JSP Examples - Scripting Elements & Directives

Q. Template text will become argument to write() method where as expression value will

become argument to print() method. What is the reason?

write() method can take only character data as argument

print() method can take any type of data as argument

Template text is always character data and hence it will become argument to write() method.

But expression value can be any type and hence we should not use write() method, compulsory

we should go for print() method which can take any type of argument.

Directives:

These can provide general information about JSP page to the JSP Engine. i.e Directives are

translation time instructions to the JSP Engine.

Syntax: <%@ directiveName attributeName=attributeValue %>

There are 3 types of directives are possible

• page directive:

<%@ page attributeName=attributeValue %>

It can be used to define page specific attributes.

• include directive:

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

To include the content of header.html at translation time

• taglib directive:

To make custom tags available to our JSP

Actions:

Actions are commands to the JSP Engine to perform certain task at runtime(execution time). There

are 9 standard actions are available.

• Scriptlet:

We can use scriptlet to place java code in the jsp.

Syntax: <%

Any Java Code

%>

Java Code inside scriptlet will be placed directly inside _jspService() method of generated servlet.

Every java statement present inside scriptlet should compulsary ends with ;

Q. Write a JSP Print hit count of the JSP?

count is local variable of _jspService()

is instance variable.

Q. What is the difference between the following?

<%! <%

int x =10; int x =10;

%> %>

In the first case x is local variable of _jspService() method where as in the second case x is instance

variable of generated servlet.

Note: It is not recommended to use scriptlets in the JSP.

• Declaration Tag:

We can use Declaration tag to declare instance variables, static variables, instance blocks, static

blocks, methods etc..

Syntax: <%!

Any java declarations

%>

These declarations will be placed directly in the generated servlet but outside of _jspService()

method.

Every Java statement inside Declaration tag should compulsary ends with semicolon(;)

demo.jsp:

CE: out cannot be resolved.

*Note:

All JSP Implicit objects are declared as local variables of _jspService() method.But declaration tag

code will be placed outside of _jspService() method. Hence we cannot use JSP implicit objects

inside Declaration tag.

But Scriptlet and expression tags code will be placed inside _jspService() method. Hence inside

scriptlet and expression tags we can use JSP Implicit objects.

Eg:

• Comments:

In the JSP, 3 types of comments are allowed.

• JSP Comments
• HTML Comments
• Java Comments
• JSP Comments:

<%-- This is JSP Comment --%>

These comments are visible only in the JSP and not visible in the remaining phases of JSP

Execution. Hence these comments also known as Hidden Comments.

It is highly recommended to use JSP Comments in the JSP.

• HTML Comments:

<!-- This is HTML Comment -->

Also known as Template text comments.

These are visible to the end user as the part of generated response source code.

Hence these are not recommended to use in the JSP.

• Java Comments:

// single line java comment

/*

multiline java comment

*/

/**

Java Doc comment

*/

Also known as scripting comments.

These are also visible in the generated servlet source code but not visible in the remaining phases.

*Note:

Among Expressions, Scriptlets, Declarations and JSP Comments, we cannot use one inside another.

i.e. Nesting of these scripting elements is not possible, otherwise we will get Compile Time Error.

Summery of JSP Comments

Source Code

Eg 2:

Eg 4:

Comparison Table of JSP Scripting Elements

inside _jspService()

Expression <%= X %> No Method OR not?

Comments

%> Yes No

<%! NA NA

Any Java

Declarations

%>

<%--

This is JSP

Comment

%>

Directives (in Detail)

Objective:

Write JSP Code that uses the following directives

Directives can provide general information about JSP page to the JSP Engine. i.e Directives are

translation time instructions to the JSP Engine.

Syntax: <%@ directiveName attributeName=attributeValue %>

page|include|taglib

There are 3 types of directives are possible

I) page directive:

page directive specifies overall properties of JSP Page to the JSP Engine.

(like Tell me about yourself? in HR Round)

Eg1: <%@ page language="java" %>

This page directive specifies to the JSP Engine that the scripting language used in this jsp is

java.

Eg2: <%@ page session="true" %>

This page directive specifies that the current jsp participating in session management.

We can use page directive any number of times, anywhere in the jsp.

The following is the list of all possible 13 attributes of page directive.

We can use import attribute to import classes and interfaces of a particular package in the

JSP.

This is similar to core java import statement.

demo.jsp without import attribute:

<h1>The Server Time is:<%= new java.util.Date() %></h1>

demo.jsp with import attribute:

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

<h1>The Server Time is:<%= new Date() %></h1>

To import multiple packages the following are various possibilities...

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

<%@ page import="java.util.,java.io." %>

Within the same JSP, we are not allowed to take any attribute multiple times with

multiple values. But we can take multiple times with same value. This rule is not

applicable for import attribute.

Q. Which of the following page directives are valid?

Note:

Inside JSP, the following packages are by default available and hence we are not required to

import these packages explicitly.

java.lang.*;

javax.servlet.*;

javax.servlet.http.*;

javax.servlet.jsp.*;

By default session object is available in every JSP. If we don't want session object then we can

make it unavailable by using page directive session attribute as follows

<%@ page session="false" %>

If we are not declaring session attribute explicitly or declaring session attribute with "true"

value, then the generated servlet code is:

HttpSession session = null;

session = pageContext.getSession();

If we declare explicitly with false value, then the above code won't be generated.

The allowed values for the session attribute are:

true TRUE True FALse durga

i.e. case insensitive String of true or false is allowed.

If we are taking any other value then we will get Translation Time error.

Eg: <%@ page session="sunny" %>

Error: invalid value for session

We have to use this attribute to specify content type of response(i.e MIME Type of Response)

<%@ page contentType="application/pdf" %>

The default value content type is : text/html

Expression Language(EL) has introduced in JSP 1.2V

The main objective of EL is to remove java code from the JSP.

EL Syntax won't be processed and just treated as plain text.

EL Syntax will be processed and print its value.

Eg: http://localhost:7777/jspdapp1/demo.jsp?user=Mallika

demo.jsp:

o/p: User Name: Mallika

demo.jsp:

o/p: User Name: ${param.user}

Note:

The default value of isELIgnored is true in JSP 1.2V.

But from JSP 2.0V onwards the default value is false.

By Default a single JSP page can be accessed by multiple threads simultaneously. Hence JSP is

not thread safe by default.

To provide Thread Safety to the JSP we should go for isThreadSafe attribute.

It means JSP page is already thread safe & it is not required generated servlet to

implement SingleThreadModel.

In this case JSP Page can process any number of client requests simultaneously.

The current JSP is not thread safe. Hence to provide thread safety, Generated servlet will

implements SingleThreadModel interface.

In this case JSP Page can process only one request at a time.

Note: The default value of isThreadSafe attribute is true.

Q. To provide Thread Safety, which of the following arrangement we have to take in the JSP?

Summary of Page Attributes

No Default Value. But the following 4

To disable Expression

To provide Thread Safety to

false

true

We can implement include mechanism either by include directive or by include action.

second.jsp:

<h1>This is Second JSP</h1>

For both including and included JSPs, a single servlet will be generated.

second.jsp:

<h1>This is Second JSP</h1>

For both including and Included JSPs, separates servlets will be generated.

Differences b/w Include Directive and Include Action

Contains 2 Attributes Page and File.

possible.

Dependent.

Logo Inclusion

(Static Include by Include Directive)

(Static Include by Include Directive) (Dynamic Include by Include Action)

(Dynamic Include by Include Action) (Dynamic Include by Include Action)

Copy Right Inclusion

(Static Include by Include Directive)

Note:

To include the content of header.jsp at translation time, the required import is:

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

To include the response of header.jsp at request processing time (run time), the required import

is:

<jsp:include page="header.jsp" flush="true"/>

Q. Which of the following inclusions are valid?

1.<%@ include page="header.jsp" %>

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

3.<jsp:include file="header.jsp" />

4.<jsp:include page="header.jsp" />

5.<jsp:include page="header.jsp" flush="true"/>

6.<%@ include file="header.jsp" flush="true"%>

Note:

page attribute is applicable for include action where as file attribute is applicable for include

directive.

flush attribute is applicable only for include action but not for include directive.