Working with Servlets

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Working with Servlets

Objectives
In this session, you will learn to:
Explore the ServletConfig interface
Explore the ServletContext interface
Implement ServletContext Inerface
Introduce session tracking
Implement session tracking

The ServletConfig interface:
Is implemented by a Web container during the initialization of a
servlet to pass the configuration information to a servlet.
Is passed to the init()method of the servlet during initialization.
Can be used to pass initialization parameters to the servlets.
The initialization parameters are passed as name-value pairs.
For example, the connection URL can be passed as an initialization
parameter of the servlet.
Exploring the ServletConfig Interface

The following table describes the various methods defined in the
ServletConfig interface:
Exploring the ServletConfig Interface (Contd.)
Method Description
String getInitParameter(String
param)
It returns a String object containing the
value of the initialization parameters.
Enumeration<String>
getInitParameterNames()
It returns the names of all the initialization
parameters as an enumeration of String
objects.
ServletContext getServletContext() It returns the ServletContext object for
the servlet in which the caller is executing.
String getServletName() It returns a String object containing the
name of the servlet instance.

The ServletContext provides the environmental information to
the servlets in which they are running.
Each Web application consists of only one ServletContext
object.
A ServletContext object is also known as a Web context.
The Web container creates an object of ServletContext at time
of deploying the project.
Following figure shows the creation of ServletContext object.
Exploring the ServletContext Interface
ServletContext Object Creation

ServletContext interface:
Exploring the ServletContext Interface (Contd.)
Method Description
public void setAttribute(String,
Object)
Binds the object with a name and stores
the name/value pair as an attribute of the
ServletContext object. If an attribute
already exists, this method replaces the
existing attribute.
public Object getAttribute(String
attrname)
Returns the object stored in the
ServletContext object with the name
passed as a parameter.
public Enumeration
getAttributeNames()
Returns an enumeration of the String
object that contains names of all the
context attributes.
public String
getInitParameter(String pname)
Returns the value of the initialization
parameter with the name passed as a
parameter.
public Enumeration
getInitParameterNames()
Returns an enumeration of String object
that contains names of all the initialization
parameters.

Usage of the ServletContext interface:
The ServletContext object provides an interface between the
container and servlet.
It can be used to get configuration information from the web.xml
file.
It can be used to set, get or remove attribute from the web.xml file.
It can be used to provide inter-application communication.
You can access ServletContext object using the
getServletContext method of:
The ServletConfig interface,
The GenericServlet class
The HttpServletRequest interface
Exploring the ServletContext Interface (Contd.)

Accessing ServletContext object using the
getServletContext method of the ServletConfig interface:
You can use the following code snippet to get the ServletContext
object:
In the preceding code snippet,
 The getServletConfig method returns the object of
ServletConfig.
 Thereafter, the conf object calls getServletContext that returns the
ServletContext object.
Implementing the ServletContext Interface
ServletConfig conf = getServletConfig();
ServletContext context = conf.getServletContext();

Consider a scenario where you need to develop a Web application
that stores the email and name of user in web.xml file. In
addition, you need to access those parameters using
ServletContext and display them, as shown in the following
figure.
Activity: Implementing the ServletContext Interface
The Expected Output

getServletContext method of the GenericServlet class:
In general, every servlet class extends HttpServlet, which is a sub
class of GenericServlet.
Use the following code snippet to access the ServletContext
object:
In the preceding code snippet, the getServletContext method
returns the ServletContext object.
Implementing the ServletContext Interface (Contd.)
ServletContext context = getServletContext();

that fulfills the following requirements:
It should provide a user interface to accept the credentials, as shown
in the following figure.
It should validate the entered details with value stored in the
web.xml file.
Activity: Implementing ServletContext Using GenericServlet
The Expected Output

It should display welcome message if the credentials are correct, as
shown in the following figure.
Activity: Implementing ServletContext Using GenericServlet (Contd.)
The Expected Output

getServletContext method of the HttpRequestServlet
interface:
You can use the following code snippet to access the
ServletContext object:
In the preceding code snippet, the req object calls the
getServletContext method that returns the ServletContext
object.
Implementing the ServletContext Interface (Contd.)
public void doGet/doPost(HttpServletRequest req,
HttpServletResponse res)
{
ServletContext ctx = req.getServletContext();
}

that fulfills the following requirements:
It should provide a user interface to accept the customer details, as
shown in the following figure:
It should store the entered details to access database.
It should display a message “Details are saved.”
Activity: Implementing ServletContext Using HttpServletRequest
The Expected Output

Session tracking is the process of maintaining information, or
state, about Web site visitors as they move from page to page.
The connection from a browser to a Web server occurs over the
stateless Hypertext Transfer Protocol (HTTP).
Therefore, the Web developer has to explicitly write code to
implement session tracking.
The mechanism to implement session tracking are:
HttpSession
Cookie
URL Rewriting
Introducing Session Tracking

HttpSession:
It is an interface that provides methods to handle session tracking.
A session object is created on the application server, usually in a Java
servlet or a Java Server Page.
The object gets stored on the application server and a unique
identifier called a session ID is assigned to it.
The object and session ID are handled by a session manager on the
application server.
Each session ID assigned by the application server has zero or more
key/value pairs tied to it.
The values are objects that you place in the session.
Implementing Session Tracking

HttpSession interface:
Implementing Session Tracking (Contd.)
Method Description
public void setAttribute(String
name, Object value)
Binds the object with a name and stores the
name/value pair as an attribute of the
HttpSession object. If an attribute already
exists, this method replaces the existing attribute.
public Object getAttribute(String
name)
Retrieves the String object specified in the
parameter, from the session object. If no object
is found for the specified attribute, the
getAttribute()method returns null.
public Enumeration
getAttributeNames()
Returns an Enumeration object that contains the
name of all the objects that are bound as
attributes to the session object.
public void
removeAttribute(String name)
Removes the object bound with the specified
name from this session.
public void invalidate() Invalidates this session and unbinds any objects
bound to it.
public Boolean isNew() Returns a Boolean with a value of true if the client
does not yet know about the session or if the
client chooses not to join the session

To use HttpSession object, you need to:
Create and retrieve session object using the getSession method of
HttpServletRequest.
Following code snippet shows an example:
The getSession method is passed a boolean value.
The false value indicates that you want to retrieve an existing
session object.
The true value lets the session manager know that a session object
needs to be created if one does not already exist.
HttpSession session = request.getSession(true);

An object of HttpSession can find out that the session is new or
can remove one.
Following code snippet shows an example:
In the above code snippet, the existing session object gets
removed and a new session object is created.
HttpSession session = request.getSession (true);
if (session.isNew() == false) {
session.invalidate();
session = request.getSession(true);
}

You need to develop a Web application that allows users to buy
products online. The application should be based on the following
guidelines:
It should provide a login page, as sown in the following figure.
Activity: Implementing Session Tracking Using HttpSession
The Expected Output

It should display a Web page to select brands of shirts, as sown in
the following figure.
Activity: Implementing Session Tracking Using HttpSession (Contd.)
The Expected Output

It should display the bill based on the selection, as shown in the
following figure.
Activity: Implementing Session Tracking Using HttpSession (Contd.)
The Expected Output

Cookie:
Is information that is stored as a name/value pair.
Is transmitted from the server to the browser.
Is a common way of session tracking.
Cookies can be used to tie specific visitors to information about
them on the server.
The Java servlet specification provides a simple cookie API that
allows you to write and retrieve cookies.
Following code snippet creates a Cookie object:
The Cookie object is valid for one hour.
The response object adds the cookie for later use.
Cookie user = new Cookie("user","Jennifer");
user.setMaxAge(3600);
response.addCookie(user);

Following code snippet retrieves the Cookie object:
The getCookies method retrieves the stored cookies.
The getName method returns the cookie name.
The getValue method returns the value stored in cookie.
String user = "";
Cookie[] cookies = request.getCookies();
if (cookies != null) {
for (int i = 0; i < cookies.length; i++) {
if (cookies[i].getName().equals("user"))
user =cookies[i].getValue();
}
}

You need to develop a Web application that accepts the name of a
user and writes it to cookie. For this, you need to create a user
interface, as shown in the following figure.
Activity: Implementing Session Tracking Using Cookie
The Expected Output

In addition, you need to read the cookie value and display it, as
shown in the following figure.
Activity: Implementing Session Tracking Using Cookie (Contd.)
The Expected Output

URL Rewriting:
is the lowest common denominator of session tracking.
Can be used for session tracking if client does not support cookies.
Involves adding data, a session ID, to the URL path that is
interpreted by the container to associate the request with a session.
In URL rewriting, users append a token or identifier to the URL of
the next Servlet or the next resource.

The HttpServletResponse interface provides following two
methods for URL Rewriting:
encodeURL(String):
 Encodes the specified URL by including the session ID in it.
 Returns the URL unchanged if encoding is not needed.
URLencodeRedirectURL(String):
 Encodes the specified URL for use in the sendRedirect method.
 Returns the URL unchanged if encoding is not needed.
 All URLs sent to the HttpServletResponse.sendRedirect
method should be run through this method. Otherwise, URL rewriting
cannot be used with browsers which do not support cookies.

Summary
In this session, you learned that:
ServletConfig is implemented by a Web container during the initialization
of a servlet to pass the configuration information to a servlet.
ServletContext provides the environmental information to the servlets in
which they are running.
Usage of the ServletContext interface are:
 It can be used to get configuration information from the web.xml file.
 It can be used to set, get or remove attribute from the web.xml file.
 It can be used to provide inter-application communication.
Session tracking is the process of maintaining information, or state, about
Web site visitors as they move from page to page.
The mechanism to implement session tracking are:
 HttpSession
 Cookie
 URL Rewriting

Summary (Contd.)
HttpSession is an interface that provides methods to handle session
tracking.
Cookie is information that is stored as a name/value pair and is transmitted
from the server to the browser.
It Is a common way of session tracking.
URL Rewriting can be used for session tracking if client does not support
cookies.
It Involves adding data, a session ID, to the URL path that is interpreted by the
container to associate the request with a session.

Working with Servlets

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