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Lesson 37
JSP Action Elements and Scope
The journey we had started of JSP is very much covered except of JSP action elements. In this handout,
we'll study the use of JSP action elements. Further also learn how and where to store JavaBean objects
that can be shared among JSP pages.
Let's first quickly look on the JSP journey to find out where we have reached.
Directive Elements
­ Provides global control of JSP ......................... <%@ %>
Scripting Elements
­ JSP comments ............................................. <%----%>
­ declarations ............................................. <%! %>
Used to declare instance variables & methods
Action Elements
­ Special JSP tags ........................................... <jsp: ..... />
JSP Action Elements
JSP action elements allow us to work with JavaBeans, to include pages at request time and to forward
requests to other resources etc.
Format
Expressed using XML syntax
-Opening tag <jsp:actionElement attribute="value" ..... >
-Body body
-Closing tag </jsp:actionElement>
Empty tags (without body) can also be used like
<jsp:actionElement attribute="value" ..... >
Some JSP Action Elements
To work with JavaBeans
-<jsp:useBean />
-<jsp:setProperty />
-<jsp:getProperty />
To include resources at request time
-<jsp:include />
To forward request to another JSP or Servlet
-<jsp:forward />
To work with applets
-<jsp:plugin />
Working with JavaBeans using JSP Action Elements
The three action elements are used to work with JavaBeans. These are discussed in detail below.
JSP useBean Action Element
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It is used to obtain a reference to an existing JavaBean object by specifying id(name of object) and scope in
which bean is stored. If a reference is not found, the bean is instantiated.
The format of this action element is:
<jsp:useBean id = "name" scope = "page|request|session|application"
class="package.Class " />
The id attribute specifies the name of the JavaBean object that is also used for later references. The scope
attribute can have one possible value out of page, request, session and application. If this attribute is
omitted, the default value of scope attribute is page. We'll discuss in detail about scope shortly.
The class attribute specifies the type of object is going to be created.
jsp:useBean is being equivalent to building an object in scriptlet. For example to build an object of
MyBeanusing scriptlet is:
<% MyBean m = new MyBean( );%>
Achieving above functionality using jsp:useBean action element will look like this:
<jsp:useBean id = "m" scope = "page" class="vu.MyBean" />
In the above code snippet, we are assuming that MyBean lies in vu package.
JSP setProperty Action Element
To set or change the property value of the specified bean. String values are converted to types of properties
by using the related conversion methods.
The format of this action element is:
<jsp:setProperty name = "beanName or id" property = "name"value ="value" />
The name attribute should match the id given in jsp:useBean. The propertyattribute specifies the name of
the property to change and the value attribute specifies the new value.
jsp:setProperty is being equivalent to following code of scriptlet. For example to change the name property
of m (instance of MyBean) using scriptlet is:
<%
m.setProperty("ali");
%>
Achieving above functionality using jsp:setProperty action element will look like this:
<jsp:setProperty name = "m" property = "name" value = "ali" />
JSP getProperty Action Element
Use to retrieves the value of property, converts it to String and writes it to output stream. The format of
this action element is:
<jsp:getProperty name = "beanName or id" property = "name" />
jsp:getProperty is being equivalent to following code of scriptlet. For example to retrieve the name property
of m (instance of MyBean) followed by writing it to output stream, scriptlet code will look like:
<%
String name = m.getName( );
out.println(name);
%>
Achieving above functionality using jsp:getProperty action element will look like this:
<jsp:getProperty name = "m" property = "name" />
Example Code: Calculating sum of two numbers by using action elements and JavaBean
This example contains index.jsp and result.jsp and one JavaBean i.e. SumBean. User will enter two
numbers on index.jsp and their sum will be displayed on result.jsp. Let's examine these one after another
SumBean.java
The SumBean has following attributes
­ firstNumber
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­ secondNumber
­ sum
The firstNumber and secondNumbers are "write-only" properties means for these only setters would be
defined. Whereas sum is a "read-only" property as only getter would be defined for it.
The SumBean also contain one additional method for calculating sum i.e. calulateSum(). After performing
addition of firstNumber with secondNumber, this method will assign the result to sum attribute.
package vu;
import java.io.*;
public class SumBean implements Serializable{
private int firstNumber;
private int secondNumber;
private int sum;
// no argument constructor
public SumBean() {
firstNumber = 0;
secondNumber = 0;
sum = 0;
}
// firstNumber & secondNumber are writeonly properties// setterspublic void setFirstNumber(int n){
firstNumber = n;
}
public void setSecondNumber(int n){
secondNumber = n;
}
// no setter for sum
// sum is a read only property
public int getSum( ){
return sum;
}
// method to calculate sum
public void calculateSum() {
sum = firstNumber + secondNumber;
}
}
index.jsp
This page will display two text fields to enter number into them.
<html>
<body>
<h2>Enter two numbers to calculate their sum</h2>
<form name="myForm" action="result.jsp">
<h3> Enter first numebr <input type="text" name="num1" /><br/>
Enter second numebr
<input type="text" name="num2" />
<br/>
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<input type="submit" value="Calculate Sum" />
</h3>
</form>
</body>
</html>
result.jsp
This page will calculate the sum of two entered numbers by the user and displays the sum back to user. The
addition is performed using SumBean
<%-- importing vu package that contains the SumBean --
%><%@page import="vu.*"%> <html> <body>
<h2>The sum is: <%-- instantiating bean using action
element -- %>
//Servlet equivalent code of useBean
SumBean sBean = new SumBean();
--%>
<jsp:useBean id="sBean" class="vu.SumBean" scope="page"/>
<%-- setting firstNumber property of sBean
using action elements
-- %>
<%-- implicit conversion from string to int as num1 is of typeString and
firstNumber is of type int--%>
<%--//Servlet equivalent code of setProperty for num1int no =
Integer.parseInt(request.getParameter("num1"));sBean.setFirstNumber(no);
--%>
<jsp:setProperty name="sBean"property="firstNumber" param="num1" />
<%--//Servlet equivalent code of setProperty for num2int no =
Integer.parseInt(request.getParameter("num2"));sBean.setSecondNumber(no);
--%>
<jsp:setProperty name="sBean"property="secondNumber" param="num2" />
<%
// calling calculateSum() method that will set the value of// sum attribute
sBean.calculateSum();
%>
<%--// servlet equivalent code of displaying sumint res = sBean.getSum();out.println(res);
--%>
<jsp:getProperty name="sBean" property="sum" />
</h2>
</body>
</html>
Sharing Beans & Object Scopes
So far, we have learned the following techniques to create objects.
-Implicitly through JSP directives
-Explicitly through actions
-Directly using scripting code
Although the beans are indeed bound to local variables, that is not the only behavior. They are also stored
in four different locations, depending on the value of the optional scope attribute of jsp:useBean. The scope
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in ServletRequest object for the duration of the current request. In other words, until you continue to
forward the request to another JSP/servlet, the beans values are available. This has been illustrated in
the following diagram.
In the diagram above, MyBean is instantiated by specifying scope = "request"that results in storing object
in ServletRequest. A value "ali" is also stored in m using setter method.
second.jsp forwards the same request (request 1) to third.jsp, since scope of m (object of MyBean) is
request, as a result third.jsp can access the values(e.g. ali) stored in m. According to the figure, third.jsp
generates a new request (request 2) and submits it to fourth.jsp. Since a new request is generated therefore
values stored in object m (e.g. ali) are not available to fourth.jsp.
session
This value means that, in addition to being bound to local variable, the bean object will be stored in the
HttpSession object associated with the current request. As you already know, object's value stored in
HttpSession persists for whole user's session. The figure below helps in understanding this concept.
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ServletContext
Summary of Object's Scopes
Let's take another view of session, request & page scopes in the next figure that helps us to understand the
under beneath things.
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The figure shows four JavaServer Pages. Each page has its own page scope. Therefore objects stored in
page scope are only available to same pages on which they are created.
Suppose page1 forwards the request to page2. Objects stored in request scope remains available to page1 as
well to page 2. Similar case is true for page 3 & page 4.
If user makes a visit to all these pages in one session, object's values stored in session scope remains
available on all these pages.
To understand the difference between sessions & application scope, consider the following figure:
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As you can conclude from the figure, for each user (client), objects are stored in different sessions.
However, in the case of application scope, all users stores objects in single place.
More JSP Action Elements
Let's talk about two important action elements. These are include& forward.
JSP include action Element
It is used to include files at request time. For example, to reuse HTML, JSP or plain text content. It's
important to note that JSP content cannot affect main page (in which output is included); only output of
included JSP is used. It also allows updating of the included content without changing the main JSP.
The jsp:include action element requires two attributes: page& flush. -page: a relative URL of
the file to be included. -flush:must have the value "true"
<jsp:include page = "relative URL" flush = "true" />
jsp:include is being equivalent to following code of scriptlet. For example to include the output of one.jsp ,
scriptlet code will look like:
<%
RequestDispatcher rd = request.getRequestDispatcher("one.jsp");rd.include(request,
response);
%>
Achieving above functionality using jsp:include action element will look like this: <jsp:include page =
"one.jsp" flush = "true" />
JSP forward action Element
It is used to forward request to another resource. The format of jsp:forward action is:
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<jsp:forward page = "one.jsp" />
jsp:forward is being equivalent to following code of scriptlet. For example to forward the request to one.jsp
, scriptlet code will look like:
<% RequestDispatcher rd = request.getRequestDispatcher("one.jsp");
rd.forward(request, response);
%>
References:
. Java A Lab Course by Umair Javed.
. Core Servlets and JavaServer Pages by Marty Hall
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