CMP 436

1
CMP 436

• Servlets Brief Introduction
• Fall 2006
• Department of Mathematics
• and Computer Science
• Lehman College, CUNY

2
What is a Servlet?

• Servlets are Java programs that serve as an
  mediating layer between an HTTP request of a
  client and applications in the Web server.
• A servlet is a dynamically loaded module that
  services requests from a Web server.
• In order to execute a servlet, a servlet
  container (engine) is needed.
• Clients can only invoke servlets via a servlet
  container. A servlet container is a key component
  in an application server.
• The main responsibility of a servlet container is
  to accept and processes requests that invoke
  servlets.
• A servlet runs entirely inside the a JVM (Java
  Virtual Machine) of a container.
• Temporary (persistent) servlets are activated
  when clients request their services
• Permanent servlets are active when their host
  servers are up.
• Servlets are designated as temporary and
  permanent through configurations of the hosting
  servers.

3
Components and Containers
Java VM
GC
Classloader
Object
Code-levelsecurity
Other VMservices
Object
Object
4
Components and Containers

• Components
• One or more objects that implement a
  well-defined application service
• Make use of component services
• Container
• Runtime entity that manages components and
  provides their services
• Components need to be
• Written within the contracts defined by the
  container
• APIs plus some rules related to the component
  services
• Deployed to containers
• Describe the components
• Deliver all the elements (classes, resources,
  etc.) that implement the components
• Provide instructions on how to manage them
• Assemble/package components into an application
  assembly

5
A Servlets Job

• Read explicit data sent by client (form data).
• Read implicit data sent by client (request
  headers).
• Generate the results.
• Send the explicit data back to client (HTML).
• Send the implicit data to client (status codes
  and response headers).

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Servlet Architecture (high level)
Web client
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Servlet Life Cycle

• The container loads and initializes the Servlet.
• The Servlet handles client requests.
• The container can remove the Servlet.
• The servlet can remain loaded to handle
  additional requests
• Incur startup costs only once, may consume server
  resources if active.
• The javax.servlet.Servlet interface introduces
  the methods supporting communication between the
  Servlets and their container.
• void init(ServletConfig config) to be executed
  when the Servlet starts running
• void destroy()
• void service(ServletRequest req,ServletResponse
  res) to be executed in response to client
  requests
• ServletConfig getServletConfig()info about the
  servlet environment, provided by the host in init
  
• String getServletInfo() information about the
  Servlet

8
Example HTTP Servlet Lifecycle
Instantiated (constructor calledby container)
Initialized init(ServletConfig)
Handle requests doGet(HttpServletRequest,
HttpServletResponse)doPost(HttpServletRequest,
HttpServletResponse)
App logichappens here
Destroyed destroy()
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The Servlet Life Cycle (contd)

• Init
• Executed once when the Servlet is first loaded
• Not called for each request.
• Service
• Called in a new thread by server for each
  request
• Dispatches to doGet, doPost, etc
• Do not override service().
• The service method gives you automatic support
  for
• HEAD requests,
• OPTIONS requests, //cache, encoding, etc
• TRACE requests.
• doGet, doPost, doXxx
• Handles GET, POST, doPut, doTrace, etc.
• Override these to provide desired behaviour.
• Destroy
• Called when server deletes Servlet instance,
• Not called after each request.

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TOMCAT

• A Java Servlet, Java ServerPage (JSP) container.
• A reference implementation of Java servlet and
  JSP.
• It is a light weight Servlet/JSP container, and
  public domain software.
• Version 3.2 supports Servlet 2.2 and JSP 1.1
• Version 4.0 supports Servlet 2.3 and JSP 1.2
• Version 5.0 supports Servlet 2.4 and JSP 2.0
• Version 5.5 requires J2SDK 5.0
• Tomcat can be used as a
• stand-alone container
• for development and debugging
• add-on to a web server
• for deployment
• supports Apache, IIS, etc.

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Tomcat Installation

• http//www.coreservlets.com/Apache-Tomcat-Tutorial
  /
• Install the JDK
• http//java.sun.com/javase/downloads/index.jsp
• Make sure JDK 1.4, 1.5, or Java SE 5.0 (JDK 5.0),
  or Java EE 5 SDK (bundled with Java 5)
• PATH environment variable must include binary
  folders (e.g., INSTALL_ROOT_DIRECTORY/jdk1.5.0/bin
  )
• Try java version and javac help to make sure
  this
• Set environment variable
• JAVA_HOME the root directory of JDK
• On Windows
• Control Panel System Advanced Environment
  Variables
• (e.g., JAVA_HOMEC\j2sdk1.5.0
• set JAVA_HOMEC\j2sdk1.5.0)
• On Linux (bash /.bash_profile)
• (e.g., export JAVA_HOME/usr/local/j2sdk1.5.0)
• Install Tomcat
• http//jakarta.apache.org/tomcat
• Current version 5.5.17
• apache-tomcat-5.5.17.zip
• apache-tomcat-5.5.17.tar.gz

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• A new subdirectory will be created
• (e.g., C\Tomact-5.5, HOME/Tomcat-5.5)
• This directory is referred to as Tomcat root or
  Tomcat home.
• Subdirectories of Tomcat home
• bin scripts to start/shutdown Tomcat etc.
• conf server configuration files
• common class/jar files available to Tomcat and
  web apps
• classes unpacked classes
• lib jar files
• shared class/jar files available to all web apps
• classes unpacked classes
• lib jar files
• logs log files
• server class/jar files for Tomcat internal use
  only
• webapps web applications (include ROOT, examples)
• work temporary working directory

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Compiling/Invoking Servlets

• Set your CLASSPATH
• Servlet JAR file (e.g., tomcat_home/common/lib/se
  rvlet-api.jar (Servlet API).
• Put your servlet classes in proper location
• Locations vary from server to server. e.g.,
• A servlet at tomcat_home/webapps/AppDir/WEB-INF/c
  lasses/HelloServlet.class can be reached through
  the URI
• http//hostport/AppDir/servlet/HelloServlet or
• http//hostport/AppDir/HelloServlet depending on
  servlet-mapping
• A servlet at tomcat_home/webapps/AppDir/WEB-INF/c
  lasses/package/HelloServlet.class can be reached
  through the URI
• http//hostport/AppDir/servlet/HelloServlet/packa
  ge.HelloServlet
• Invoke your Servlets
• http//host8080/AppDir/servlet/ServletName
• Custom URL-to-servlet mapping (via web.xml).

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Package Abstraction
Servlet
Generic Servlet
HttpServlet
YourOwnServlet
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Servlet Directory

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class HelloServlet extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• String docType
• "lt!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML
  4.0 "
• "Transitional//EN\"gt\n"
• out.println(docType
• "ltHTMLgt\n"
• "ltHEADgtltTITLEgtHellolt/TITLEgtlt/HEADgt
  \n"
• "ltBODY BGCOLOR\"FDF5E6\"gt\n"
• "ltH1gtHellolt/H1gt\n"
• "lt/BODYgtlt/HTMLgt")

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Compiling

• javac classpath LIB/servlet-api.jar
• Hellox.java
• Or setup CLASSPATH
• Or add additional lib option if you use IDE
  (e.g., JBuilderX)
• When you change the Servlet, usually it may not
  be enough just to compile it.
• You may need to stop tomcat and restart tomcat to
  make tomcat reload the Servlet and not use the
  old version stored in memory.

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Directory Structure
18
Directory Structure (cont.)
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web.xml

• lt!DOCTYPE web-app
• PUBLIC "-//Sun Microsystems, Inc.//DTD Web
  Application 2.3//EN"
• "http//java.sun.com/dtd/web-app_2_3.dtd"gt
• ltweb-appgt
• ltservletgt
• ltservlet-namegtServlet1lt/servlet-namegt
• ltservlet-classgtHelloServletlt/servlet-classgt
• lt/servletgt
• ltservletgt
• ltservlet-namegtServlet2lt/servlet-namegt
• ltservlet-classgtcoreservlets.HelloServlet2lt/ser
  vlet-classgt
• lt/servletgt
• ltservletgt
• ltservlet-namegtServlet3lt/servlet-namegt
• ltservlet-classgtcoreservlets.HelloServlet3lt/ser
  vlet-classgt
• lt/servletgt
• ltservlet-mappinggt
• ltservlet-namegtServlet1lt/servlet-namegt