Wt unit 3

UNIT-3
Introduction to Servlets

Concepts of CGI
What is CGI ?
The Common Gateway Interface, or CGI, is a set of standards
that define how information is exchanged between the web
server and a custom script.
The Common Gateway Interface, or CGI, is a standard for
external gateway programs to interface with information
servers such as HTTP servers.

Concepts of CGI
CGI technology enables the web server to call an external
program and pass HTTP request information to the external
program to process the request.
For each request, it starts a new process.

Concepts of CGI
CGI Architecture Diagram

Disadvantages of CGI
There are many problems in CGI technology:
1. If number of clients increases, it takes more time for sending
response.
2. For each request, it starts a process and Web server is limited
to start processes.
3. It uses platform dependent language e.g. C, C++, perl.

Concepts of Servlets
What is a Servlet?
Servlet can be described in many ways, depending on the context.
•Servlet is a technology i.e. used to create web application.
•Servlet is an API that provides many interfaces and classes
including documentations.
•Servlet is an interface that must be implemented for creating any
servlet.
•Servlet is a class that extend the capabilities of the servers and
respond to the incoming request. It can respond to any type of
requests.
•Servlet is a web component that is deployed on the server to
create dynamic web page.

Advantage of Servlet
There are many advantages of servlet over CGI.
The basic benefits of servlet are as follows:
•better performance: because it creates a thread for each request
not process.
•Portability: because it uses java language.
•Robust: Servlets are managed by JVM so we don't need to worry
about memory leak, garbage collection etc.
•Secure: because it uses java language..

Advantage of Servlet
The web container creates threads for handling the multiple requests to the
servlet.
Threads have a lot of benefits over the Processes such as they share a common
memory area, lightweight, cost of communication between the threads are low.

Life Cycle of a Servlet (Servlet Life Cycle)
The web container maintains the life cycle of a servlet instance.
Let's see the life cycle of the servlet:
Servlet class is loaded.
Servlet instance is created.
init method is invoked.
service method is invoked.
destroy method is invoked.

Life Cycle of a Servlet (Servlet Life Cycle)

Servlet Life Cycle methods
The init() Method:
The init method is called only once.
It is called only when the servlet is created, and not
called for any user requests afterwards.
The init method definition looks like this −
public void init() throws ServletException
{
// Initialization code...
}

Servlet Life Cycle methods(cont..)
The service() Method
The service() method is the main method to perform the
actual task.
The servlet container (i.e. web server) calls the service()
method to handle requests coming from the client(
browsers) and to write the formatted response back to
the client.
The service() method checks the HTTP request type
(GET, POST, PUT, DELETE, etc.) and calls doGet, doPost,
doPut, doDelete, etc. methods as appropriate.

The service() Method
Here is the signature of this method −
public void service(ServletRequest request, ServletResponse response) throws
ServletException, IOException
{
…..
…
}
We have nothing to do with service() method but you override
either doGet() or doPost() depending on what type of request
you receive from the client.

The destroy() Method
The destroy() method is called only once at the end of
the life cycle of a servlet.
This method gives your servlet a chance to close
database connections, halt background threads, write
cookie lists or hit counts to disk, and perform other such
cleanup activities.
public void destroy()
{ // Finalization code...
}

Servlet Life Cycle Scenario

Sample Code of Servlets
//Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
// Extend HttpServlet class
public class HelloWorld extends HttpServlet {
private String message;
public void init() throws ServletException {
// Do required initialization
message = "Hello World";
}
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// Set response content type
response.setContentType("text/html");
// Actual logic goes here.
PrintWriter out = response.getWriter();
out.println("<h1>" + message + "</h1>");
}
public void destroy() {
// do nothing.
}
}

Compiling Servlets
Compiling a Servlet:
1. Let us create a file with name HelloWorld.java with the code
shown above.
2. Place this file at C:ServletDevel (in Windows) or at
/usr/ServletDevel (in Unix).
3. This path location must be added to CLASSPATH before
proceeding further.
4. Assuming your environment is setup properly, go
in ServletDevel directory and compile HelloWorld.java as follows
−
$ javac HelloWorld.java

Compiling Servlets(con..)
5. If the servlet depends on any other libraries, you have to
include those JAR files on your CLASSPATH as well.
6. Include only servlet-api.jar JAR file because if you are not
using any other library in Hello World program.
7. If everything goes fine, above compilation would
produce HelloWorld.class file in the same directory.
Next section would explain how a compiled servlet would be
deployed in production.

Servlets Deployment
Servlet Deployment:
Step 1: By default,
A servlet application is located at the path
<Tomcat-installationdirectory>/webapps/ROOT
and
the class file would reside in
<Tomcat-installationdirectory>/webapps/ROOT/WEB-
INF/classes.
If you have a fully qualified class name,
of com.myorg.MyServlet then this servlet class must be
located in WEB-INF/classes/com/myorg/MyServlet.class.

Servlets Deployment(con..)
Step 2:
let us copy HelloWorld.class into
<Tomcat-installationdirectory>/webapps/ROOT/WEB-
INF/classes
Step 3:
create following entries in web.xml file located in
<Tomcat-installation-directory>/webapps/ROOT/WEB-INF/

Servlets Deployment(cont..)
<servlet>
<servlet-name>HelloWorld</servlet-name>
<servlet-class>HelloWorld</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HelloWorld</servlet-name>
<url-pattern>/HelloWorld</url-pattern>
</servlet-mapping>
Above entries to be created inside <web-app>...</web-app> tags
available in web.xml file.

Step 4:
Start tomcat server using
<Tomcat-installationdirectory>binstartup.bat (on
Windows) or
<Tomcat-installationdirectory>/bin/startup.sh (on
Linux/Solaris etc.) and
Step 5:finally type
http://localhost:8080/HelloWorld
in the browser's address box.

Servlet API
The javax.servlet and javax.servlet.http packages
represent interfaces and classes for servlet api.
The javax.servlet package contains many
interfaces and classes that are used by the servlet or
web container. These are not specific to any
protocol.
The javax.servlet.http package contains interfaces
and classes that are responsible for http requests
only.

There are many interfaces in javax.servlet package. They are as
follows:
1. Servlet
2. ServletRequest
3. ServletResponse
4. RequestDispatcher
5. ServletConfig
6. ServletContext
7. SingleThreadModel
8. Filter
9. FilterConfig
10. FilterChain
11. ServletRequestListener
12. ServletRequestAttributeListener
13. ServletContextListener
14. ServletContextAttributeListener

There are many classes in javax.servlet package. They are as follows:
1. GenericServlet
2. ServletInputStream
3. ServletOutputStream
4. ServletRequestWrapper
5. ServletResponseWrapper
6. ServletRequestEvent
7. ServletContextEvent
8. ServletRequestAttributeEvent
9. ServletContextAttributeEvent
10. ServletException
11. UnavailableException

Interfaces in javax.servlet.http package
1. HttpServletRequest
2. HttpServletResponse
3. HttpSession
4. HttpSessionListener
5. HttpSessionAttributeListener
6. HttpSessionBindingListener
7. HttpSessionActivationListener
8. HttpSessionContext (deprecated now)

Classes in javax.servlet.http package
1. HttpServlet
2. Cookie
3. HttpServletRequestWrapper
4. HttpServletResponseWrapper
5. HttpSessionEvent
6. HttpSessionBindingEvent
7. HttpUtils (deprecated now)

Parameters in Servlets
The parameters are the way in which a client or user can
send information to the Http Server

For example,
in a login screen, we need to send to the server, the user
and the password so that it validates them.

For example,
How does the client or the Browser send these parameters
using the methods GET or POST,
The first thing we are going to do is to create in our site a page
"login.html" with the following content:
<html>
<body>
<form action="login" method="get">
<table>
<tr>
<td>User</td>
<td><input name="user" /></td>
</tr>
<tr>
<td>password</td>
<td><input name="password" /></td>
</tr>
</table>
<input type="submit" />
</form>
</body>
</html>

Then, we create a Servlet which receives the request
in /login , which is the indicated direction in
the action attribute of the tag <form> of login.html

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LoginServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String user = req.getParameter("user");
String pass = req.getParameter("password");
if ("edu4java".equals(user) && "eli4java".equals(pass)) {
response(resp, "login ok");
} else {
response(resp, "invalid login");
}
}

private void response(HttpServletResponse resp, String msg)
throws IOException {
PrintWriter out = resp.getWriter();
out.println("<html>");
out.println("<body>");
out.println("<t1>" + msg + "</t1>");
out.println("</body>");
out.println("</html>");
}
}

We modify web.xml to link /login with this Servlet.
<web-app>
<servlet>
<servlet-name>timeservlet</servlet-name>
<servlet-class>com.edu4java.servlets.FirstServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>login-servlet</servlet-name>
<servlet-class>com.edu4java.servlets.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>timeservlet</servlet-name>
<url-pattern>/what-time-is-it</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>login-servlet</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
</web-app>

Session Tracking and Cookies
Session Tracking:
Keeping track of a sequence of related requests sent
by a client to perform some designated task is known
as “Session Tracking”
Cookies are one of the solutions to session tracking.

Session Tracking and Cookies(cont..)
Cookies:
A cookie is a small piece of information that is persisted
between the multiple client requests.
A cookie has
 a name, a single value, and
 optional attributes such as a comment, path and
 domain qualifiers, a maximum age, and
 a version number.

How Cookies Work:

How Cookies Work:
•By default, each request is considered as a new request.
•we add cookie with response from the servlet.
•So cookie is stored in the cache of the browser.
•After that if request is sent by the user, cookie is added with
request by default.
•we recognize the user as the old user.

Types of Cookie
There are 2 types of cookies in servlets.
1. Non-persistent cookie
2. Persistent cookie
Non-persistent cookie
It is valid for single session only. It is removed each time
when user closes the browser.
Persistent cookie
It is valid for multiple session . It is not removed each
time when user closes the browser. It is removed only if
user logout or signout.

Advantage of Cookies
Simplest technique of maintaining the state.
Cookies are maintained at client side.
Disadvantage of Cookies
It will not work if cookie is disabled from the browser.
Only textual information can be set in Cookie object.

Cookie class
javax.servlet.http.Cookie class provides the functionality
of using cookies. It provides a lot of useful methods for
cookies.

Constructor of Cookie class
Cookie() : constructs a cookie.
Cookie(String name, String value) : constructs a cookie
with a specified name and value.

Method Descriptionpublic
Public void setMaxAge(int expiry) Sets the maximum age
of the cookie in seconds.
public String getName() Returns the name of the cookie.
The name cannot be changed after creation.
public String getValue() Returns the value of the cookie.
public void setName(String name) changes the name
of the cookie.
public void setValue(String value) changes the value
of the cookie

How to send Cookies to the Client( 3 steps )
1) Create a Cookie object.
Cookie c = new Cookie("userName",”Vits");
2) Set the maximum Age:
c.setMaxAge(1800); // value in seconds
3) Place the Cookie in HTTP response header:
response.addCookie(c);

How to Read Cookies from the Client/browser
Cookie c[]=request.getCookies(); //c.length gives the cookie
count
for(int i=0;i<c.length;i++)
{
out.print("Name: "+c[i].getName()+" & Value:
"+c[i].getValue());
}

How to delete Cookies from the Client/browser
If you want to delete a cookie then you simply need to follow up following
three steps −
Read an already existing cookie and store it in Cookie object.
Set cookie age as zero using setMaxAge() method to delete an existing
cookie
Add this cookie back into response header.

How to delete Cookies from the Client/browser
Let's see the simple code to delete cookie.
It is mainly used to logout or sign out the user.
Cookie ck=new Cookie("user",""); //deleting value of cookie
ck.setMaxAge(0); //changing the maximum age to 0 seconds
response.addCookie(ck); //adding cookie in the response

Http Sessions
A session contains information specific to a particular user
across the whole application.
In such case, container creates a session id for each user.
The container uses this id to identify the particular user.
This unique ID can be stored into a cookie or in a request
parameter.
The HttpSession stays alive until it has not been used for
more than the timeout value specified in tag in deployment
descriptor file.
The default timeout value is 30 minutes, this is used if you
don’t specify the value in tag.

Http Sessions(cont..)

This is how you create a HttpSession object.
protected void doPost(HttpServletRequest req,
HttpServletResponse res) throws ServletException,
IOException
{
HttpSession session = req.getSession();
}

How you can store user information in
HttpSession object.
setAttribute(): method stores user information session
object
later when needed this information can be fetched from
the session.

How you can store user information in
HttpSession object.
Example: storing username, email-id and user age in
session with the attribute name uName, uemailId and
uAge respectively.
session.setAttribute("uName", "Chaitanya");
session.setAttribute("uemailId", "myemailid@gmail.com");
session.setAttribute("uAge", "30");

TO get the value from session we use the
getAttribute() method of HttpSession interface.
Example:
String userName = (String) session.getAttribute("uName");
String userEmailId = (String) session.getAttribute("uemailId");
String userAge = (String) session.getAttribute("uAge");

Sr.No. Method & Description
1
public Object getAttribute(String name)
This method returns the object bound with the specified
name in this session, or null if no object is bound under
the name.
2
public Enumeration getAttributeNames()
This method returns an Enumeration of String objects
containing the names of all the objects bound to this
session.
3
public long getCreationTime()
This method returns the time when this session was
created, measured in milliseconds since midnight January
1, 1970 GMT.
4
public String getId()
This method returns a string containing the unique
identifier assigned to this session.

5
public long getLastAccessedTime()
This method returns the last accessed time of the session,
in the format of milliseconds since midnight January 1,
1970 GMT
6
public int getMaxInactiveInterval()
This method returns the maximum time interval
(seconds), that the servlet container will keep the session
open between client accesses.
7
public void invalidate()
This method invalidates this session and unbinds any
objects bound to it.
8
public boolean isNew()
This method returns true if the client does not yet know
about the session or if the client chooses not to join the
session.

9
public void removeAttribute(String name)
This method removes the object bound with the specified
name from this session.
10
public void setAttribute(String name, Object value)
This method binds an object to this session, using the
name specified.
11
public void setMaxInactiveInterval(int interval)
This method specifies the time, in seconds, between client
requests before the servlet container will invalidate this
session.

Wt unit 3

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