the HttpSession object). If you

,call getServletContext on it.

4. Use the objects. – The attribute name is usuall com ared to a stored name to see if it

is the one you are monitoring. The attribute value is used in anapplication-specific manner. The session is usually used to read

previously stored attributes (getAttribute) or to store new or .

5. Declare the listener. – Use listener and listener-class in web.xml as before.

46



Example: Monitoring Yacht

public class YachtWatcher

private String orderAttributeName

= "orderedItem";

private String purchaseAttributeName

= "purchasedItem";

private String itemName = "yacht";

47


/** Checks for initial ordering and final purchase of

* acht. Records "Customer ordered a acht" if the

* orderedItem attribute matches "yacht".

* Records "Customer finalized purchase of a yacht" if the

* purchasedItem attribute matches "yacht".

*/

public void attributeAdded(HttpSessionBindingEvent event){

, , ,

" ordered a ");

checkAttribute(event, purchaseAttributeName, itemName,

" finalized purchase of a ");

}

48




/** Checks for order cancellation: was an order for "yacht"

* cancelled? Records "Customer cancelled an order for

* a yacht" if the orderedItem attribute matches "yacht".

*/

public void attributeRemoved(HttpSessionBindingEvent event)

{

checkAttribute(event, orderAttributeName, itemName,

" "

}

49


/** Checks for item replacement: was "yacht" replaced

* b some other item? Records "Customer chan ed to a new

* item instead of a yacht" if the orderedItem attribute

* matches "yacht".

*/

public void attributeReplaced

(HttpSessionBindingEvent event) {

, , ,

" changed to a new item instead of a ");

}

50




private void checkAttribute

,

String orderAttributeName,

String keyItemName,

String currentAttributeName = event.getName();

String currentItemName = (String)event.getValue();

&& currentItemName.equals(keyItemName)) {

ServletContext context =

event. etSession . etServletContext

context.log("Customer" + message +

keyItemName + ".");

}

}

}

51


Log File

2001-11-07 11:50:59 Customer ordered a yacht.

2001-11-07 11:51:06 Customer changed to a new item instead of a yacht.

2001-11-07 11:52:37 Customer cancelled an order for a yacht.

2001-11-07 11:53:05 Customer finalized purchase of a yacht.


2001-11-07 11:53:50 Customer cancelled an order for a yacht.



- - : : us omer cance e an or er or a yac .



52



Using Multiple Cooperating

• Some tasks cannot be accomplished withany one listener type

– A combination is required

• ,purchases of the current daily special – -

information about the session attributes that store dailyspecials

– erv e on ex r u e s ener o mon or c anges othe attributes that store the information

– Htt SessionAttributeListener to kee a runnin count of orders for the daily special.

53


54




55

Summary

• Available listeners – Servlet context listeners.

• Notified when servlet context is initialized and destroyed.

– .• Notified when context attributes are

added/removed/replaced

– ess on s eners.• Notified when sessions are created, invalidated, or timed

out.

– Session attribute listeners.• Notified when session attributes are

added/removed/replaced

• Steps – Implement interface, override methods, access the

56



© 2009 Marty Hall

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06 Lifecycle Listeners