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Web Design & Development CS506

Lesson 35

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Web Design & Development CS506

Code Example: Use of Implicit Objects

The following example constitutes of 4 JSP pages. These are index.jsp, controller.jsp, web.jsp and java.jsp.

The user will select either the option of "java" or "web" from index.jsp, displayed in the form of radio

buttons and submits the request to controller.jsp. Based on the selection made by the user, controller.jsp

will redirect the user to respective pages (web.jsp or java.jsp).

The flow of the example is shown below in the pictorial form.

The code of these entire pages is given below.

index.jsp

<html>

<body>

<h2>

Select the page you want to visit</h2>

<h3>

Web Design & Develoment

</h3> <br>

<h3>

Java

</h3>

<br>

</form>

</body>

</html>

controller.jsp

<html>

<body>

// reading parameter "page", name of radio button using

// implicit object request

String pageName = request.getParameter("page");

// deciding which page to move on based on "page" value

// redirecting user by using response implicit object

if (pageName.equals("web")) {

response.sendRedirect("web.jsp");

} else if (pageName.equals("java") ) {

response.sendRedirect("java.jsp");

}

</body>

</html>

web.jsp

<html>

<body>

// use of out implicit object, to generate HTML

out.println( "<h2>" +

"Welcome to Web Design & Development Page" +

"</h2>"

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Web Design & Development CS506

);

</body>

</html>

java.jsp

<html>

<body>

// use of out implicit object, to generate HTML

out.println( "<h2>" +

"Welcome to Java Page" +

"</h2>"

);

</body>

</html>

The details of remaining 5 implicit objects are given below:

session

This variable is of type HttpSession, used to work with session object.

application

This variable is of type ServletContext. Allows to store values in key-value pair form that are

shared by all servlets in same web application

config

This variable is of type ServletConfig. Represents the JSP configuration options e.g. init-

parameters etc.

Page Context

This variable is of type javax.servlet.jsp.PageContext, to give a single point of access to

many of the page attributes. This object is used to stores the object values associated with

this object.

exception

This variable is of type java.lang.Throwable. Represents the exception that is passed to JSP

error page.

page

This variable is of type java.lang.Object. It is synonym for this.

JSP Directives

JSP directives are used to convey special processing information about the page to JSP container. It affects

the overall structure of the servlet that results from the JSP page. It enables programmer to:

Specify page settings

To Include content from other resources

To specify custom-tag libraries

Format

<%@ directive {attribute="val"}* %>

In JSP, there are three types of directives: page, include & taglib. The formats of using these are:

page: <%@ page {attribute="val"}* %>

include: <%@ include {attribute="val"}* %>

taglib: <%@ taglib {attribute="val"}* %>

JSP page Directive

Give high level information about servlet that will result from JSP page. It can be used anywhere in the

document. It can control

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Web Design & Development CS506

Which classes are imported

What class the servlet extends

What MIME type is generated

How multithreading is handled

If the participates in session

Which page handles unexpected errors etc.

The lists of attributes that can be used with page directive are:

language = "java"

extends = "package.class"

import = "package.*,package.class,..."

session = "true | false"

Info = "text"

contentType = "mimeType"

isThreadSafe = "true | false"

errorPage = "relativeURL"

isErrorPage = "true | false"

Some example uses are:

To import package like java.util

<%@page import="java.util.*" info="using util package" %>

To declare this page as an error page

<%@ page isErrorPage = "true" %>

To generate the excel spread sheet

<%@ page contentType = "application/vnd.ms-excel" %>

JSP include Directive

Lets you include (reuse) navigation bars, tables and other elements in JSP page. You can include files at

Translation Time (by using include directive)

Request Time (by using Action elements, discussed in next handouts)

Format

<%@include file="relativeURL"%>

Purpose

To include a file in a JSP document at the time document is translated into a servlet. It may contain JSP

code that affects the main page such as response page header settings etc.

Example Code: using include directive

This example contains three JSP pages. These are index.jsp, header.jsp & footer.jsp. The header.jsp will

display the text of "web design and development" along with current date. The footer.jsp will display only

"virtual university". The outputs of both these pages will be included in index.jsp by using JSP include

directive.

header.jsp

<%@page import="java.util.*"%>

<html>

<body>

<h3> Web Desing & Development </h3>

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Web Design & Development CS506

</marquee>

</body>

</html>

footer.jsp

<html>

<body>

</marquee>

</body>

</html>

index.jsp

<html>

<body>

// includes the output of header.jsp

<%@include file="header.jsp" %>

<TR><TH></TH><TH>Apples<TH>Oranges

<TR><TH>First Quarter<TD>2307<TD>4706

<TR><TH>Second Quarter<TD>2982<TD>5104

<TR><TH>Third Quarter<TD>3011<TD>5220

<TR><TH>Fourth Quarter<TD>3055<TD>5287

</TABLE>

// includes the output of footer.jsp

<%@include file="footer.jsp" %>

</body>

</html>Example Code: setting content type to generate excel spread sheet

In this example, index.jsp is modified to generate excel spread sheet of the last example. The change is

shown in bold face.

index.jsp

// setting content type to generate excel sheet using page directive

<%@page contentType="application/vnd.ms-excel" %>

<html>

<body>

// includes the output of header.jsp

<%@include file="header.jsp" %>

<TR><TH></TH><TH>Apples<TH>Oranges

<TR><TH>First Quarter<TD>2307<TD>4706

<TR><TH>Second Quarter<TD>2982<TD>5104

<TR><TH>Third Quarter<TD>3011<TD>5220

<TR><TH>Fourth Quarter<TD>3055<TD>5287

</TABLE>

// includes the output of footer.jsp

<%@include file="footer.jsp" %>

</body>

</html>

JSP Life Cycle Methods

The life cycle methods of JSP are jspInit(), _jspService() and jspDesroy(). On receiving each

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