Java Web Programming [2/9] : Servlet Basic

Module 2: Servlet Basics

Thanisa Kruawaisayawan
Thanachart Numnonda
www.imcinstitute.com

Objectives
 What is Servlet?
 Request and Response Model
 Method GET and POST
 Servlet API Specifications
 The Servlet Life Cycle
 Examples of Servlet Programs

2

What is a Servlet?

 Java™ objects which extend the functionality of a
HTTP server
 Dynamic contents generation
 Better alternative to CGI
 Efficient
 Platform and server independent
 Session management
 Java-based

3

Servlet vs. CGI
Servlet CGI
 Requests are handled by  New process is created
threads. for each request
 Only a single instance (overhead & low
will answer all requests scalability)
for the same servlet  No built-in support for
concurrently (persistent sessions
data)

4

Servlet vs. CGI (cont.)
Request CGI1
Child for CGI1

Request CGI2 CGI
Based Child for CGI2
Webserver
Request CGI1
Child for CGI1

Request Servlet1
Servlet Based Webserver

Request Servlet2 Servlet1
JVM
Request Servlet1 Servlet2

5

Single Instance of Servlet

6

Servlet Request and Response
Model
Servlet Container
Request

Browser
HTTP Request
Servlet
Response

Web Response
Server

7

What does Servlet Do?
 Receives client request (mostly in the form of
HTTP request)
 Extract some information from the request
 Do content generation or business logic process
(possibly by accessing database, invoking EJBs,
etc)
 Create and send response to client (mostly in the
form of HTTP response) or forward the request to
another servlet or JSP page
8

Requests and Responses

 What is a request?
 Informationthat is sent from client to a server
 Who made the request

 Which HTTP headers are sent

 What user-entered data is sent

 What is a response?
 Information that is sent to client from a server
 Text(html, plain) or binary(image) data
 HTTP headers, cookies, etc
9

HTTP

 HTTP request contains
 Header
 Method
 Get: Input form data is passed as part of URL
 Post: Input form data is passed within message body

 Put

 Header

 request data
10

Request Methods
 getRemoteAddr()
 IP address of the client machine sending this request
 getRemotePort()
 Returns the port number used to sent this request
 getProtocol()
 Returns the protocol and version for the request as a string of the form
<protocol>/<major version>.<minor version>
 getServerName()
 Name of the host server that received this request
 getServerPort()
 Returns the port number used to receive this request

11

HttpRequestInfo.java
public class HttpRequestInfo extends HttpServlet {{
public class HttpRequestInfo extends HttpServlet
::
protected void doGet(HttpServletRequest request,
HttpServletResponse response)throws ServletException, IOException {{
HttpServletResponse response)throws ServletException, IOException
response.setContentType("text/html");
PrintWriter out == response.getWriter();
PrintWriter out response.getWriter();

out.println("ClientAddress: "" ++ request.getRemoteAddr() ++
out.println("ClientAddress: request.getRemoteAddr()
"<BR>");
"<BR>");
out.println("ClientPort: "" ++ request.getRemotePort() ++ "<BR>");
out.println("ClientPort: request.getRemotePort() "<BR>");
out.println("Protocol: "" ++ request.getProtocol() ++ "<BR>");
out.println("Protocol: request.getProtocol() "<BR>");
out.println("ServerName: "" ++ request.getServerName() ++ "<BR>");
out.println("ServerName: request.getServerName() "<BR>");
out.println("ServerPort: "" ++ request.getServerPort() ++ "<BR>");
out.println("ServerPort: request.getServerPort() "<BR>");

out.close();
out.close();
}}
::
}}

12

Reading Request Header
 General
 getHeader
 getHeaders
 getHeaderNames
 Specialized
 getCookies
 getAuthType and getRemoteUser
 getContentLength
 getContentType
 getDateHeader
 getIntHeader
13

Frequently Used Request Methods

 HttpServletRequest methods
 getParameter() returns value of named
parameter
 getParameterValues() if more than one value
 getParameterNames() for names of parameters

14

Example: hello.html
<HTML>
:
<BODY>
<form action="HelloNameServlet">
Name: <input type="text" name="username" />
<input type="submit" value="submit" />
</form>
</BODY>
</HTML>

15

HelloNameServlet.java

public class HelloNameServlet extends HttpServlet {{
public class HelloNameServlet extends HttpServlet
::
HttpServletResponse response)
throws ServletException, IOException {{
throws ServletException, IOException
out.println("Hello "" ++ request.getParameter("username"));
out.println("Hello request.getParameter("username"));
out.close();
out.close();
}}
::
}}

16

HTTP GET and POST
 The most common client requests
 HTTP GET & HTTP POST
 GET requests:
 User entered information is appended to the URL in a query string
 Can only send limited amount of data
 .../chap2/HelloNameServlet?username=Thanisa
 POST requests:
 User entered information is sent as data (not appended to URL)
 Can send any amount of data

18

TestServlet.java
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class TestServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
throws ServletException, IOException {

PrintWriter out = response.getWriter();

out.println("<h2>Get Method</h2>");
}
}

19

Steps of Populating HTTP
Response
 Fill Response headers
 Get an output stream object from the response
 Write body content to the output stream

20

Example: Simple Response
Public class HelloServlet extends HttpServlet {
public void doGet(HttpServletRequest request,

// Fill response headers

// Get an output stream object from the response
PrintWriter out = response.getWriter();

// Write body content to output stream
out.println("<h2>Get Method</h2>");
}
}

21

Servlet API Specifications
http://tomcat.apache.org/tomcat-7.0-doc/servletapi/index.html

22

Servlet Interfaces & Classes
Servlet

GenericServlet HttpSession

HttpServlet

ServletRequest ServletResponse

HttpServletRequest HttpServletResponse

23

CounterServlet.java

::
public class CounterServlet extends HttpServlet {{
public class CounterServlet extends HttpServlet
private int count;
private int count;
::
HttpServletResponse response) throws ServletException, IOException {{
HttpServletResponse response) throws ServletException, IOException
count++;
count++;
out.println("Count == "" ++ count);
out.println("Count count);
out.close();
out.close();
}}
::
}}

24

Servlet Life-Cycle
Is Servlet Loaded?

Http
request
Load Invoke
No

Http
response Yes
Run
Servlet
Servlet Container

Client Server
25

Servlet Life Cycle Methods
service( )

init( ) destroy( )
Ready
Init parameters

doGet( ) doPost( )
Request parameters 26

The Servlet Life Cycle
 init
 executed once when the servlet is first loaded
 Not call for each request
 Perform any set-up in this method
 Setting up a database connection

 destroy
 calledwhen server delete servlet instance
 Not call after each request
 Perform any clean-up
 Closing a previously created database connection

27

doGet() and doPost() Methods
Server HttpServlet subclass

doGet( )
Request

Service( )

Response doPost( )

Key: Implemented by subclass
28

Servlet Life Cycle Methods
 Invoked by container
 Container controls life cycle of a servlet
 Defined in
 javax.servlet.GenericServlet class or
 init()
 destroy()

 service() - this is an abstract method

 javax.servlet.http.HttpServlet class

 doGet(), doPost(), doXxx()
 service() - implementation
29

Implementation in method service()
protected void service(HttpServletRequest req, HttpServletResponse
resp)
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
...
doGet(req, resp);
...
} else if (method.equals(METHOD_HEAD)) {
...
doHead(req, resp); // will be forwarded to doGet(req,
resp)
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
...
} 30
}

Username and Password Example

31

Acknowledgement
Some contents are borrowed from the
presentation slides of Sang Shin, Java™
Technology Evangelist, Sun Microsystems,
Inc.

32

Thank you

thananum@gmail.com
www.facebook.com/imcinstitute
www.imcinstitute.com

33

Java Web Programming [2/9] : Servlet Basic

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