1. Servlet Fudamentals

2. Revisiting the Three-tier Model
Web-based e-commerce applications are usually built according to the “Three-tier Model”, which includes:
The First-tier: Web Client
The Second-tier: Server-side Application
The Third-tier: Database Management Systems (DBMS)

3. Three-Tier Model First-tier : Web Clients Third-tier (DBMS) :
Database
Cluster
Client Browser
First-tier : Web Clients
Second-tier : Server-side Web Applications
Web Server
Application Server
Third-tier (DBMS) :
Database Cluster(s)
SQL
Server
Other
databases
Oracle
DB2
Internet
Connectivity

4. Server-side programming technologies
There are numerous ways to implement server-side applications. Common examples include:
CGI (Common Gateway Interface)
ASP (Active Server Page)
Java Servlets
PHP, Perl, & other server programming languages

5. Common Gateway Interface (CGI)
Early Web page designs were “static” in the sense that a client can only request for a static HTML document from the Web server
Later, CGI programming techniques were introduced to remove this constraint by providing dynamic Web pages via server-side interaction, as shown in Fig.

6. Static Web Page Retrieval (Fig. 4.2)
 HTTP Request
 HTTP Document
 Retrieve Document
 Request for
Web Document
Web Documents
Web Browser
Web Server

7. CGI-based Web Application (Fig. 4.3)
 Get Data
 HTTP Request
 HTTP Document
 Output
(HTML)
 HTML forms to invoke CGI scripts
CGI Scripts/
Applications
Web Browser
Web Server
Database
 Return data

8. Disadvantages of CGI programs
Each new request activates a new process to run the CGI program
Creating a process requires time and resource so CGI programs is not as scaleable a solution
Also, CGI programs may raise security problems.

9. Java Servlet
A servlet is a small piece of server-side application, which can be viewed as the server-side analog of an applet.
In a typical servlet application, a servlet-enabled Web server receives an HTTP request from the client.
It then forwards the request to the servlet engine for performing the necessary operations as specified by the program.
Finally it returns a response (e.g., HTML document) to the client via the Web server (Fig. 4.5)

10. Typical Web-based Servlet Interaction (Fig. 4.5)
Servlet Engine
HTTP Request
HTTP Response
Web Browser
Web Server
Database

11. What can you build with Servlets?
Search Engines
E-Commerce Applications
Shopping Carts
Product Catalogs
Intranet Applications
Groupware Applications:
bulletin boards
file sharing

12. Servlets vs. CGI A Servlet does not run in a separate process.
A Servlet stays in memory between requests.
There is only a single instance of a servlet which answers all requests concurrently.
Browser 1
Web
Server
Browser 2
Browser N
Servlet

13. How Servlets Work is servlet No Receive loaded? Request Yes is servlet
current? No
Load Servlet
Yes
Send
Response
Process Request

14. Servlet Architecture The client makes a request via HTTP
Web
Server
Servlet
Container
Client
(web browser)
HTTP request
Servlet
HTTP response
The client makes a request via HTTP
The web server receives the requests and forwards it to the servlet
If the servlet has not yet been loaded, the web server loads it into the JVM and executes it
The servlet receives the HTTP request and performs some type of process
The servlet returns a response to the web server
The web server forwards the response to the client

15. Servlets follow a three-phase life cycle:
Servlet Life Cycle
Servlets follow a three-phase life cycle:
1) initialization
2) service
3) destruction

16. Life Cycle: Initialization
A servlet container:
loads a servlet class during startup, or
when the servlet is needed for a request
After the Servlet class is loaded, the container will instantiate it.

17. Life Cycle: Initialization
Initialization is performed by container before any request can be received.
Persistent data configuration, heavy resource setup (such as JDBC connection) and any one-time activities should be performed in this state.
The init() method will be called in this stage.

18. Life Cycle: Service 1
The service method is defined for handling client request.
The Container of a servlet will call this method every time a request for that specific servlet is received.

19. Life Cycle: Service
The Container generally handles concurrent requests with multithreads.
All interactions with response and requests will occur within this phase until the servlet is destroyed.

20. Life Cycle: Service Method
The service() method is invoked to every request and is responsible for generating the response to that request.
The service() method takes two parameters:
javax.servlet.ServletRequest
javax.servlet.ServletResponse
public void service ( ServletRequest request,
ServletResponse response )
throws IOException {
. . . }

21. Life Cycle: Destruction
When the servlet container determines that the servlet should be removed, it calls the destroy method of the servlet.
The servlet container waits until all threads running in the service method have been completed or time out before calling the destroy method.

22. Types of Servlet Generic Servlet Http Servlet javax.servlet (package)
extends javax.servlet.Servlet
service method
Http Servlet
javax.servlet.http (package)
extends javax.servlet.HttpServlet
doget(), doPost()….

23. Types of servlets (cont..)
Generic servlet
service(Request, Response) throws ServletException, IOException
HttpServlet
doGet(HttpServletRequest req, HttpServletResponse res)
doPost(HttpServletRequest req, HttpServletResponse res)

24. Servlet APIs
Every servlet must implement javax.servlet.Servlet interface
Most servlets implement the interface by extending one of these classes
javax.servlet.GenericServlet
javax.servlet.http.HttpServlet

25. Generic Servlet & HTTP Servlet
Client
request
Server
service ( )
response
HTTPServlet
Browser
request
doGet
( )
HTTP Server
service ( )
response
doPost
( )

26. Interface javax.servlet.Servlet
The Servlet interface defines methods
to initialize a servlet
to receive and respond to client requests
to destroy a servlet and its resources
to get any startup information
to return basic information about itself, such as its author, version and copyright.
Developers need to directly implement this interface only if their servlets cannot (or choose not to) inherit from GenericServlet or HttpServlet.
Life
Cycle
Methods

27. GenericServlet - Methods
void init(ServletConfig config)
Initializes the servlet.
void service(ServletRequest req, ServletResponse res)
Carries out a single request from the client.
void destroy()
Cleans up whatever resources are being held (e.g., memory, file handles, threads) and makes sure that any persistent state is synchronized with the servlet's current in-memory state.
ServletConfig getServletConfig()
Returns a servlet config object, which contains any initialization parameters and startup configuration for this servlet.
String getServletInfo()
Returns a string containing information about the servlet, such as its author, version, and copyright.

28. HTTPServlet
A general servlet knows nothing about the HyperText Transfer Protocol (HTTP), which is the major protocol used for Internet.
A special kind of servlet, HTTPServlet, is needed to handle requests from HTTP clients such as web browsers.
HTTPServlet is included in the package javax.servlet.http as a subclass of GenericServlet.

29. Hello World Example import java.io.*; import javax.servlet.*;
import javax.servlet.http.*;
public class HelloWorld extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException
res.setContentType("text/html");
PrintWriter out = res.getWriter();
out.println ("<!DOCTYPE HTML PUBLIC \”-//W3C//DTD HTML
4.0 Transitional//EN\”>");
out.println("<HTML><HEAD><TITLE>Hello
World</TITLE></HEAD>");
out.println(“<BODY><BIG>Hello World
</BIG></BODY></HTML>");
out.close(); }

30. Hello World Example import java.io.*; import javax.servlet.*;
import javax.servlet.http.*;
public class HelloWorld extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException
res.setContentType("text/html");
PrintWriter out = res.getWriter();
out.println ("<!DOCTYPE HTML PUBLIC \”-//W3C//DTD HTML
4.0 Transitional//EN\”>");
out.println("<HTML><HEAD><TITLE>Hello
World</TITLE></HEAD>");
out.println(“<BODY><BIG>Hello World
</BIG></BODY></HTML>");
out.close(); }

32. DEMO