Web Application Architecture

Web Application
Architecture
(based J2EE 1.4 Tutorial)

1

In this session, I am going to talk about what a typical Web application
looks like. I will also show you how to build a Web application by going
through step by step guideline. Most of the contents in this presentation is
made from “Chapter 4: Web Application” tutorial section of Java WSDP.

1

Disclaimer & Acknowledgments
? Even though Sang Shin is a full-time employee of
Sun Microsystems, the contents here are created
as his own personal endeavor and thus does not
reflect any official stance of Sun Microsystems.
? Sun Microsystems is not responsible for any
inaccuracies in the contents.
? Acknowledgments
– The slides of this presentation are made from “Web
Application” section of J2EE 1.4 tutorial written by Sun
Microsystems

2

2

Agenda
? Web application, components and Web container
? Technologies used in Web application
? Web application development and deployment
steps
? Web Application Archive (*.WAR file)
– *.WAR directory structure
– WEB-INF subdirectory
? Configuring Web application
– Web application deployment descriptor (web.xml file)

3

So this is the list of topics we are going to talk about in this session.

First, we will define a few terms: “web application”, “web components” and “web
container”. Next, we will talk about the steps you will follow to develop and
deploy a web application. Then we will talk about “Web Application Archive”,
which is called WAR file in short. We will look into the directory structure of a
WAR file especially a meta sub-directory called WEB-INF.

A Web application has a accompanying configuration file called web.xml. This
file is referred to as the deployment descriptor of the web application and contains
all the configuration information of the application that web container needs to
know in order to deploy and run the application properly. Understanding the
web.xml file is important since you are going to have to construct web.xml file as
part of development of your web application.

Then we will go through some life-cycle operations that you can perform over
your web applications, for example, installing and deploying, listing all web
applications that are currently running on the target web container and updating,
removing, undeploying them.

3

Web Application &
Web Components &
Web Container

4

So let's understand a few terms.

4

Web Components & Container
Applet Container Web Container EJB Container

Applet HTTP/ JSP Servlet RMI EJB
HTTPS

J2SE

RMI/IIOP

RMI/IIOP
JavaMail JavaMail

JDBC

JDBC
JMS

JTA

JNDI
JNDI

JMS
JTA
App Client JAF
Container JAF
App HTTP/ J2SE
Client HTTPS

RMI
RMI/IIOP

JDBC
JNDI
JMS

J2SE J2SE

Database
5

I talked about the container and component model of J2EE architecture during the
J2EE overview session. Under component and container architecture of J2EE, the
business logic (typically as EJB's), contents or presentations (typically as web-tier
components such as Servlet and JSP components) are captured in the form of
components and those components are then run within corresponding host execution
environment called containers. (I am using the term “components” a bit loosely here
for Servlet and JSP since they are not really components in a strict sense.)

In the web-tier container, the components are constructed in the form of web
components and web components can be either in the form of servlet or JSP pages
while in the EJB-tier container, the components are constructed in the form of EJB
beans.

5

Web Components & Container
? Web components are in the form of either
Servlet or JSP (along with JavaBean's and
custom tags)
? Web components run in a Web container
– Tomcat and Resin are popular web containers
– All J2EE compliant app servers (Sun Java System
App Server) provide web containers
? Web container provides system services to
Web components
– Request dispatching, security, and life cycle
management
6

So just to repeat what I just said, web components are in the form of either servlet or
JSP pages. And web components run in a web container. And there are a few popular
web containers out there, for example, Tomcat and Resin are two most popular web
containers.

I said that containers provide host execution environment for the components providing
some system services. For web components, the web container provides several system
services, for example, receiving service requests from clients and then dispatching them
to appropriate web components, and life cycle management of your components. For
example, the web container can figure out when it is the time to create an instance of
servlet class to handle the incoming service request and when to remove the instance.

6

Web Application & Components
? Web Application is a deployable package
– Web components (Servlets and JSP's)
– Static resource files such as images
– Helper classes
– Libraries
– Deployment descriptor (web.xml file)
? Web Application can be represented as
– A hierarchy of directories and files (unpacked form) or
– *.WAR file reflecting the same hierarchy (packed form)

7

OK. We have talked a bit on what a web component is, and how web components run
in a web container. Now let's talk about a web application. A web application is
basically a collection of everything that constitutes a deployable package. The
deployable package contains multiple web components, that is, multiple servlets and
JSP pages, and static resource files such as images that are part of displayable HTML
pages, helper classes that are used by the web components, and library classes and
finally deployment descriptor (web.xml file).

A Web application is defined as a hierarchy of directories and files in a standard layout
as specified in Servlet specification. Such a hierarchy can be accessed in its unpacked
form, where each directory and file exists in the file system separately, or in a packed
form known as a Web Application Archive, or WAR file. The former format is more
useful during development, while the latter is used when you distribute your application
to the target platform.

7

Technologies Used
In Web Application

8

.

8

Web Request Handling

9

In the Java 2 platform, web components provide the dynamic extension capabilities for
a web server. Web components are either Java servlets, JSP pages, or web service
endpoints. The interaction between a web client and a web application is illustrated in
the figure of the slide.

The client sends an HTTP request to the web server. A web server that implements Java
Servlet and JavaServer Pages technology converts the request into an
HTTPServletRequest object. This object is delivered to a web component, which can
interact with JavaBeans components or a database to generate dynamic content. The
web component can then generate an HTTPServletResponse or it can pass the request to
another web component. Eventually a web component generates a
HTTPServletResponse object. The web server converts this object to an HTTP response
and returns it to the client.

9

Java Web Application Technologies

10

Servlets are Java programming language classes that dynamically process requests and
construct responses. JSP pages are text-based documents that execute as servlets but
allow a more natural approach to creating static content. Although servlets and JSP
pages can be used interchangeably, each has its own strengths. Servlets are best suited
for service-oriented applications (web service endpoints are implemented as servlets)
and the control functions of a presentation-oriented application, such as dispatching
requests and handling nontextual data. JSP pages are more appropriate for generating
text-based markup such as HTML, Scalable Vector Graphics (SVG), Wireless Markup
Language (WML), and XML.

Since the introduction of Java Servlet and JSP technology, additional Java technologies
and frameworks for building interactive web applications have been developed. These
technologies and their relationships are illustrated in the figure.

10

Web Application
Development and
Deployment Steps
(using hello2 example under
J2EE 1.4 tutorial)

11

Now let's go over the steps needed for the development and deployment of
a web application.

11

Web Application Development
and Deployment Steps
1.Write (and compile) the Web component code
(Servlet or JSP) and helper classes referenced
by the web component code
2.Create any static resources (for example,
images or HTML pages)
3.Create deployment descriptor (web.xml)
4.Build the Web application (*.war file or
deployment-ready directory)
5.Deploy the web application into a Web
container
? Web clients are now ready to access them via URL 12

So this is the list of steps you will follow in order to develop and deploy a web
application. First, you will write and compile the web component code. Web
component code is either servlet Java code or JSP pages and helper classes that are used
by the web components. You also need to create any static resources such as images or
HTML pages. Next, you will create deployment descriptor, that is, the web.xml file.

Then you will build web application. As was mentioned, a web application can be
either in the form of *.war file (packed form) or laid-out form of *.war file (unpacked
form) as was discussed in previous slide. A *.war file will be a compressed file with the
same layout as the build directory.

Then you will deploy the web application to the web container. We will discuss the
difference between “installation” and “deployment” later on. (There is a fine distinction
between the two.)

Once a web application is deployed, then client can access the web components through
the HTML browser. So let's go over each of these steps one by one.

12

1. Write and compile the Web
component code
? Create development tree structure
? Write either servlet code or JSP pages
along with related helper code
? Create build.xml for Ant-based build (and
other application development life-cycle
management) process
? IDE (i.e. NetBeans) handles all these
chores
13

So the first step is to write and compile web component code. Now just like any other
application development, you might want to create development directory structure that
fits your need. As we will see in the following slide, people have recommended
directory structure they found suitable for web application development. (This is only a
recommendation, of course.)

Once you create an appropriate development directory structure, you can then write
servlet or JSP along with related helper classes and other pieces. Now another thing
people find quite useful is to use ANT build tool for the development and deployment
tasks of web applications. As we will see later on, ANT is basically platform
independent make utility. And the file that contains the build instructions that ANT
uses is called build.xml file.

13

Development Tree Structure
? Keep Web application source separate from
compiled files
– facilitate iterative development
? Root directory (example from hello2 sample
code from J2EE 1.4 tutorial)
– build.xml: Ant build file
– src: Java source of servlets and JavaBeans
components
– web: JSP pages and HTML pages, images

14

In order to facilitate iterative development/deployment and also in order to keep source
code separate from compiled files, the source code for the tutorial examples is stored in
the following structure under each application directory, for example, for hello1 web
application:

? build.xml - Ant build file
? context.xml - Optional application configuration file
? src - Java source of servlets and JavaBeans components

? web - JSP pages and HTML pages, images

The Ant build files (build.xml) distributed with the examples contain

*targets for creating an unpacked WAR structure in the build subdirectory of the hello1
application
*targets for copying and compiling files from the src directory to the build directory
*targets for invoking the web container's manager commands to install, reload,
remove, deploy, and undeploy applications.

14

Example: hello2 Tree Structure
(before “ant build” command)
? Hello2
– src/servlets
? GreetingServlet.java

? ResponseServlet.java

– web
? WEB-INF

– web.xml
? duke.waving.gif

– build.xml
15

So this is an example directory structure of hello1 web application you find in the Java
WSDP. There are two servlet Java source files. And there is a web directory called web
which contains WEB-INF directory and other static resource files.

You will code each web component in the src directory.

15

2. Create any static resources
? HTML pages
– Custom pages
– Login pages
– Error pages
? Image files that are used by HTML pages or
JSP pages
– Example: duke.waving.gif

16

The next step is to create any static resources that will be used by the web components.
And they include some static HTML pages or image files that are used by the HTML
pages or JSP pages. And an example of image is duke that you see quite often in
various Java-related web pages.

Static resources such as image file are stored in the application's web directory.

16

3. Create deployment
descriptor (web.xml)
? Deployment descriptor contains
deployment time runtime instructions to
the Web container
– URL that the client uses to access the web
component
? Every web application has to have it

17

The next step is to create deployment descriptor, web.xml file. By the way, the name of
this file has to be web.xml file. Now web.xml file contains deployment time and
runtime instructions to the web container. For example, you can specify the URN that
clients of your web applications will use to access your web components. (We will talk
about this URN later on in detail.) Every web application has to have a web.xml file.

The web.xml file is coded and stored in the application's webWEB-INF directory.

17

4. Build the Web application
? Either *.WAR file or unpacked form of
*.WAR file
? Build process is made of
– create build directory (if it is not present) and its
subdirectories
– compile Java code into build/WEB-INF/classes
directory
? Java classes reside under ./WEB-INF/classes
directory
– copy web.xml file into build/WEB-INF directory
– copy image files into build directory
18

Once you have all the pieces, you can create a web application. As mentioned before, a
web application is basically a deployable package that contains everything that is
needed for the deployment.

As mentioned before, a web application is either in the form or *.war file (packed) or
laid out form of the war file (unpacked). Typically a build process for the web
application involves creating “build” directory which functions as the root directory for
the compile time generated pieces such as java classes and pieces that are needed to
create *.war file such as web.xml file.

For example, the compiled Java classes will end up residing in “build/WEB-
INF/classes” directory and web.xml will be copied into “build/WEB-INF” directory and
the image files will be copied into the “build” directory itself. Basically the “build”
directory is the reflection of directory structure that a particular web container expects
(in unpacked form).

18

Example: hello2 Tree Structure
(after “asant build” command)
? Hello1
– src
– web
– build.xml
– build
? WEB-INF

– classes
? GreetingServlet.class

? ResponseServlet.class

– web.xml
? duke.waving.gif
19

So this is an example of hello1 web application. Once the build process is completed
via “ant build” command, “build” directory gets created if it is not present before, and
GreetingServlet.class and ResponseServlet.class class files are put into “classes”
subdirectory of WEB-INF directory and web.xml file is copied from “web” directory to
“build/WEB-INF” directory and image file is copied from “web” directory into “build”
directory. Now the build directory is now ready to be taken either as a source directory
for creating *.war file (packed form) or can be copied over to web container (unpacked
form) for deployment.

19

5. Deploy Web application

? Deploy the application over deployment
platform such as Sun Java System App Server
or Tomcat
? 3 ways to deploy to Sun Java System App
server
– asadmin deploy --port 4848 --host localhost –
passwordfile "c:j2eetutorial14examplescommonadmin-
password.txt" --user admin hello2.war (asant deploy-war)
– App server admin console
– NetBeans

20

Once you built a ready-to-deployable web application, you can now deploy the
application over the web container.

20

6. Perform Client Access to
Web Application
? From a browser, go to URL of the Web
application

21

Once your web application has been deployed, then a client (typically a browser) can try
to access the web resources by specifying the appropriate URN address.

21

http://localhost:8080/hello1/greeting

22

So for the hello1 example web application, a client will specify
http://localhost:8080/hello1/greeting to access the hello1 web resource.

22

Running Web Application

23

This is response html page that gets displayed as a result of the web resource
handling of the first HTTP request that comes from the client.

23

Web Application
Archive (*.WAR)

24

OK, now let's talk about WAR file.

24

Web Application
? Web application can be deployed in two
different forms
– a *.war file or
– an unpacked directory laid out in the same
format as a *.war file (build directory)
? Use *.war file when you have to deploy on
a remote machine
– asant deploy-war command

25

As was mentioned a couple of times already, a web application can be installed or
deployed in two different forms, either as a *.war file (packed form) or unpacked
directory that reflects the same directory structure of the *.war file.

Now when you “install”, you can use either form while when you do “deploy” you can
use only *.war file. And I will explain why this is the case later on.

25

What is *.WAR file?
? Ready to deploy'able package over web
container
? Similar to *.jar file
? Contains things to be deployed
– Web components (servlets or JSP's)
– Server-side utility classes
– Static Web presentation content (HTML, image,
etc)
– Client-side classes (applets and utility classes)
? Reflects contents in build directory
26

So what is war file? It is a ready-to-deployable package over web container. It is like
*.jar file and obviously contains things that need to be deployed over the container such
as web service components (servlets, JSP pages), server side helper or utility classes,
static resources, and sometimes applet classes that would be needed by the client.

As was mentioned before, it reflects the contents of the “build” directory we talked
about.

26

Document Root & Context
? Document Root of the Web application
– Top-level directory of WAR
– Contains JSP pages, client-side classes and
archives, and static Web resources are stored
– Also contains WEB-INF directory
? A context is a name that gets mapped to
the document root of a Web application
– /hello1 is context for hello1 example
– Distinguishes a Web application in a single Web
container
– Has to be specified as part of client URL 27

Now let's talk about *.war file's directory structure a bit. The top level directory of the
war file is called “document root” of the web application. And the “document root”
directory contains JSP pages, client side classes and archive file such as applet code and
static web resources.

The document root also contains WEB-INF directory.

Now one terminology you have heard many times (but maybe were not sure what it
exactly is) is what is called “web application context” or “context” in short. Basically
the context is the name of the document root of your web application. For example,
/hello1 is the context name of hello1 example. Context is a way to distinguish various
web applications in a single web container.

The context has to be specified as part of “request path” that is specified by the client.
(We will talk about “request path” later on in this presentation.)

27

Directory Structure of
*.WAR file

28

This picture shows the directory structure of *.war file. Please note that the
“WebApplicationName” directory reflects the Document root, thus the context.

28

Directory Structure of
*.WAR file

29

.

29

How to Create *.WAR file?
? 3 different ways
– Use IDE (NetBeans)
– Use ant tool after putting proper build
instruction in build.xml file
? “asant create-war” (under J2EE 1.4

tutorial)
– Use “jar cvf <filename>.war .” command
under build directory

30

So how do you create a *.war file? There are 3 different ways to create *.war file. If
you are using an IDE, creating and deploying *.war file gets handled via one or two
clicks.

For J2EE 1.4 tutorial sample applications, you can use “asant create-war” command
with proper build scripts. Or you can use “jar” command to create the war file. Jar
command in this case is like zip utility.

30

Example: Creating hello2.war via
“asant create-war” command
C:j2eetutorial14exampleswebhello2>asant create-war
Buildfile: build.xml
...
create-war:
[echo] Creating the WAR....
[delete] Deleting:
C:j2eetutorial14exampleswebhello2assemblewarhello2.war
[delete] Deleting directory
C:j2eetutorial14exampleswebhello2assemblewarWEB-INF
[copy] Copying 1 file to
C:j2eetutorial14exampleswebhello2assemblewarWEB-INF
[copy] Copying 2 files to
C:j2eetutorial14exampleswebhello2assemblewarWEB-INFclasses
[war] Building war:
C:j2eetutorial14exampleswebhello2assemblewarhello2.war
[copy] Copying 1 file to C:j2eetutorial14exampleswebhello2 31

31

Example: Creating hello2.war via
jar command
C:j2eetutorial14exampleswebhello2build>jar cvf hello2.war .
added manifest
adding: duke.waving.gif(in = 1305) (out= 1295)(deflated 0%)
adding: servlets/(in = 0) (out= 0)(stored 0%)
adding: servlets/GreetingServlet.class(in = 1680) (out= 887)(deflated 47%)
adding: servlets/ResponseServlet.class(in = 1090) (out= 572)(deflated 47%)

C:j2eetutorial14exampleswebhello2build>jar xvf hello2.war
created: META-INF/
extracted: META-INF/MANIFEST.MF
extracted: duke.waving.gif
created: servlets/
extracted: servlets/GreetingServlet.class
extracted: servlets/ResponseServlet.class

32

So in this example, I am creating hello2.war file using “jar” command. Basically you
“cd” to “build” directory and then use “jar” command to pick up all the pieces
underneath it into the “hello2.war” file. Please don't forget the dot(.) at the end in this
example.

In the lower part of the slide, I use “jar tvf” command just to show the contents of the
newly created *.war file.

32

WEB-INF Directory
? Subdirectory of Document root
? Contains
– web.xml : Web application deployment descriptor
– JSP tag library descriptor files
– Classes : A directory that contains server-side
classes: servlets, utility classes, and JavaBeans
components
– lib : A directory that contains JAR archives of
libraries (tag libraries and any utility libraries
called by server-side classes)
33

Now what is this WEB-INF directory? WEB-INF directory contains web.xml file, JSP
tag library descriptor files (we have not learned this yet so don't worry about it if you
don't understand), and “classes” directory that contains servlet classes and “lib”
directory that contains various jar files that could contain Java classes that are used by
the web components and their helper classes.

33

HTTP request URL & Web
component URL (alias) & Context
? Request URL: User specified access point of
a web resource
– http://[host]:[port]/[request path]?[query string]
– [request path] is made of context and web component's
URL
– http://localhost:8080/hello1/greeting?username=Monica
? Context: Name of the root document of a web
application – Identifies a particular application
on that server
– /hello1 is context 34

Now I would like to talk about how a client can access the web service and explain
some related terms.

First “request URL” is user specified access point of a web resource. This is something
that is being passed to the web components as part of HTTP request. The typical
request URL is made of

http://[host]:[port]/[request path]?[query string]

The [request path] is made of “context” of the web application and the “URN of the web
component”. For example, in the case hello1 example, “/hello1” is context and
“/greeting” is the URN of the web component. So the [request path] will look like
“/hello1/greeting”. Now as mentioned before, context is the name of the root document
of the web application. And as a web application developer/deployer, you can choose
any name as context name for your application. The context name is specified in the
web.xml deployment descriptor.

34

Configuring
Web Application via
web.xml

35

Now let's talk about how to configure your web application using web.xml
deployment descriptor.

35

Configuring Web Application
? Configuration information is specified in
web.xml (Web Applications Deployment
Descriptor)

36

Web applications are configured via elements contained in Web application deployment
descriptor (web.xml file). You can either manually create web.xml using a text editor
of your choice or use a tool.

So let's go over some configuration elements that can be specified in web.xml file.

36

Web Applications Deployment
Descriptor (web.xml)
? Prolog
? Alias Paths
? Context and Initialization Parameters
? Event Listeners
? Filter Mappings
? Error Mappings
? Reference to Environment Entries, Resource
environment entries, or Resources

37

So this is the list of things that can constitute web.xml file. The most important
configuration information that you need to understand is what is called alias paths,
which define which URL is mapped to which web components.

37

Web Applications Deployment
Descriptor (web.xml)
? Case sensitive
? Order sensitive (in the following order)
– icon, display-name, description, distributable
– context-param, filter, filter-mapping
– listener, servet, servlet-mapping, session-config
– mime-mapping, welcome-file-list
– error-page, taglib, resource-env-ref, resource-ref
– security-constraint, login-config, security-role
– env-entry, ejb-ref, ejb-local-ref
38

Not only elements in web.xml are case sensitive, they are also order-sensitive. In other
words, the elements in web.xml file has to be defined in a specific order that is specified
in the Servlet specification. This slide shows the ordered list of configuration elements
if they are present in the web.xml (many of them are optional).

38

Prolog (of web.xml)
? Every XML document needs a prolog
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD
Web Application 2.3//EN" "http://java.sun.com/dtd/web-
app_2_3.dtd">

39

Since the deployment descriptor is an XML document, it requires a prolog. The prolog of
the web.xml file is as follows:

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web
Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">

39

Alias Paths (of web.xml)
? When a request is received by Servlet container,
it must determine which Web component in a
which web application should handle the request.
It does so by mapping the URL path contained in
the request to a Web component
? A URL path contains the context root and alias
path
– http://<host>:8080/context_root/alias_path
? Alias Path can be in the form of either
– /alias-string (for servlet) or
– /*.jsp (for JSP)
40

When a request is received by Tomcat it must determine which Web component should
handle the request. It does so by mapping the URL path contained in the request to a
Web component. A URL path contains the context root and an alias path

http://<host>:8080/context_root/alias_path

Before a servlet can be accessed, the Web container must have least one alias path for
the component. The alias path must start with a / and end with a string or a wild card
expression with an extension (*.jsp, for example). Since Web containers automatically
map an alias path that ends with *.jsp, you do not have to specify an alias path for a JSP
page unless you wish to refer to the page by a name other than its file name.

40

Alias Paths (of web.xml)
<servlet>
<servlet-name>greeting</servlet-name>
<display-name>greeting</display-name>
<description>no description</description>
<servlet-class>GreetingServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>response</servlet-name>
<display-name>response</display-name>
<description>no description</description>
<servlet-class>ResponseServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>greeting</servlet-name>
<url-pattern>/greeting</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>response</servlet-name>
<url-pattern>/response</url-pattern>
</servlet-mapping> 41

So in order to set up mappings of Hello application in the Web deployment descriptor,
you must add “servlet” and “servlet-mapping” elements to the web.xml file. (By the
way, to define an alias for a JSP page instead of a servlet, you must replace the servlet-
class subelement with a jsp-file subelement in the servlet element.)

41

Context and Initialization
Parameters (of web.xml)
? Represents application context
? Can be shared among Web components in a WAR file
<web-app>
...
<context-param>
<param-name>
javax.servlet.jsp.jstl.fmt.localizationContext
</param-name>
<param-value>messages.BookstoreMessages</param-value>
</context-param>
...
</web-app>

42

The Web components in a WAR share an Java object that represents their application
context, which is referred to as context object. And you can set name-value pairs to the
context object so that all web components in the same web application can have access
to the name-value pairs that are saved in the context object. In order to initialize the
name-value pairs, you add a “context-param” or “init-param element” to the web.xml
file. The “context-param” is a sub-element of the top-level web-app element. The “init-
param” is a subelement of the servlet element. This slide shows an element used to
declare a context parameter that sets the resource bundle used in the example discussed
in Chapter 16 of Java WSDP Servlet tutorial:

42

Event Listeners (of web.xml)
? Receives servlet life-cycle events

<listener>
<listener-class>listeners.ContextListener</listener-class>
</listener>

43

You can also specify life cycle events, which we will talk about when we talk about
“Servlet” later on in this course. (If you don't understand life cycle events for now, that
is fine.)

Life-cycle events are only specified in the deployment descriptor if you have them
coded as part of your application.

43

Filter Mappings (of web.xml)
? Specify which filters are applied to a
request, and in what order

<filter>
<filter-name>OrderFilter<filter-name>
<filter-class>filters.OrderFilter<filter-class>
</filter>
<filter-mapping>
<filter-name>OrderFilter</filter-name>
<url-pattern>/receipt</url-pattern>
</filter-mapping>

44

This is the filter mappings. Filters are software components that intercept incoming
HTTP requests and outgoing HTTP responses and perform whatever functions such as
caching, logging, compression, or transformations. And these filters can be chained and
plugged during deployment time to achieve desired behavior. And which filters ought
to be plugged in and how they get chained are specified in the deployment descriptor.

Filter and filter-mapping descriptors are only coded in the deployment descriptor if you
have servlet filter components. These are optional elements in the deployment
descriptor.

Again we will talk about “filter” when we talk about “advanced servlets”. And you
don't really have to understand filters for now.

44

Error Mappings (of web.xml)
? Maps status code retURLed in an HTTP
response to a Java programming language
exception retURLed by any Web
component and a Web resource

<error-page>
<exception-type>exception.OrderException</exception-type>
<location>/errorpage.html</location>
</error-page>

45

Error mapping information is also specified in the deployment descriptor. That is, an
application exception can be mapped into a custom error page. Or an HTTP response
can be mapped to a Java exception, like in the example above. Otherwise, a default
error page will be displayed.

45

References (of web.xml)
? Need when web components make
references to environment entries,
resource environment entries, or resources
such as databases
? Example: declare a reference to the data
source
<resource-ref>
<res-ref-name>jdbc/BookDB</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
</resource-ref>
46

Now references. When web components make references to environment entries or
resource environment entries, or resources such as databases, you can also specify
information on those in the deployment descriptor. And in this example, a declaration to
the data source is specified.

46

Example web.xml of hello2
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" version="2.4"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/
xml/ns/j2ee/web-app_2_4.xsd">
<display-name>hello2</display-name>
<servlet>
<display-name>GreetingServlet</display-name>
<servlet-name>GreetingServlet</servlet-name>
<servlet-class>servlets.GreetingServlet</servlet-class>
</servlet>
<servlet>
<display-name>ResponseServlet</display-name>
<servlet-name>ResponseServlet</servlet-name>
<servlet-class>servlets.ResponseServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>GreetingServlet</servlet-name>
<url-pattern>/greeting</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>ResponseServlet</servlet-name>
<url-pattern>/response</url-pattern>
</servlet-mapping>
</web-app> 47

So this is the complete web,xml file of the hello1 web application. Given that hello1 is
rather simple web application, the only thing its deployment descriptor specifies is
servlet and its mapping to URNs. For example, the part in the blue color specifies
information about the servlet such as name and servlet classes and the part in the red
color specify the mapping between servlet and their URNs. These URNs are the ones
that clients use when they access the web components. And when the web container
receives the URN from a client, it knows which servlet classes to invoke by looking at
the deployment descriptor's servlet mapping elements.

Other elements of the deployment descriptor and how they are used are referenced in
future classes.

47

Web Application Architecture

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