UNIT-3 Servlet

Servlet
 life cycle of a servlet.
 The Servlet API
 Handling HTTP Request
 Handling HTTP Response
 Session Tracking
 using Cookies

Intro…
– Servlets are small programs that execute on the server side of a
web connection.
– Just as applets dynamically extend the functionality of a web
browser.
– servlets dynamically extend the functionality of a web server.

Early days of the Web
– A server could dynamically construct a page by creating a separate process to
handle each client request.
– The process would open connections to one or more databases in order to
obtain the necessary information.
– It communicated with the web server via an interface known as the Common
Gateway Interface (CGI).
– CGI allowed the separate process to read data from the HTTP request and write
data to the HTTP response.

Cont.,
– A variety of different languages were used to build CGI programs. These
included C, C++, and Perl.
– CGI suffered serious performance problems.
– It was expensive in terms of processor and memory resources to create a
separate process for each client request.
– It was also expensive to open and close database connections for each
client request.
– In addition, the CGI programs were not platform-independent.
– Therefore, other techniques were introduced. Among these are servlets.

Servlets offer several advantages in
comparison with CGI
– First, performance is significantly better. Servlets execute within the address space of a
web server.
– It is not necessary to create a separate process to handle each client request.
– Second, servlets are platform-independent because they are written in Java.
– Third, the Java security manager on the server enforces a set of restrictions to protect
the resources on a server machine.
– Finally, the full functionality of the Java class libraries is available to a servlet. It can
communicate with applets, databases, or other software via the sockets and RMI
mechanisms that you have seen already.

The Life Cycle of a Servlet
– Three methods are central to the life cycle of a servlet.
These are:
– init( )
– service( )
– destroy( )

User scenario to understand
when these methods are called
– First, assume that a user enters a Uniform Resource Locator (URL) to a web browser.
The browser then generates an HTTP request for this URL. This request is then sent
to the appropriate server.
– Second, this HTTP request is received by the web server. The server maps this
request to a particular servlet. The servlet is dynamically retrieved and loaded into the
address space of the server.
– Third, the server invokes the init( ) method of the servlet. This method is invoked
only when the servlet is first loaded into memory. It is possible to pass initialization
parameters to the servlet so it may configure itself.

Cont.,
– Fourth, the server invokes the service( ) method of the servlet. This method is called to
process the HTTP request. the servlet to read data that has been provided in the HTTP
request.
–
– It may also formulate an HTTP response for the client. The servlet remains in the
server’s address space and is available to process any other HTTP requests received
from clients. The service( ) method is called for each HTTP request.
– Finally, the server may decide to unload the servlet from its memory. The algorithms
by which this determination is made are specific to each server. The server calls the
destroy( ) method to relinquish any resources.

Using Tomcat for Servlet Development
– To create servlets, you will need access to a servlet
development environment.
– The one used by is Tomcat, Tomcat is an open-source product
maintained by the Jakarta Project of the Apache Software
Foundation.
– It contains the class libraries, documentation, and runtime
support that you will need to create and test servlets.

A Simple Servlet
– To become familiar with the key servlet concepts, we will begin by
building and testing a simple servlet. The basic steps are the following:
1. Create and compile the servlet source code. Then, copy the
servlet’s class file to the proper directory, and add the servlet’s
name and mappings to the proper web.xml file.
2. Start Tomcat.
3. Start a web browser and request the servlet.

Create and Compile the Servlet Source Code
import java.io.*;
import javax.servlet.*;
public class HelloServlet extends GenericServlet {
public void service(ServletRequest request,ServletResponse
response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter pw = response.getWriter();
pw.println("<B>Hello!</B>");
pw.close();
} }

Start Tomcat
– Start Tomcat as explained earlier. Tomcat must be running
before you try to execute a servlet.
– Start a Web Browser and Request the Servlet
– Start a web browser and enter the URL shown here:
http://localhost:8080/servlets-examples/servlet/HelloServlet
– Alternatively, you may enter the URL shown here:
http://127.0.0.1:8080/servlets-examples/servlet/HelloServlet
– This can be done because 127.0.0.1 is defined as the IP address
of the local machine.

The Servlet API
– Two packages contain the classes and interfaces that are required to
build servlets. These are:
– javax.servlet
– javax.servlet.http.
– They constitute the Servlet API.
– Keep in mind that these packages are not part of the Java core
packages.
– Instead, they are standard extensions provided by Tomcat.

The javax.servlet Package
– The javax.servlet package contains a number of interfaces
and classes that establish the framework in which. The
ServletRequest and ServletResponse interfaces are also
very important:
– Servlet
– ServletConfig
– ServletContext
– ServletRequest
– ServletResponse

The following summarizes the core classes that are
provided in the javax.servlet :
–GenericServlet
–ServletInputStream
–ServletOutputStream
–ServletException
–UnavailableException

<html>
<body>
<center>
<form name="Form1“ method="post“ action="http://localhost:8080/servlets-
examples/servlet/PostParametersServlet">
<table>
<tr>
<td><B>Employee</td>
<td><input type=textbox name="e" size="25" value=""></td>
</tr>
<tr>
<td><B>Phone</td>
<td><input type=textbox name="p" size="25" value=""></td>
</tr>
</table>
<input type=submit value="Submit">
</form>
</body> </html>

import java.io.*;
import java.util.*;
public class PostParametersServlet extends GenericServlet {
public void service(ServletRequest request,ServletResponse response)
// Get enumeration of parameter names.
Enumeration e = request.getParameterNames();
// Display parameter names and values.
while(e.hasMoreElements()) {
String pname = (String)e.nextElement();
pw.print(pname + " = ");
String pvalue = request.getParameter(pname);
pw.println(pvalue);
}
pw.close();
}
}

Compile the servlet. Next, copy it to the appropriate
directory, and update the web.xml file, as previously
described.
Then, perform these steps to test this example:
1. Start Tomcat (if it is not already running).
2. Display the web page in a browser.
3. Enter an employee name and phone number in the text
fields.
4. Submit the web page.
After following these steps, the browser will display a
response that is dynamically generated by the servlet.

The javax.servlet.http Package
– The javax.servlet.http package contains a number of interfaces
and classes that are commonly used by servlet developers.
– You will see that its functionality makes it easy to build servlets
that work with HTTP requests and responses.
– The following table summarizes the core interfaces that are
provided in this package:
Interface Description
HttpSer vletRequest Enables ser vlets to read data from an HTTP request.
HttpSer vletResponse Enables ser vlets to write data to an HTTP response.
HttpSession Allows session data to be read and written.
HttpSessionBindingListener Informs an object that it is bound to or unbound from a
session.

Class Description
Cookie Allows state information to be stored on a client machine.
HttpServlet Provides methods to handle HTTP requests and responses.
HttpSessionEvent Encapsulates a session-changed event.
HttpSessionBindingEvent Indicates when a listener is bound to or unbound from a session
value, or that a session attribute changed.
 The following table summarizes the core classes that are provided in this package.
 The most important of these is HttpServlet. Servlet developers typically extend this
class in order to process HTTP requests.

 The HttpServletRequest Interface
The HttpServletRequest interface enables a servlet to obtain information about
a client request.
 The HttpServletResponse Interface
The HttpServletResponse interface enables a servlet to formulate an HTTP
response to a client. Several constants are defined. These correspond to the different
status codes that can be assigned to an HTTP response.
For example, SC_OK indicates that the HTTP request succeeded,
SC_NOT_FOUND indicates that the requested resource is not available.
 The HttpSession Interface
The HttpSession interface enables a servlet to read and write the state
information that is associated with an HTTP session. All of these methods throw an
IllegalStateException if the session has already been invalidated.

Handling HTTP Requests and Responses
– The HttpServlet class provides specialized methods that handle the various types of
HTTP requests.
– A servlet developer typically overrides one of these methods. These methods are:
 doDelete( )
 doGet( )
 doHead( )
 doOptions( )
 doPost( )
 doPut( )
 doTrace( )
– However, the GET and POST requests are commonly used when handling form input.

<html>
<body>
<center>
<form name="Form1“ action="http://localhost:8080/servlets-examples/servlet/ColorGetServlet">
<B>Color:</B>
<select name="color" size="1">
<option value="Red">Red</option>
<option value="Green">Green</option>
<option value="Blue">Blue</option>
</select>
<br><br>
</form>
</body>
</html>

import java.io.*;
import javax.servlet.http.*;
public class ColorGetServlet extends HttpServlet {
public void doGet(HttpServletRequest request,HttpServletResponse response)
String color = request.getParameter("color");
pw.println("<B>The selected color is: </b> ");
pw.println(color);
pw.close();
}
}

<html>
<body>
<center>
<form name="Form1“ method="post“
action="http://localhost:8080/servlets-examples/servlet/ColorPostServlet">
<B>Color:</B>
<select name="color" size="1">
<option value="Red">Red</option>
<option value="Green">Green</option>
<option value="Blue">Blue</option>
</select>
<br><br>
</form>
</body>
</html>

import java.io.*;
import javax.servlet.http.*;
public class ColorPostServlet extends HttpServlet {
public void doPost(HttpServletRequest request,HttpServletResponse
response) throws ServletException, IOException {
String color = request.getParameter("color");
pw.println("<B>The selected color is: </b> ");
pw.println(color);
pw.close();
}
}

UNIT-3 Servlet

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UNIT-3 Servlet