Java Servlet

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Java Servlets

• Web Engineering

Faraz Hashmi, BSIT-5th Eve, 3011
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Servlet

• Servlets are protocol- and platform-independent
  server side components, written in Java, which
  dynamically extend Java-enabled servers.
• They have become more and more popular as they
  benefit from all the advantages of the Java
  programming language in particular Architecture
  and Platform Independence and the ability to
  build robust and secure applications.

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Servlet

• A Servlet, in simple terms, is a Java program
  running under a web server taking a 'request'
  object as an input and responding back by a
  'response' object. Typically a web browser will
  send the request in HTTP format. The Servlet
  container will convert that into a request
  object. Similarly the response object - populated
  by the Servlet is converted into an HTTP response
  by the Servlet container.
• This mechanism makes the browser - web server
  interaction very easy. Java Servlets are more
  efficient, easier to use, more powerful, more
  portable, and cheaper than traditional CGI and
  many alternative CGI-like technologies.
• (More importantly, Servlet developers tend to get
  paid more than Perl programmers)

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Servlet Mechanism

• A servlet is a Java class and thus needs to be
  executed in a Java Virtual Machine by a service
  called a servlet engine. The servlet engine loads
  the servlet before it can be used. The servlet
  then stays loaded until it is unloaded or the
  servlet engine is shut down.

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Servlet Mechanism
JVM
Request
Client/ Browser
Server
Servlet
Response
Extends the functionality of the server
by Generating HTML pages dynamically
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Servlet Types

• Servlets are based on two main packages
• javax.servlet
• javax.servlet.http
• GenericServlet
• For writing protocol independent servlets
• HttpServlet
• Extends from GenericServlet class
• Adds functionality for writing HTTP specific
  servlets

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Servlet class hierarchy
Object
GenericServlet
ServletResponse
ServerRequest
javax.servlet
HttpServletRequest
HttpServlet
HttpServletResponse
javax.servlet.http
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Software Requirements

• To use Java servlets, following softwares will be
  needed
• J2SE
• Additional J2EE based libraries for servlets such
  as servlet-api.jar, jsp-api.jar. You can
  download these APIs separately but they are also
  available with the web server youll be using.
• A capable servlet web engine (web server)

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Environment Setup

• Steps
• Download the Apache Tomcat Server
• Install Tomcat
• Set the JAVA_HOME variable
• Set the CATALINA_HOME variable
• Set the CLASSPATH variable
• Test the Server

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Servlet Advantages Disadvantages

• The advantage of Servlets is,
• Portability
• o Portable across operating systems and across
  web servers
• Power
• o Harness the full power of the core Java
  APIs networking and URL access, multithreading,
  image manipulation, data compression, JDBC,
  object serialization, internationalization
• Efficiency Endurance
• o Memory resident, so invocation highly
  efficientno process to spawn or interpreter to
  invoke

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Servlet Advantages Disadvantages

• Safety
• o Support safe programming since inherit
  Javas strong type safety, exception-handling
  mechanism
• Elegance
• o Code is clean, object-oriented, modular, and
  simple (i.e.. Session tracking, cookie)
• Integration
• o Tightly integrated with the servertranslate
  file paths, perform logging, check authorization,
  and MIME type mapping

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Servlet Advantages Disadvantages

• The disadvantage of Servlets is,
• Web Administrator will need to learn how to
  install and maintain Java Servlets
• Tedious uses of out.println() statements
• o Can be remedied by using Java Server Page
  (JSP)

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Types of HTTP Requests

• Get
• Post
• Delete
• Options
• Put
• Trace

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How HTTP Sends Request

• Some HTTP request types
• Get -- Attr/Val pairs attached after ? of URL
• E.g. http//www.gmail.com/register?name
  ali
• Post -- Attr/Val pairs attatched with the
  request body

Client
Server
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HTTP Request Example
Request parameters etc.
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Servlet Model
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Servlet Life Cycle

• Initialize
• Service
• Destroy

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HelloWorld Servlet

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class HelloWorldServlet extends
  HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse
  response)
• throws ServletException,
  IOException
• PrintWriter out response.getWriter()
• out.println(Hello World!)

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Web.xml

• lt?xml version"1.0" encoding"ISO-8859-1"?gt
• ltweb-appgt
• ltservletgt
• ltservlet-namegtHelloWorldServletlt/servlet-n
  amegt
• ltservlet-classgtHelloWorldServletlt/servlet-
  classgt
• lt/servletgt
• ltservlet-mappinggt
• ltservlet-namegtHelloWorldServletlt/servlet-n
  amegt
• lturl-patterngt/myfirstservletlt/url-patterngt
  
• lt/servlet-mappinggt
• lt/web-appgt