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J2EE Overview
Web Programming
CSCI 4300
J2EE multi-tier architecture
• Servlet: Java class loaded into Web server
• JSP page: enhanced HTML page that is compiled into a
servlet for deployment
• JavaBean: data holding class (part of Model in MVC)
J2EE Architecture
• Applets and special clients: downloaded,
connect back to server
• Enterprise Java Beans: long-term, applicationscope data storage and business logic
Web Application Layout
Mirroring packages with directories
Web Application Descriptor,
web.xml
• Defines servlets and maps them to URL
paths
• Declares the welcome file for the app
• Specifies pages or servlets to handle
various errors
• Provides initialization parameters to
servlets
• Provides links to external resources such
as database connections
Web.xml 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>
• You can validate your web.xml using the W3C
validator service (recommended1)
• When you change your web.xml, you must ask
Chris or me to reload your web-app
Web.xml root element
<web-app>
… web application contents.
</web-app>
• Can contain:
– Servlet definitions
– Servlet paths
– Parameters
– Etc, etc.
Servlet element
<servlet>
<servlet-name>GonzoServlet</servlet-name>
<servlet-class>cs4300.Gonzo</servlet-class>
</servlet>
Or
<servlet>
<servlet-name>GonzoServlet</servlet-name>
<jsp-page>gonzo.jsp</jsp-page>
</servlet>
Servlet-mapping element
<servlet-mapping>
<servlet-name>GonzoServlet</servlet-name>
<url-pattern>/gonzo</url-pattern>
</servlet-mapping>
If the web application root is
http://ganesha.cs.uga.edu/myapp,
GonzoServlet is accessed at
http://ganesha.cs.uga.edu/myapp/gonzo
Welcome page
<welcome-file-list>
<welcome-file>
index.jsp
</welcome-file>
</welcome-file-list>
• Index.jsp is the file served at the URL
http://ganesha.cs.uga.edu/myapp
Error page elements
<error-page>
<error-code>404</error-code>
<location>notFound.jsp</location>
</error-page>
<error-page>
<exception-type>
org.xml.sax.SaxParseException
</exception-type>
<location>parseError.jsp</location>
</error-page>
Basic JSP page
<%@ page contentType="text/html; charset=UTF-8" %>
<html>
<head>
<Static HTML code…>
• Declarations
• Java expressions
• Scriptlets (chunks of Java code)
Try to minimize the mixing of HTML with
Java (violates Model 2 architecture)
Declaration, scriptlet, and
expression
<%! int i; String[] userName %>
<table border=1>
<% for (i=0;i<userName.length;i++) { %>
<tr><td><%=i+1 %> </td>
<td> <%=userName[i] %> </td></tr>
<% } %>
</table>
Equivalent HTML from previous
<table border=1>
<tr><td>1</td><td>Michael Adams</td></tr>
<tr><td>2</td><td>Dan Everett</td></tr>
<tr><td>1</td><td>Hunter Thompson</td></tr>
</table>
•
But note that we try to avoid such mixing of
Java and HTML!
Model 2 Architecture (MVC)
Helper Object Scopes
JavaBean
• Helper class that holds information
• Separates Java code from HTML
• Implements Model component of MVC
architecture
• Useful for holding session data
JSP Address Book Interface
JavaBean Example
• Store functionality: user enters name -email pair
for storage in bean
• Fetch functionality: user enters name, bean
replies with email
JSP-Bean communication
• Make <INPUT> element names in JSP
correspond to variable names in Bean
• Write Bean class with setX() and getX()
setter and getter methods
• <jsp:setProperty> sends inputs to Bean
• <jsp:getProperty> gets values from Bean
Acknowledgements
• J2EE architecture diagrams: Sun J2EE tutorial,
http://java.sun.com/j2ee/1.4/docs/tutorial/doc/index.html
• Object scopes: jGuru Java Server Pages fundamentals,
http://java.sun.com/developer/onlineTraining/JSPIntro/contents.html#JSPIntro6_3
