CST252 Network Software Design

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CST252 Network Software Design

• Lecture 10 Web Applications

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Admin

• Results on noticeboard
• 1st Coursework (30)
• 1st Phase Test (20)
• Second Phase Test
• Monday 20th May 1300 - 1400
• 4th Floor of CS building
• Second coursework due 13th May

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Resources

• Java Tutorial (java.sun.com)
• Specialised Trail Servlets
• Course Website
• users.wmin.ac.uk/lancasd/CCTM91
• Book
• Cay Horstmann and Gary Cornell. Core Java
• Volume 2 chapter 3
• Books specifically on JSP/Servlets

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Last Lecture Review

• Applets
• browser capabilities
• Simple graphics
• rectangles and ellipses
• colours
• Security
• Jar files
• More Graphics and Events

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Lecture 10 Overview

• Web Applications
• Sending Data to a Web Server
• GET/POST methods
• Web html Form
• Server-Side Processing
• Servlets
• JSP

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Web Applications

• For many network applications it is better to use
  HTTP rather than to invent an application
  protocol and work at socket level
• eg the On-line quiz of your coursework
• But not always - eg peer to peer
• A web application is a dynamic extension of a web
  server

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Dynamic Web Server

• In the beginning - web pages were static, the web
  server just returned fixed html files
• Dynamical pages
• response page depends on data sent
• A Web Form
• the way of gathering and sending data to server
• Servlets
• programs on the server computer that use the data
  returned to create a new web page

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Sending Data to a Web Server

• A Web Form
• History - a hack on top of CERN system, NCSAs
  Mosaic 2.0 CGI scripts
• How to attach the data
• GET and POST methods for sending data
• in URL
• in body

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html form

• ltHTMLgt
• ltHEADgt
• ltTITLEgtThree Parameter Testlt/TITLEgt
• lt/HEADgt
• ltBODYgt
• ltFORM METHODGET ACTION"/cgi-bin/docalc"gt
• First Parameter ltINPUT TYPE"TEXT"
  NAME"param1"gtltBRgt
• Second Parameter ltINPUT TYPE"TEXT"
  NAME"param2"gtltBRgt
• Third Parameter ltINPUT TYPE"TEXT"
  NAME"param3"gtltBRgt
• ltCENTERgt ltINPUT TYPE"SUBMIT"gt lt/CENTERgt
• lt/FORMgt
• lt/BODYgt
• lt/HTMLgt

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Form as web page
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GET method

• Attach the data as parameters to the URL
• http//host/script?params
• Parameters must be encoded (avoid spaces)
• Simple, but limited in length
• METHODGET is default

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POST Method

• Parameters are sent in the body of an http
  request after the header and blank line
• Still need to encode them
• More manageable if there are many parameters
• METHODPOST is not default - must specify

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POST from a Java program

• Open a URLConnection and write name/value pairs
  to the output stream
• URL url new URL(http/host/script)
• URLConnection ucon url.openConnection()
• ucon.setDoOutput(true)
• PrintWriter out new PrintWriter(ucon.getOutputSt
  ream())
• out.print(name1URLEncoder.encode(value1))
  
• out.print(name2URLEncoder.encode(value2)\n
  )
• out.close()

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Custom Clients

• Could write a java client that acts as a custom
  client for a web applications
• Sends data using POST
• Receives HTTP
• Server is implemented through a dynamic web
  server
• eg Census data

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Server Side Processing

• Old style CGI
• often in Perl
• Servlets
• servlets are to web servers as applets are to
  browsers
• JSP
• simplified technique for pages that are mostly
  static with small dynamic parts

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Servlets vs CGI

• Efficient and Scalable
• new threads not processes (JVM permanently
  running)
• Convenient
• sessions - store data between requests
• Secure
• CGI is known to have loopholes
• Portable
• Java with standard Servlet API

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DateServlet.java

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class HelloWWW extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServeletException, IOException
• response.setContentType(text/html)
• PrintWriter out response.getWriter()
• String now new Date().toString()
• out.println( ltHTMLgt\n
• ltHEADgtltTITLEgtDate Servletlt/TITLEgtlt/HEADgt\n
• ltBODYgt\n
• ltH1gtltpgtThe current time is now
• lt/pgtlt/H1gt\n
• lt/BODYgtlt/HTMLgt)

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DateServlet notes

• Just creates a web page with the date
• Includes, and extends
• Only need to code a single method doGet()
• HttpServletRequest and HttpServletResponse
  parameters
• setContentType
• Create html as String, and pass to out
• Here the returned information does not depend on
  any user input so ignore request
• overrides abstract method in httpServlet

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Installing Servlets

• Run by the web server
• Different Web Servers require different setup
  (now mainly Tomcat)
• Tomcat (Apache)
• Need a deployment descriptor web.xml
• Can package to a .war file
• Not covered here

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JWSDP

• Java WebServer Development Package
• Convenient package of utilities for web services
  development
• tomcat
• ant
• XML support
• SOAP support

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Servlet Life Cycle

• init() called when servlet is first created,
  but not on further requests
• one off initialisation, eg connection pools
• service() checks what kind of request it is and
  passes to doPost(), doGet()..
• dont touch, put code into doXXX()
• destroy() before removing the servlet from
  memory

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getParameter()

• A method to get the parameters sent from an Html
  form
• ltFORM ACTION/servlet/ParamTestgt
• First Parameter ltINPUT TYPETEXT
  NAMEparam1gtltBRgt
• Second Parameter ltINPUT TYPETEXT
  NAMEparam2gtltBRgt
• Third Parameter ltINPUT TYPETEXT
  NAMEparam3gtltBRgt
• ltCENTERgt ltINPUT TYPESUBMITgt lt/CENTERgt
• lt/FORMgt
• Access each parameter using the name field
  appearing in the form
• String p1 request.getParameter(param1)

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Proxy Servlet

• To allow an applet to obtain information from
  other sources
• Recall applet security
• Applets can only call home
• A solution, is to provide a forwarding service or
  proxy running on the web server machine
• Implement as a servlet ProxySvr.java
• Compare C and Perl versions

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ProxySvr.java

• Applet makes a get request
• http//www.yourserver.com/proxySvr?URLhttp//www
  .wmin.ac.uk/
• index.html
• public class ProxySvr extends HttpServlet
• public void doGet(HttpServletRequest request,
• HttpServletResponse response)
• throws ServletException, IOException
• PrintWriter out response.getWriter()
• String query request.getParameter(URL)
• query URLDecoder(query)
• URL url new URL(query)
• BufferedReader in new BufferedReader(
• new InputStreamReader(url.openStream()))
• String line
• while( (linein.readLine()) ! null )
• out.println(line)
• out.flush()
• // All exception handling ignored

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JavaServer Pages (JSP)

• Servlets are a powerful way of dynamically
  creating web pages
• But tedious, if the most of the page is static,
  with just one line changed (eg customer name)
• JSP allows you to use ordinary web authoring
  tools to create a static page, and embed a few
  dynamic lines
• Other competing technologies ASP,PHP..

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date.jsp

• lthtmlgt
• ltheadgt
• lttitlegtData JSPlt/titlegt
• lt/headgt
• ltbodygt
• lth1gtDate JSPlt/h1gt
• ltpgtThe current time is
• lt new java.util.Date() gt
• lt/pgt
• lt/bodygt
• lt/htmlgt

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Using JSP

• The places where dynamic content is to be added
  are marked with special HTML-like tags lt...gt
• Easier to deploy than servlets
• Server automatically changes .jsp files into
  regular servlets
• translation and compilation stages
• eg Tomcat - inspect the intermediate java code

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JSP Elements

• Expressions lt expression gt evaluated and
  placed in the servlet output
• Scriptlets lt code gt inserted into a method
  called by the servlet service() method
• Declarations lt! code gt inserted into the body
  of the servlet

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Separate Computation (.java) and Presentation
(.html)

• date.jsp contained a bit of java code, this is
  rarely a good idea, input from
• programmer
• graphic designer
• Better to perform computations in a separate java
  class
• Easiest if these java classes are JavaBeans

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JavaBean Components

• JavaBeans are ordinary java classes that follow a
  particular naming convention for methods
  accessing properties
• getpropertyname()
• setpropertyname(Type newValue)
• JSP has shortcuts for using this kind of java
  object and accessing methods
• ltjspuseBean ...../gt

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time.jsp

• ltjspuseBean idformatter classTimeFormatterBe
  an/gt
• ltjspsetProperty nameformatter propertydate
• valuelt new java.util.Date() gt/gt
• lthtmlgt
• ltheadgt
• lttitlegtData JSPlt/titlegt
• lt/headgt
• ltbodygt
• lth1gtDate JSPlt/h1gt
• ltpgtThe current time is
• ltjsp.getProperty nameformatter
• propertytime /gt
• lt/pgt
• lt/bodygt
• lt/htmlgt

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TimeFormatterBean.java

• import java.text.DateFormat
• import java.util.Date
• public class TimeFormatterBean
• public TimeFormatterBean()
• timeFormatter DateFormat.getTimeInstance()
  
• public void setDate(Date aDate)
• theDate aDate
• public String getTime()
• String timeStr timeFormatter.format(theDate)
  
• return timeStr
• private DateFormat timeFormatter
• private Date theDate

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Notes

• time.jsp
• first two lines construct the bean and initialise
  the Date property
• jsp.getProperty directive calls the get method
  for the time property
• TimeFormatterBean.java
• two parameters
• date (set method to initialise)
• time (get method)
• Uses DateFormat class to get just the time as a
  String, not the whole Date

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Handling Request Parameters in JSP

• Find out what the user typed into the input
  fields of an html form using
• lt request.getParameter(paramname) gt
• Doesnt matter if it was POST or GET
• Convenient shorthand for passing request input to
  JavaBean set methods

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Sessions

• HTTP is a stateless protocol
• browser sends request to web server
• web server sends reply then disconnects
• In many cases (eg shopping cart), would like the
  system to remember what was added on previous
  pages
• JSP scopesession uses the same JavaBean for
  all pages rather than a new one for each page

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Information from the Web

• Harvesting information
• Parsing html
• At the mercy of changes of page format
• SOAP
• XML
• A modern way of storing content
• platform independent
• text based

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Web Services

• Request for information via a web application,
  typically BtoB
• eg Stock price
• Access using SOAP which transfers the information
  as XML
• Allows communication even if using different
  information systems and data formats

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XML

• Extensible Markup Language
• To transmit data from one program to another in a
  structured form
• Encoding independent of any particular
  programming language
• Text comprehensible to humans

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XML

• Strict formatting rules
• ltcoingt
• ltvaluegt0.5lt/valuegt
• ltnamegtHalf Dollarlt/namegt
• lt/coingt
• Java API to manipulate XML JAXP (1.4)
• Parsing and analysing
• SAX line by line calling (user defined) methods
  as each construct is encountered
• DOM create whole tree in memory, then analyse

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Simple Object Access Protocol

• SOAP
• messages with immediate reply/ delayed reply
• Java API
• JAXM
• JAX-RPC
• Business registry access

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Summary

• HTML forms GET and POST
• Java programs that do this
• Dynamic Server
• Servlets
• JSP