Lecture 2

The Servlet Technology
Inside Servlets
Writing Servlet Applications

What Is a Servlet?
“A servlet is a server-side Java replacement for CGI
scripts and programs. Servlets are much faster and
more efficient than CGI and they let you unleash
the full power of Java on your web server, including
back-end connections to databases and object
repositories through JDBC, RMI, and CORBA”
– Atlanta Java Users Group (http://www.ajug.org/meetings/jun98.html)

2

Ways to use Servlet
A simple way:
Servlet can process data which was POSTed
over HTTPS using an HTML FORM, say, an
order-entry, and applying the business logic used
to update a company's order database.

Some other uses:
Since servlets handle multiple requests concurrently, the
requests can be synchronized with each other to support
collaborative applications such as on-line conferencing.
One could define a community of active agents, which
share work among each other. The code for each agent
would be loaded as a servlet, and the agents would pass
data to each other.
One servlet could forward requests other servers. This
technique can balance load among several servers which
mirror the same content.

Server-side Java for the web
a servlet is a Java program which outputs an html page; it is a
server-side technology
HTTP Request
HTTP Response
browser

Server-side Request
Response Header +
Html file
web server

Java Servlet
Java Server Page

servlet container
(engine) - Tomcat

Servlet Structure
Java Servlet Objects on Server Side
Managed by Servlet Container
Loads/unloads servlets
Directs requests to servlets
Request → doGet()

Each request is run as its own thread

A servlet Is a Server-side Java Class
Which Responds to Requests

doGet()

doPost()
service()
doPut()

doDelete()

8

Servlet Basics
Packages:
javax.servlet, javax.servlet.http
Runs in servlet container such as Tomcat
Tomcat 4.x for Servlet 2.3 API
Tomcat 5.x for Servlet 2.4 API

Servlet lifecycle
Persistent (remains in memory between requests)
Startup overhead occurrs only once
init() method runs at first request
service() method for each request
destroy() method when server shuts down

Web App with Servlets

GET …

Servlet

HEADERS
BODY

doGet()
…
…

Servlet Container

Life-cycle of a servlet
init()
service()
destroy()
It can sit there simply servicing requests with no
start up overhead, no code compilation. It is
FAST
It may be stateless – minimal memory

11

5 Simple Steps for Java Servlets
1. Subclass off HttpServlet
2. Override doGet(....) method
3. HttpServletRequest
getParameter("paramName")

4. HttpServletResponse
set Content Type
get PrintWriter
send text to client via PrintWriter

5. Don't use instance variables

HTTP: Request &
Response
Client sends HTTP request (method) telling
server the type of action it wants performed
GET method
POST method

Server sends response

14

Http: Get & Post

GET method
For getting information from server
Can include query string, sequence with additional
information for GET, appended to URL
e.g. Http://www.whoami.Com/Servlet/search?Name=Inigo&ID=1223344

15

Http: Get & Post

POST method
designed to give information to server
all information (unlimited length) passed as part of
request body
• invisible to user
• cannot be reloaded (by design)

16

Interaction with Client
HttpServletRequest
String getParameter(String)
Enumeration getParameters(String[])

HttpServletResponse
Writer getWriter()
ServletOutputStream getOutputStream()

Handling GET and POST Requests

HttpServlet
Implements Servlet
Receives requests and sends responses to a web
browser
Methods to handle different types of HTTP requests:
handles GET requests
doPost() handles POST requests
doPut() handles PUT requests
doDelete() handles DELETE requests
doGet()

10/12/11

G53ELC: Servlets

18

HttpServlet Request Handling

19

Handling HttpServlet
Requests
service() method not usually overridden
doXXX() methods handle the different request types
Needs to be thread-safe or must run on a STM
(SingleThreadModel) Servlet Engine
multiple requests can be handled at the same time

20

Simple Counter Example
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class SimpleCounter extends HttpServlet
{
int count = 0;
public void doGet(HttpServletRequest req, HttpServletResponse res) throws
ServletException, IOException
{
res.setContentType(“text/plain”);
PrintWriter out = res.getWriter();
count++;
out.println(“This servlet has been accessed “ + count +
“ times since loading”);
}
}

21

Multithreading solutions
Synchronize the doGet() method
public synchronized void doGet(HttpServletRequest req,
HttpServletResponse res)

servlet can’t handle more than one GET request at a
time

Synchronize the critical section
PrintWriter out = res.getWriter();
synchronized(this)
{
count++;
out.println(“This servlet has been accessed “ + count + “ times
since loading”);
}

22

Forms and Interaction
<form method=get action=“/servlet/MyServlet”>
GET method appends parameters to action URL:
/servlet/MyServlet?userid=Jeff&pass=1234
This is called a query string (starting with ?)

Username: <input type=text name=“userid” size=20>
Password: <input type=password name=“pass”
size=20>
<input type=submit value=“Login”>

RequestHeaderExample.java
import
import
import
import

java.io.*;
java.util.*;
javax.servlet.*;
javax.servlet.http.*;

public class RequestHeaderExample extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
Enumeration e = request.getHeaderNames();
while (e.hasMoreElements()) {
String name = (String)e.nextElement();
String value = request.getHeader(headerName);
out.println(name + “ = “ + value );
}
}
}

Accessing Request Components
getParameter("param1")
getCookies() => Cookie[]
getContentLength()
getContentType()
getHeaderNames()
getMethod()

Environment Variables
JavaServlets do not require you to use the
clunky environment variables used in CGI
Individual functions:
PATH_INFO
REMOTE_HOST
QUERY_STRING
…

req.getPathInfo()
req.getRemoteHost()
req.getQueryString()

Setting Response Components
Set status first!
setStatus(int)

HttpServletResponse.SC_OK...
sendError(int, String)
sendRedirect(String url)

Setting Response Components
Set headers
setHeader(…)
setContentType(“text/html”)

Output body
PrintWriter out = response.getWriter();
out.println("<HTML><HEAD>...")

 Install Web Server on your machine such as Tomcat, Appache, ..
 Create Home Page as CV which should contains:
 Your Name as title
 You Image in the right hand side
 Your previous courses listed in Table

 Another linked Page for Accepting your friends which should
contains form to accept your friends information:








Name (Text Field)
Birth of Date (Text Field)
Country ( Drop Down List)
E-mail ( Text Field)
Mobile ( Text Field)
Gender (Radio Button with Male or Female)
Save request of friends into file

31

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