Lecture 10 Object Oriented Programming in Java

June 1, 2000 Object Oriented Programming in Java (95-707)Advanced Topics

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Lecture 10Object Oriented Programming in Java

Advanced TopicsServlets


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Today’s Lecture

• Trail: Servlets• Lesson:overview,Cient Interaction, LifeCycle,... • http://web2.java.sun.com/docs/books/tutorial/servlets/index.html

• Additional Linkhttp://developer.java.sun.com/developer/onlineTraining/Servlets/Fundamentals/

introduction.html


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Servlet Architecture

ClientBrowser

Web server with servlet engine

HTTP Servlet

1: send http request

2: call servlet with request info

5: send HTTP response

4: return informationfrom servlet engine 3: perform logic


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1: send HTTP request

• Handled by regular html tags• examples:

– simplehttp://localhost:8080/servlet/snoop

– with parametershttp://localhost:8080/servlet/snoop?username=alain

– from a form post<form action=“/servlet/snoop” method=“post”>

<input type="text" name="username" value=""><input type="text" name=”password" value="">

</form>


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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

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class HelloWorldServlet extends HttpServlet {

…

}

Example from HelloWorldServlet:


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Initialization of Servlet

• Example from PhoneServlet.javapublic 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");

...

} <form action=“/servlet/PhoneServlet” method=“post”><input type="text" name=”name" value=""><input type="text" name=”number" value=""><input type="text" name=”oncoder" value="">

</form>


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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("<HEAD><TITLE> Phone Servlet Output </TITLE></HEAD>");

out.println("<BODY BGCOLOR=\"#FFFFFF\">");

out.println("<h1>Phone Records</h1>");

out.println("<p>");

...

)


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Create response

• Example from PhoneServlet: public void doGet (HttpServletRequest req, HttpServletResponse res)


{

res.setContentType("text/html");

ServletOutputStream out = res.getOutputStream();

String title = "Phones";

...

out.println("<HEAD><TITLE> Phone Servlet Output </TITLE></HEAD>");

out.println("<BODY BGCOLOR=\"#FFFFFF\">");

out.println("<h1>Phone Records</h1>");

out.println("<p>");

...

)


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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)


{

HttpSession session = req.getSession(true);

...

//Here's the meat

Integer ival = (Integer) session.getValue("sessiontest.counter");

if (ival==null) ival = new Integer(1);

else ival = new Integer(ival.intValue() + 1);

session.putValue("sessiontest.counter", ival);

...

)

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Lecture 10 Object Oriented Programming in Java