CS4273: Distributed System Technologies and Programming I

1
CS4273 Distributed System Technologies and
Programming I

• Lecture 7 Java Networking

2
Web Networking in Java

• Applets Send Data back to Web Servers
• In many situations applets need to send data back
  to the server for processing. Generally there are
  two ways for applets to send data back to their
  home sites (note applets can only communicate to
  their home sites)
• Launch the application server on the web site and
  applets communicate with it via sockets. But
  youre usually not allowed to launch your server
  on web site.
• Use CGI (common gateway interface). It is well
  defined in HTTP protocol and most commonly used.

3
A simple ASP example

• lt_at_ Language "VBScript" gt
• lthtmlgtltheadgtlttitle gt
• An ASP Example
• lt/titlegtlt/headgt
• ltbody bgcolorwhitegt
• ltform name"form1" methodPOST"gt
• ltinput name"TextBox1" type"text" id"TextBox1"
  /gt
• lt
• ss Request.Form("TextBox1")
• gt
• ltinput name"TextBox2" type"text" id"TextBox2"
  value"lt ss gt" /gt
• ltinput type"submit" name"Button1"
  value"Button" id"Button1" /gt
• lt/formgt
• lt/bodygtlt/htmlgt

4
CGI Program working with HTML forms

• lthtmlgtltheadgt lttitlegt Order Form lt/titlegt lt/headgt
• ltbodygt
• lth1 aligncentergt Order Form lt/h1gt
• ltform action "http//www.cs.cityu.edu.hk/jia/fo
  rmcgi.cgi"
• methodPOSTgt
• Name ltinput name"customer" size47gt ltpgt
• Street Address ltinput name"address" size40gt ltpgt
• Credit Card No. ltinput name"cardno" size10gt
• Expire Date ltinput name"expire" size6gt
• M/C ltinput name"cc" typeradio
  value"mastercard"gt
• Visa ltinput name"cc" typeradio
  value"visacard"gt
• Ship by express ltinput name"express"
  typecheckboxgt
• ltinput typesubmit value"submit address"gt ltpgt
• lt/bodygt lt/htmlgt

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A simple example of a CGI program

• main(argc, argv)
• int argc char argv
• char data FILE fp
• // header required by http protocol
• printf("Content-type text/plain\n")
• printf("\n")
• scanf("s", data) // read from browser
• // write data to a file
• fp fopen("form.dat", "w")
• fprintf(fp, "s\n", data)
• // send reply back to browser
• printf(Your order is processed. Thanks!\n)

6
Data format of HTML forms

• Data received from the above example is sent back
  to the originating web server of the form as a
  single line string, where fields are separated by
  , space are replaced by and other symbols
  by ASCII code
• customerXiaohuaJiaaddressCSdept2CCityUof
  HKcardno12345667expire012F97ccvisacardexp
  resson

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CGI Program for HTML forms

• cgi programs can be developed in any language.
  After being compiled, name the executable file
  with the extension .cgi.
• put the cgi program in the same directory as the
  webpage (it can be in other dir, so long as you
  specify correctly in the form). This program will
  be started by the http server when a client
  clicks submit button of the form.
• The cgi program reads the form data from the
  stdin and its stdout has been redirected to the
  client browser.
• It processes the form data and sends replies back
  to the web browser (the replies is usually in
  HTML format, so that the browser can display the
  response nicely).

8
Applets Communicate with CGI Programs

• applets can take inputs from the user, send
  request to and recv replies from a cgi program
  via the web server using HTTP protocol.
• applets create a socket connecting back to the
  originating http server. Then they use POST
  method of http protocol to send data back
• out.println("POST " "/jia /cgi/appletcgi.cgi
  HTTP/1.0")
• out.println("Content-type plain/text")
• out.println("Content-length " data.length())
• out.println("") // header ends with \r
• out.println(data) // data must be packed into
  one line
• http server strips the header from the packet
  recved from applets and passes only the data to
  cgi program.
• cgi program (in the same format as that for HTML
  forms) recv data from applets through stdin and
  sends replies back through stdout.
• cgi program will be started by the http server
  each time when an applet POSTs data.

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Example Of Applet Interacts With CGI Program

• // one run of this protocol sends one line
  str
• out.println("POST /jia/cgi/appletcgi.cgi
  HTTP/1.0")
• out.println("Content-type plain/text")
• out.println("Content-length "
  (sdata.length()1))
• out.println("")
• out.println(sdata i)
• while ((line in.readLine()) ! null)
• sourceArea.append(line"\n")
• out.close() in.close()

• public class appletCGI extends Applet implements
  ActionListener
• public void init()
• ..
• add("North", send new JButton("send"))
• send.addActionListener(this)
• sourceArea new TextArea(30, 80)
• public void public void actionPerformed
• (ActionEvent e)
• String sdata "HelloJava"
• if (e.getSource() send)
• s new Socket("personal.cs.cityu.edu.hk",80
  )
• in new BufferedReader(new
• InputStreamReader(s.getInputStream()))
• out new PrintWriter(s.getOutputStream(),
  true)

10
A CGI program working with Applet

• main(argc, argv)
• int argc char argv
• char sdata256, rdata256 FILE fp
• printf("Content-type text/plain\n")
• printf("\n")
• fp fopen("form.dat", "a") // append mode
• scanf("s\r", rdata)
• fprintf(fp, "s\n", rdata)
• printf(Echoed from CGI s\n", rdata)
  
• fclose (fp)

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What A CGI Program Can Do

• After a CGI program is started by the http
  server, it receives data from applets (or HTML
  forms). It processes users inputs by
• connect to database servers to retrieve client
  requested information (convert client query into
  database query commands), or
• save the processed data to a local file, or
• connect to mail server to send emails to clients.
  
• Note CGI is an ordinary application server. It
  can communicate with any other servers running
  anywhere.
• CGI is a gateway to bridge applets
• to the outside of the world!

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Implementation of Email Order

• cgi program can request a mail server to send the
  data received from an applet to an email-account
  as a mail.
• cgi program interacts with the mail server via
  SMTP (simple mail transfer protocol) to send a
  mail. Applet cant access mail svr directly due
  to security.
• SMTP is an ASCII text protocol. You can find the
  format of SMTP commands by using telnet to
  connect to an email server.
• Important telnet is a great debugging
• tool for Internet programming!!!

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Implementation of Email Order (Cont.)

• class mailOrder
• public static void main(String args )
• boolean more true
• String str null
• s new Socket("mars.cs.cityu.edu.hk",25)
• in new BufferedReader(
• new InputStreamReader(s.getInputStream()))
  
• out new PrintWriter(s.getOutputStream(),
  true)
• str in.readLine() // read reply from
  svr
• System.out.println(str) // for debug
• // followings are SMTP commands
• out.println("HELO ss4e.cs.ust.hk\r" )
• str in.readLine() // read reply from svr
• System.out.println(str)
• out.println("MAIL FROM jcao\r" )
• // read reply from mail svr
• out.println("RCPT TO eechan_at_cityu.edu.hk\r" )
  
• // read reply from mail svr

• out.println( "DATA\r" ) // start of data
• out.println( "this is 2nd test!\r" ) // email
  body
• out.println( ".\r" ) // end of email body
• out.println( "QUIT\r" ) // quit SMTP
• .

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CGI program in Java

• Since the above cgi program (mailOrder) is
  written in Java and a Java program needs the Java
  interpreter to execute it, a cgi shell script is
  needed to execute the Java code
• !/bin/sh
• echo Content-type text/plain
• echo
• /usr/local/jdk/bin/java mailOrder
• echo QUERY_STRING