Web Applications Basics

Introduction to Web• Web features

• Clent/Server• HTTP• HyperText Markup Language• URL addresses

• Web server - a computer program that is responsible for accepting HTTP requests from clients and serving them HTTP responses

• Web application - a dynamic extension of a web or application server

Web Applications & Components• Two types of web applications:

• Presentation-oriented (HTML, XML pages)• Service-oriented (Web services)

• Web components provide the dynamic extension capabilities for a web server:• Java servlets• JSP pages• Web service endpoints

Web Application Interaction• [client] sends an HTTP request to the web server

• [web server] HTTP request HTTPServletRequest

• This object is delivered to a web component, which can interact with JavaBeans or a DB to generate dynamic content

• [web component] generates an HTTPServletResponse or pass the request to another web component

• [web server] HTTPServletResponse HTTP response

• [web server] returns HTTP response to the client

Web Application Interaction

Web Components• Servlets - Java classes that dynamically process

requests and construct responses

• JSP pages - text-based documents that execute as servlets but allow a more natural approach to creating static content

• Appropriate usage• Servlets - service-oriented applications, control

functions• JSP - generating text-based markup

(HTML, SVG, WML, XML)

Java Web Application Technologies

Java Servlet technology is the foundation of all the web application technologies

Web Containers• Web components are supported by the services of

a runtime platform called a web container

• In J2EE, a web container "implements the web component contract of the J2EE architecture“

• Web container services: • request dispatching • security • concurrency• life-cycle management• naming, transactions, email APIs

Web Container Examples• Non-commercial

• Apache Tomcat• Jetty

• Commertial• Sun Java System Application Server • BEA WebLogic Server• Oracle Application Server• WebSphere

• Open source• JBoss

Deployment• Web components have to be installed or

deployed to the web container

• Aspects of web application behaviour can be configured during application deployment

• The configuration information is maintained in a XML file called a web application deployment descriptor

Web Application Development• A web application consists of:

• Web components• Static resource files (such as images)• Helper classes and libraries

• The process for creating and running a web application is different from that of traditional stand-alone Java classes

Development Cycle1. Develop the web component code2. Develop the web application deployment

descriptor3. Compile the web application components and

helper classes referenced by the components4. Optionally package the application into a

deployable unit5. Deploy the application into a web container6. Access a URL that references the web

application

Web Modules• According to Java EE architecture and Java

Servlet Specification:

• Web components and static web content files such as images are called web resources

• A web module is the smallest deployable and usable unit of web resources

• Web module corresponds to a web application

• A web module has a specific structure

Web Module Structure• The top-level directory

of a web module is the

document root of

the application

• The document root contains:• JSP pages• client-side classes • client-side archives• static web resources

Web Module Structure

• The document root contains

a subdirectory /WEB-INF/

• web.xml: web application

deployment descriptor

• lib: JAR archives of

libraries called by

server-side classes

Web Module Structure

• classes: server-side classes: • servlets• utility classes• JavaBeans components

• tags: tag files, which are

implementations of

tag libraries

Configuring Web Applications• Web applications are configured

via /WEB-INF/web.xml file

• Configuration options:• Map URLs to web components• Set initialization parameters• Map errors to error screens• Declare welcome files• Declare resource references

Mapping URLs to Web Components• When a request is received by the web container

it must determine which web component should handle the request

• Need to add a servlet definition and a servlet mapping for each servlet to web.xml file<servlet>

<servlet-name>ServletName</servlet-name>

<servlet-class>ServletClass</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>ServletName</servlet-name>

<url-pattern>/path</url-pattern>

</servlet-mapping>

Initialization Parameters• It's possible to pass initialization parameters to the

context or to a web component

• Context parameters: <context-param>

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

<param-value>value</param-value>

</context-param>

• Servlet parameters (within servlet definition):<init-param>

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

<param-value>value</param-value>

</init-param>

Handling Errors• Web container generates default error page

• You can specify custom default page to be displayed instead

• Steps to handle errors• Create appropriate error html pages for error

conditions• Modify the web.xml accordingly

Example: Setting Error Pages<error-page>

<exception-type>exception.BookNotFoundException

</exception-type><location>/errorpage1.html</location>

</error-page><error-page>

<exception-type>exception.BooksNotFoundException

</exception-type><location>/errorpage2.html</location>

</error-page><error-page>

<exception-type>exception.OrderException

</exception-type><location>/errorpage3.html</location>

</error-page>

Example: web.xml<?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"><web-app>

<display-name>Your team project name</display-name> <description>Team N servlets</description> <context-param> <param-name>name_of_context_initialization_parameter</param-name> <param-value>value_of_context_initializtion_parameter</param-value> </context-param>

<servlet> <servlet-name>YourServlet</servlet-name> <servlet-class>YourServletClass</servlet-class>

</servlet> <servlet-mapping>

<servlet-name>YourServlet</servlet-name> <url-pattern>/YourServlet</url-pattern>

</servlet-mapping></web-app>

WAR Files• A web module can be deployed as an

unpacked file structure or can be packaged in a JAR file known as a Web Archive File

• WAR file can be created by:• executing jar command• using Ant target• using IDE (Eclipse for instance)• using Maven

Setting a Context Root• A context root identifies a web application

in a Java EE server

• The server is responsible for mapping URL’s that start with a specific prefix to the location of a web application

• Usually this is done with a web server configuration file

Using Maven & Jetty

• A convenient way to develop, build, deploy and run Web application is by using:

• Maven build tool

http://maven.apache.org/

• Jetty web server http://www.mortbay.org/

Creating Directory Structure• Maven 2 supports the notion of creating a

complete project template with a simple command

• To create Web project template need to use maven-archetype-webapp archetype

mvn archetype:create

-DgroupId=com.maven2example

-DartifactId=maven2example_webapp

-DarchetypeArtifactId=maven-archetype-webapp

Maven Web Directory Structure

<root>/src/main/webapp/

- directory structure for a WAR

Packaging• Executing the command

mvn package

creates a WAR file

Running with Jetty• It’s easy to run application by using Jetty

plugin for Maven

• Jetty is an open-source, standards-based, full-featured web server implemented entirely in Java

• First created in 1995

• Latest version 6.1.5 / July 23, 2007

Running with Jetty• Add the Jetty plugin to the pom.xml

<build>

<finalName>maven2example_webapp</finalName>

<plugins>

<plugin>

<groupId>org.mortbay.jetty</groupId>

<artifactId>maven-jetty-plugin</artifactId>

<version>6.0.1</version>

</plugin>

</plugins>

</build>

Running with Jetty• Execute mvn jetty:run command

>mvn jetty:run

[INFO] Scanning for projects...

[INFO] Searching repository for plugin with prefix: 'jetty'.

[INFO] -----------------------------------------------------

---

[INFO] Building maven2example_webapp Maven Webapp

[INFO] task-segment: [jetty:run]

[INFO] -----------------------------------------------------

...

[INFO] Starting jetty 6.0.1 ...

...

[INFO] Started Jetty Server

• Stop by Ctrl+C

Opening the ApplicationOpen your web browser to

http://localhost:8080/

Opening the ApplicationValid URL ishttp://localhost:8080/maven2example_webapp/

Resources• J2EE Tutorial “Getting Started with Web

applications”

http://java.sun.com/javaee/5/docs/tutorial/doc/bnadr.html

• Building Web Applications with Maven 2

http://today.java.net/pub/a/today/2007/03/01/building-web-applications-with-maven-2.html

• Filmiņa par Web 2 (5 minūtes)http://www.youtube.com/watch?v=6gmP4nk0EOE