Transcript Lecture 10 Object Oriented Programming in Java Advanced Topics Servlets

Lecture 10
Object Oriented Programming in Java
Advanced Topics
Servlets
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
1
Today’s Lecture
• Trail: Servlets
• Lesson:overview,Cient Interaction, LifeCycle,...
• http://web2.java.sun.com/docs/books/tutorial/servlets/index.html
• Additional Link
http://developer.java.sun.com/developer/onlineTraining/Servlets/Fundamentals/introduction.ht
ml
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
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Servlet Architecture
Web server with servlet engine
Client
Browser
1: send http request
HTTP Servlet
5: send HTTP
response
June 1, 2000
2: call servlet
with request
info
4: return
information
from
servlet engine
Object Oriented Programming in Java (95-707)
Advanced Topics
3: perform logic
3
1: send HTTP request
• Handled by regular html tags
• examples:
– simple
http://localhost:8080/servlet/snoop
– with parameters
http://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>
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
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2: call servlet with request info
• Handled by the web server
• transparent to the developer
June 1, 2000
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Advanced Topics
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3: perform logic
• Handled by a custom java class which inherits
from the Servlet class
June 1, 2000
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Advanced Topics
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4: return response from servlet engine
• Handled by the web server
• transparent to the developer
June 1, 2000
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Advanced Topics
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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, ...
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
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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)
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
9
Base Class: HttpServlet
Example from HelloWorldServlet:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HelloWorldServlet extends HttpServlet {
…
}
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
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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();
}
}
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
11
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
Objectwhen
Oriented Programming
(95-707)
•
is called
we havein Java
HTTP
POST
June doPost
1, 2000
Advanced Topics
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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
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
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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>
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
14
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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Advanced Topics
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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("<HEAD><TITLE> Phone Servlet Output
</TITLE></HEAD>");
out.println("<BODY BGCOLOR=\"#FFFFFF\">");
out.println("<h1>Phone Records</h1>");
out.println("<p>");
...
)
June 1, 2000
Object Oriented Programming in Java (95-707)
Advanced Topics
16
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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Advanced Topics
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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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Advanced Topics
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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("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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