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Answer: HTTP is a stateless protocol. Each request is independent of the previous one. However, in some applications, it is necessary to save state information so that information can be collected from several interactions between a browser and a server. Sessions provide such a mechanism.

A session can be created via the

getSession( )

method of

HttpServletRequest

. An

HttpSession

object is returned. This object can store a set of bindings that associate names with objects. The

setAttribute( )

,

getAttribute( )

,

getAttributeNames( )

, and

removeAttribute( )

methods of

HttpSession

manage these bindings. It is important to note that session state is shared among all the servlets that are associated with a particular client.

The following servlet illustrates how to use session state. The

getSession( )

method gets the current session. A new session is created if one does not already exist. The

getAttribute( )

method is called to obtain the object that is bound to the name “date”. That object is a

Date

object that encapsulates the date and time when this page was last accessed. (Of course, there is no such binding when the page is first accessed.) A

Date

object encapsulating the current date and time is then created. The

setAttribute( )

method is called to bind the name “date” to this object.

When you first request this servlet, the browser displays one line with the current date and time information. On subsequent invocations, two lines are displayed. The first line shows the date and time when the servlet was last accessed. The second line shows the current date and time.

Answer: The

ServletRequest

interface includes methods that allow you to read the names and values of parameters that are included in a client request. We will develop a servlet that illustrates their use. The example contains two files. A web page is defined in

PostParameters.htm

, and a servlet is defined in

PostParametersServlet.java

.

The HTMLsource code for

PostParameters.htm

is shown in the following listing. It defines a table that contains two labels and two text fields. One of the labels is Employee and the other is Phone. There is also a submit button. Notice that the action parameter of the form tag specifies a URL. The URL identifies the servlet to process the HTTP POST request.

The source code for

PostParametersServlet.java

is shown in the following listing. The

service( )

method is overridden to process client requests. The

getParameterNames( )

method returns an enumeration of the parameter names. These are processed in a loop. You can see that the parameter name and value are output to the client. The parameter value is obtained via the

getParameter( )

method.

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.

Answer: Now, let’s develop a servlet that illustrates how to use cookies. The servlet is invoked when a form on a web page is submitted.

The HTML source code for

AddCookie.htm

is shown in the following listing. This page contains a text field in which a value can be entered. There is also a submit button on the page. When this button is pressed, the value in the text field is sent to

AddCookieServlet

via an HTTP POST request.

The source code for

AddCookieServlet.java

is shown in the following listing. It gets the value of the parameter named “data”. It then creates a

Cookie

object that has the name “MyCookie” and contains the value of the “data” parameter. The cookie is then added to the header of the HTTP response via the

addCookie( )

method. A feedback message is then written to the browser.

The source code for

GetCookiesServlet.java

is shown in the following listing. It invokes the

getCookies( )

method to read any cookies that are included in the HTTP GET request. The names and values of these cookies are then written to the HTTP response. Observe that the

getName( )

and

getValue( )

methods are called to obtain this information.

Compile the servlets. Next, copy them 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

AddCookie.htm

in a browser.

3. Enter a value for

MyCookie

.

4. Submit the web page.

After completing these steps, you will observe that a feedback message is displayed by the browser.

Next, request the following URL via the browser:

http://localhost:8080/servlets-examples/servlet/GetCookiesServlet

Observe that the name and value of the cookie are displayed in the browser.