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Saturday, January 21, 2017

JSP - JavaBeans

A JavaBean is a specially constructed Java class written in the Java and coded according to the JavaBeans API specifications.
Following are the unique characteristics that distinguish a JavaBean from other Java classes:

It provides a default, no-argument constructor.
It should be serializable and implement the Serializable interface.
It may have a number of properties which can be read or written.
It may have a number of "getter" and "setter" methods for the properties.

JavaBeans Properties:

A JavaBean property is a named attribute that can be accessed by the user of the object. The attribute can be of any Java data type, including classes that you define.
A JavaBean property may be read, write, read only, or write only. JavaBean properties are accessed through two methods in the JavaBean's implementation class:

Method	Description
getPropertyName()	For example, if property name is firstName, your method name would be getFirstName() to read that property. This method is called accessor.
setPropertyName()	For example, if property name is firstName, your method name would be setFirstName() to write that property. This method is called mutator.

A read-only attribute will have only a getPropertyName() method, and a write-only attribute will have only a setPropertyName() method.

JavaBeans Example:

Consider a student class with few properties:

package com.tutorialspoint;

public class StudentsBean implements java.io.Serializable
{
   private String firstName = null;
   private String lastName = null;
   private int age = 0;

   public StudentsBean() {
   }
   public String getFirstName(){
      return firstName;
   }
   public String getLastName(){
      return lastName;
   }
   public int getAge(){
      return age;
   }
   public void setFirstName(String firstName){
      this.firstName = firstName;
   }
   public void setLastName(String lastName){
      this.lastName = lastName;
   }
   public void setAge(Integer age){
      this.age = age;
   }
}

Accessing JavaBeans:

The useBean action declares a JavaBean for use in a JSP. Once declared, the bean becomes a scripting variable that can be accessed by both scripting elements and other custom tags used in the JSP. The full syntax for the useBean tag is as follows:

<jsp:useBean id="bean's name" scope="bean's scope" typeSpec/>

Here values for the scope attribute could be page, request, session or application based on your requirement. The value of the id attribute may be any value as a long as it is a unique name among other useBean declarations in the same JSP.
Following example shows its simple usage:

<html>
<head>
<title>useBean Example</title>
</head>
<body>

<jsp:useBean id="date" class="java.util.Date" /> 
<p>The date/time is <%= date %>

</body>
</html>

This would produce following result:

The date/time is Thu Sep 30 11:18:11 GST 2010

Accessing JavaBeans Properties:

Along with <jsp:useBean...>, you can use <jsp:getProperty/> action to access get methods and <jsp:setProperty/> action to access set methods. Here is the full syntax:

<jsp:useBean id="id" class="bean's class" scope="bean's scope">
   <jsp:setProperty name="bean's id" property="property name"  
                    value="value"/>
   <jsp:getProperty name="bean's id" property="property name"/>
   ...........
</jsp:useBean>

The name attribute references the id of a JavaBean previously introduced to the JSP by the useBean action. The property attribute is the name of the get or set methods that should be invoked.
Following is a simple example to access the data using above syntax:

<html>
<head>
<title>get and set properties Example</title>
</head>
<body>

<jsp:useBean id="students" 
                    class="com.tutorialspoint.StudentsBean"> 
   <jsp:setProperty name="students" property="firstName"
                    value="Zara"/>
   <jsp:setProperty name="students" property="lastName" 
                    value="Ali"/>
   <jsp:setProperty name="students" property="age" 
                    value="10"/>
</jsp:useBean>

<p>Student First Name: 
   <jsp:getProperty name="students" property="firstName"/>
</p>
<p>Student Last Name: 
   <jsp:getProperty name="students" property="lastName"/>
</p>
<p>Student Age: 
   <jsp:getProperty name="students" property="age"/>
</p>

</body>
</html>

Let us make StudentsBean.class available in CLASSPATH and try to access above JSP. This would produce following result:

Student First Name: Zara 

Student Last Name: Ali 

Student Age: 10

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