Object Oriented Programming in Java (95-707) Advanced Topics

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Lecture 10Object Oriented Programming in Java

• Advanced Topics
• Servlets

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Todays Lecture

• Trail Servlets
• Lessonoverview,Cient Interaction, LifeCycle,...
• http//web2.java.sun.com/docs/books/tutorial/servl
  ets/index.html
• Additional Link
• http//developer.java.sun.com/developer/onlineTrai
  ning/Servlets/Fundamentals/introduction.html

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Servlet Architecture
Web server with servlet engine
Client Browser
2 call servlet with request info
1 send http request
HTTP Servlet
5 send HTTP response
4 return information from servlet engine
3 perform logic
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1 send HTTP request

• Handled by regular html tags
• examples
• simple
• http//localhost8080/servlet/snoop
• with parameters
• http//localhost8080/servlet/snoop?usernamealain
  
• from a form post
• ltform action/servlet/snoop methodpostgt
• ltinput type"text" name"username" value""gt
• ltinput type"text" namepassword" value""gt
• lt/formgt

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2 call servlet with request info

• Handled by the web server
• transparent to the developer

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3 perform logic

• Handled by a custom java class which inherits
  from the Servlet class

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4 return response from servlet engine

• Handled by the web server
• transparent to the developer

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5 final step

• The browser renders the html page which is
  returned
• In this class we will only deal with HTML, but
  others types could be returned by servlets
• I.e. audio, images, ...

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Overall Servlet API

• base class is HttpServlet
• APIs to
• initialize servlets
• receive http requests
• retrieve information on the request
• send a response to the browser
• maintain session across multiple invocations
  (remember that HTTP is session-less)

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Base Class HttpServlet
Example from HelloWorldServlet

• import java.io.
• import javax.servlet.
• import javax.servlet.http.
• public class HelloWorldServlet extends
  HttpServlet

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Initialization of Servlet

• Example from PhoneServlet.java
• public class PhoneServlet extends HttpServlet
• private Hashtable phones new Hashtable()
• private String dataFilePath "phonelist"
• // Public Methods
• public void init(ServletConfig conf)
• throws ServletException
• super.init(conf)
• String param getInitParameter("phonelist"
  )
• if (param ! null)
• dataFilePath param
• ...
• readFromFile()

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HTTP GET and POST

• Example from PhoneServlet
• public class PhoneServlet extends HttpServlet
• public void init(ServletConfig conf) throws
  ServletException
• ...
• public void doGet(HttpServletRequest req,
  HttpServletResponse res)
• throws ServletException, IOException
• ...
• public void doPost(HttpServletRequest req,
  HttpServletResponse res)
• throws IOException
• ...
• doGet is called when we have HTTP GET
• doPost is called when we have HTTP POST

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Inside doGet and doPost

• Three usual steps
• read request data such as input parameters
• set response headers
• length, type
• write the response data

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Get Request Data and Parameters

• Example from PhoneServlet
• public void doPost (HttpServletRequest req,
  HttpServletResponse res)throws IOException
• ...
• String name, number, opcode
• name req.getParameter("name")
• number req.getParameter("number")
• opcode req.getParameter("opcode")
• ...

ltform action/servlet/PhoneServlet
methodpostgt ltinput type"text" namename"
value""gt ltinput type"text" namenumber"
value""gt ltinput type"text" nameoncoder"
value""gt lt/formgt
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Set Response Headers

• Example from PhoneServlet
• public void doGet (HttpServletRequest req,
  HttpServletResponse res)
• throws ServletException, IOException
• res.setContentType("text/html")
• ServletOutputStream out res.getOutputStream()
  
• String title "Phones"
• ...
• out.println("ltHEADgtltTITLEgt Phone Servlet Output
  lt/TITLEgtlt/HEADgt")
• out.println("ltBODY BGCOLOR\"FFFFFF\"gt")
• out.println("lth1gtPhone Recordslt/h1gt")
• out.println("ltpgt")
• ...
• )

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Create response

• Example from PhoneServlet
• public void doGet (HttpServletRequest req,
  HttpServletResponse res)
• throws ServletException, IOException
• res.setContentType("text/html")
• ServletOutputStream out res.getOutputStream()
  
• String title "Phones"
• ...
• out.println("ltHEADgtltTITLEgt Phone Servlet Output
  lt/TITLEgtlt/HEADgt")
• out.println("ltBODY BGCOLOR\"FFFFFF\"gt")
• out.println("lth1gtPhone Recordslt/h1gt")
• out.println("ltpgt")
• ...
• )

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HTTP protocol is session-less

• HTTP protocol is session-less
• between two requests the web server does not
  remember the caller
• protocol is as follows
• browser connects to HTTP server
• browser makes a request for a web resource
• web server looks for the resource
• web server forwards the resource to the browser
• web server disconnects the browser

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Sessions with Servlets

• The servlet engine fills-in the gap and provides
  a way to keep a session across multiple requests
  to a web server
• Mechanism is handled by the servlet engine on
  demand by the servlets
• The base class is HttpSession

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HttpSession

• Example from SessionServlet.java
• public void doGet (HttpServletRequest req,
  HttpServletResponse res)
• throws ServletException, IOException
• HttpSession session req.getSession(true)
• ...
• //Here's the meat
• Integer ival (Integer) session.getValue("sess
  iontest.counter")
• if (ivalnull) ival new Integer(1)
• else ival new Integer(ival.intValue() 1)
• session.putValue("sessiontest.counter", ival)
• ...
• )