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Ch03 Servlets - Objectives
• Introduce concepts of Java Servlet Web
components
• Support Environments for Java Servlets
• Discuss the functionality of Java Servlets
• Discuss the Java Servlet API
• Discuss the Java Servlet Debugging
• Provide step-by-step tutorials on building,
deploying, and using Java Servlet
components
Java Servlets
• Java Servlets technology provides an HTTP based
request and response paradigm on Web servers.
Java Servlets can handle generic service requests
and respond to the client’s requests.
• Applications include embedded system, wireless
communication, and any other generic
request/response application.
• Java Web HTTP Servlets take HTTP requests,
process them, and respond to the client with
process results via HTTP protocol.
Java Servlets (cont.)
• A Servlet object is supported by a Servlet container,
which is supported by an HTTP Web server.
• The Servlet container is responsible of managing the
life cycle of a Servlet object. Functions of the Servlet
container include taking input requests from clients;
instantiating an instance of the Servlet class; passing
the requests to the Servlet object and letting the
Servlet object process the requests; forwarding the
results to clients.
• The results returned to the client by a Servlet Web
component may be in a format of TEXT/HTML,
TEXT/XML, TEXT/PLAIN, IMAGE/JPEG, or in other
binary types of formats.
Java Servlets (cont.)
• A Servlet component can delegate the requests to
its back-end tier such as a database management
system, RMI, EAI, or other Enterprise Information
System (EIS). In this case the Servlet plays a role of
the middle tier in a 3-tier architecture.
• A Servlet is deployed as a middle tier just like other
Web component such as JSP components. The
Servlet Web components are always parts of the
Web application.
• For example, in J2EE, Servlet Web components and
Java Server Pages (JSP) Web components can be
deployed in a .WAR file; EJB components can be
deployed in a .JAR file
Java Servlets (cont.)
• All together, an Enterprise application can be
deployed in an .EAR file, which consists of .JAR
files and .WAR files.
• The Servlet components are building block
components which always work together with other
components such as JSP components, JavaBean
components, Enterprise Java Bean (EJB)
components, and Web service components.
• A Servlet component is also a distributed
component, which can provide services to remote
clients and also access remote resources.
Support Environments for Java Servlets
• A Java Servlet application is supported by its
Servlet container. The container may be an add-on
Servlet container or standalone container, which
comes as a part of a Web server.
• Since a Java Servlet itself is a Java class. It needs
Java API support, specifically the Java Servlet API
that is available in an archive file called servletapi.jar in Tomcat.
Support Environments for Java Servlets
• The Apache Tomcat web server is the official
reference implementation of Servlet containers,
supporting Servlets and JSP.
• Tomcat itself can be a standalone Web server and
can also be an add-on Servlet/JSP
engine/container for other web servers.
• Tomcat 5.x is the newest edition at the time of this
writing, which supports Servlet 2.4 and JSP 2.0.
Support Environments for Java Servlets
• The Tomcat Web server is an open source Servlet
container originally developed by Sun
Microsystems.
• There are many other Web servers supporting
Servlets and JSP, such as Sun’s Java Web server,
and Macromedia s JRun, Caucho Resin, and Jetty.
• Many application servers, like Sub Java System
Application Server, BEA WebLogic and IBM
WebSphere, Oracle Application Server, Pramati
Server, JBoss also support Servlets and JSP.
Web Server Configuration (server.xml)
• An XML format file called server.xml is used to
control and configure the behavior and setting of the
Tomcat Web server. This file is located in the conf
subdirectory of the Tomcat installation directory.
• Some common changes needed in the configuration
of Tomcat Web server may be:
• 1. Reset the server port number where the Servlet
class or other Web component will listen for requests.
• <Connector port=”80” … />
• where 8080 is the initial port number. It is replaced
by 80 to make it much more convenient for clients to
access Servlets since 80 is the default HTTP port.
Web Server Configuration (server.xml)
2. Turn on the Servlet reloading so that you don’t need to reload the
recompiled Servlet. <DefaultContext reloadable=”true”/> is inserted in
the Service tag.
A sample of server.xml.
<Server port="8005" shutdown="SHUTDOWN" debug="0">
<Service name="Catalina">
<Connector port="80" maxThreads="150" minSpareThreads="25“
maxSpareThreads="75" enableLookups="false"
redirectPort="8443" acceptCount="100" debug="0"
connectionTimeout="20000" disableUploadTimeout="true" />
<Engine name="Catalina" defaultHost="localhost" debug="0“>
<Host name="localhost" debug="0" appBase="webapps"
unpackWARs="true" autoDeploy="true">
. . .
<DefaultContext reloadable="true" />
<Context path="" docBase="ROOT" debug="0" />
</Host>
</Engine>
</Service>
</Server>
Java Servlet Deployment Descriptor
(web.xml)
• An XML format file called web.xml in WEB-INF subdirectory
of your Web application directory is used to control and
configure the behavior and setting of Java Servlets in your
specific Web application. There is another server-wide
web.xml in the same place as server.xml.
• The server-wide configurations will apply to all Web
components on the server. As each application is deployed,
this file is processed first, followed by the "/WEBINF/web.xml" deployment descriptor from your own
applications. The application specific resource
configurations should go in the "/WEB-INF/web.xml" in
your application.
• A sample Web server-wide web.xml for Tomcat 5.
Java Servlet Deployment Descriptor
(web.xml) (cont.)
web.xml example “myPackage”
<?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>Welcome to Tomcat</display-name>
<description>Welcome to Tomcat</description>
<servlet>
<servlet-name>myServlet</servlet-name>
<servlet-class>myPackage.myServlet</servlet-class>
Java Servlet Deployment Descriptor
(web.xml) (cont.)
<init-param>
<param-name>key1</param-name>
<param-value>value1</param-value>
</init-param>
<init-param>
<param-name>key2</param-name>
<param-value>value2</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>myServlet</servlet-name>
<url-pattern>/GetMyServlet</url-pattern>
</servlet-mapping>
</web-app>
A common change is to modify or add a servlet mapping.
Basics of Java Servlets
• The Java Servlet is a server-side Web component
which takes a HTTP request from a client, handles
it, talks to a database, talks to a JavaBean
component, and responds a HTTP response or
dispatch the request to other Servlets or JSP
components.
• Servlets can dynamically produce text-based HTML
markup contents and binary contents as well based
on the client’s request. Since a Servlet is a Web
component it needs be deployed in a Servlet
supporting web server with its deployment
descriptor.
Java Servlet architecture
• A Java Servlet is just a typical Java class which
extends an abstract class HttpServlet.
• The HttpServlet class extends another abstract
class GenericServlet . The GenericServlet class
implements three interfaces: javax.servlet.Servlet,
javax.servlet.ServletConfig, and
java.io.Serializable.
Java Servlet
Class Hierarchy