Servlets

1
Servlets

• Server Side Web Based Java Programming

2
What are Servlets

• Extensions to Java enabled servers
• Servlet containers e.g.
• Tomcat
• Jetty
• Server side replacement for CGI
• Dynamically loaded module
• Runs inside java virtual machine

3
Why Use Servlets

• Efficient
• Servlet loaded and Init method called once
• Service method called everytime servlet requested
• Persistent
• As servlet stays running between requests, can
  maintain client state
• Portable
• Java byte code not tied to a particular platform
• Secure
• Can run within a security manager

4
Servlet Interface

• init method
• Called once when the servlet instantiated
• service method
• Called every time a client request is received
• destroy method
• Called when service is shutdown
• GenericServlet implements this interface

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HttpServlet

• Has init, service and destroy methods
• doGet method
• Override the doGet method to process get requests
• doPost method
• Override the doPost method to process get
  requests
• A processRequest method
• Can be called by post and get methods

6
Using Netbeans to Create Servlets

• Create a New Web Project
• Choose a Web Application
• Choose location for Project and Files
• Accept defaults
• This creates the necessary structure for
  deploying jsps and servlets to the Netbeans built
  in tomcat servlet container
• Right click sources directory
• Create Servlet
• Uncomment text and run file
• Or a Web Application with sources (if servlet
  code already exists)
• Right click servlet in sources directory
• Run file
• Right click sources directory
• Create Servlet
• Specify Servlet Name and URL pattern
• Note URL pattern shows how the servlet will be
  called
• Uncomment text and run file
• Edit the servlet to actually print some relevant
  content

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Using Netbeans to Compile and Run

• Compile the servlet
• Right click servlet and compile
• Test the servlet by running it
• Right click servlet and select Run File
• In browser select appropriate URL (e.g.
  http//localhost8080/servlet/myServlet)
• Note Look at the web.xml file in the WEB-INF
  directory to see the servlet mappings that have
  been defined

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HelloWorld Servlet

• protected void processRequest(HttpServletRequest
  request, HttpServletResponse response) throws
  ServletException, IOException
• response.setContentType("text/html")
• PrintWriter out response.getWriter()
• out.println("lthtmlgtltheadgtlttitlegtServletlt/titlegtlt/
  headgt")
• out.println("ltbodygt")
• out.println("Hello World")
• out.println("lt/bodygtlt/htmlgt")
• out.close()

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Servlet Objects

• Request Object
• Contains references to HTTP Request variables
• E.g. methods
• getParameter read paramater values from Request
  Object
• Following not normally used
• getReader Retrieve request body as a Character
  Input Stream
• getInputStream Retrieve request body as a bit
  Input Stream
• Response Object
• Encapsulates HTTP Response
• E.g. methods
• setContentType
• getWriter Retrieve response body as a Character
  Output Stream
• getOutputStream Retrieve response body as a bit
  Output Stream

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Processing Form Variables

• Use request objects getParameter(parameter)
  method
• Returns string value of parameter
• if more than one value only the first is given
• getParameterNames()
• Returns enumeration of parameter names
• getParameterValues(parameter)
• Returns string array of parameter values
• Useful for multiple values

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Session Variables

• Create a session
• HttpSession theSession req.getSession(true)
• Retrieve session ID
• theSession.getId()
• Retrieve given session variable value
• getAttribute(Name)
• Store a Session variable value
• void setAttribute(Name, Chris)
• Remove a session variable
• void removeAttribute(Name)
• Retrieve enumeration of session variable names
• Enumeration getAttributeNames()

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Accessing Databases

• Load the database driver
• Class.forName("org.gjt.mm.mysql.Driver").newInstan
  ce()
• Connect to database
• theConnection DriverManager.getConnection("jdbc
  mysql//127.0.0.13306/Test2")
• Create a statement object
• Statement theStatementtheConnection.createStateme
  nt()
• Execute a query
• ResultSet theResulttheStatement.executeQuery("sel
  ect from sRecord")
• Loop through results
• while(theResult.next()).
• Accessing returned result elements
• theResult.getString(1) ..
• Close the resources
• Catch errors and exceptions in a try ..
  catch() block

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Database Pooling

• Allows Servlets to share connections
• Pooled Connections stay open, more efficient
• Faster response from servlets that interact with
  databases
• Available in jdk 1.4.2
• Pooled Datasource is initialised in the init
  method
• Lookup process can take time, so just done once
• Access to Datasource is via a synchronised block,
  so multiple servlet calls do not access same
  datasource

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Additional Servlet Examples

• response.sendRedirect(Location)
• Uses http headers to request browser to load
  another page
• Location is a string with the absolute URL
• rdrequest.getRequestDispatcher(Resource)
• rd.forward(request,response)
• Forwards the current request to a resource on the
  current server. Does not use http.

15
Java Server Pages (JSPs)

• Similar concept to PHP, ASP etc
• Java code is embedded in HTML documents
• lt gt
• Tags used to embed code
• HelloWorld JSP example

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JSP scriptlets

• Can add multiple scriplets on the JSP page
• All the scriplets share the same context
• Can access variables declared in different
  scriplets on the same page
• All variables are local to service method
• JSP comments
• lt-- This is a JSP Comment --gt
• Declare class level variables
• lt! String name "Chris" gt

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JSP directives

• Directives added to page using lt_at_ gt tags
• Used to import classes for the page
• lt_at_ page import"java.util." gt
• Declare HTTP headers, such as content type etc.
• lt_at_page contentType"text/html"gt
• Include Files into Current Context
• lt_at_ include file"Date.jsp" gt

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Additional JSP directives

• lt_at_ page autoFlush"true" gtTells JSP to flush
  the page buffer when it fills
• lt_at_ page buffersizekb gtGives the size of the
  page buffer in kb or none for no buffer
• lt_at_ page errorPage"path" gtDefines a page to
  display if an error occurs in the JSP page
• lt_at_ page info"description" gtGives a brief
  description for the page
• lt_at_ page isErrorPage"true" gtGives an error page
  access to the exception implicit variable
• lt_at_ page isThreadSafe"true" gtTells the JSP that
  multiple pages can execute in parallel
• lt_at_ page language"lang" gtSets the JSP script
  language to lang
• lt_at_ page session"true" gtTells JSP that the page
  participates in a session

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JSP Implicit Objects

• request
• response
• out
• session
• etc

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JSP tags

• Rather than using scriplets and java code
• Code action can be included as HTML tags (useful
  for Web development IDEs)
• ltjspinclude page"Date.jsp"gt
• ltjspparam name"ParamName1" value"ParamValue1"
  /gt
• lt/jspincludegt
• ltjspforward page"PrintDate.jsp"/gt
• Forwarding redirects to current page clearing any
  content that already has been buffered
• Exception generated when content already has been
  sent
• Increase size of buffer if this might be a
  problem
• lt_at_ page buffer"12kb" gt

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Additional Tags

• jspplugin
• To include an applet using the java plugin
• jspusebean
• Discussed later for using a javabean class
• ltjspparamgt
• Used inside include, forward and plugin tags

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JavaBeans

• Reusable software component
• Seperate business logic from displayed HTML
• Features of JavaBeans
• a public class,
• a public constructor with no arguments
• public get and set methods to read and write to
  properties.
• Make JavaBean part of a package
• most servlet/JSP containers require it

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JavaBean tags

• ltjspuseBean id "myBean" scope"session" class
  myPkg.myBean" /gt
• Specify which class the myBean referes to
• ltjspsetProperty name "myBean" property
  "firstName value "Chris" /gt
• Set a particular property value
• ltjspgetProperty name "myBean" property
  "fullName" /gt
• Get a particular property value
• ltjspsetProperty name"BeanName" property"" /gt
• Used to set all properties available from form
  variables

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Tag Libraries

• It is possible to create your own JSP tags
• This is done by creating a Tag Handler class
• A Tag Library descriptor file is also required so
  that the interface to the tag library is well
  defined
• Custom tags
• allow tag based content creation tools to easily
  add program elements to your code

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JSP Standard Template Library (JSTL)

• Part of JSP 1.2 and 2.0 specification
• Available on version of tomcat in netbeans 6,
  otherwise add JSTL files to the applications lib
  directories
• All scripting uses tag based syntax
• Suitable for tools that automatically generate
  tags
• E.g.
• lt_at_ taglib uri"http//java.sun.com/jstl/core"
  prefix"c" gt
• ltcforEach var"i" begin"1" end"10" step"1"gt
• ltcout value"i" /gt ltbr /gt
• lt/cforEachgt
• Web.xml file must be modified with the location
  of the tdl files
• lttaglibgt
• lttaglib-urigthttp//java.sun.com/jstl/corelt/tag
  lib-urigt
• lttaglib-locationgt/META-INF/c.tldlt/taglib-locat
  iongt
• lt/taglibgt

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More JSTL

• ltcset var"foo" scope"session" value"..."/gt
• ltcif test"!empty param.Add"gt . lt/cifgt
• ltcchoosegt   
• ltcwhen test"input '1'"gt...lt/cwhengt
• ltcotherwisegt     ...   lt/cotherwisegt
• lt/cchoosegt
• ltcforEach var"item" items"sessionScope.item"
  gt   ... lt/cforEachgt

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Summary
Main points to remember Servlet
Concepts Request and response objects Session
variables Database access JSPs JavaBeans Tag
Libraries JSTL