JavaBean

Not to be confused with Enterprise JavaBean.

JavaBeans are reusable software components for Java. Practically, they are classes written in the Java programming language conforming to a particular convention. They are used to encapsulate many objects into a single object (the bean), so that they can be passed around as a single bean object instead of as multiple individual objects. A JavaBean is a Java Object that is serializable, has a nullary constructor, and allows access to properties using getter and setter methods.

1 Advantages of JavaBeans
2 JavaBeans API
3 JavaBean conventions
4 JavaBean Example
5 See also
6 External links

Advantages of JavaBeans

A Bean obtains all of the benefits of Java's "write once, run anywhere" paradigm.
The properties, events, and methods of a Bean that are exposed to another application can be controlled.
Auxiliary software can be provided to help configure a Bean.
The configuration settings of a Bean can be saved in a persistent storage and can be restored at a later time.
A Bean may register to receive events from other objects and can generate events that are sent to other objects.

JavaBeans API

The JavaBeans functionality is provided by a set of classes and interfaces in the java.beans package.

Interface	Description
AppletInitializer	Methods in this interface are used to initialize Beans that are also applets.
BeanInfo	This interface allows the designer to specify information about the events,methods and properties of a Bean.
Customizer	This interface allows the designer to provide a graphical user interface through which a bean may be configured.
DesignMode	Methods in this interface determine if a bean is executing in design mode.
ExceptionListener	A method in this interface is invoked when an exception has occurred.
PropertyChangeListener	A method in this interface is invoked when a bound property is changed.
PropertyEditor	Objects that implement this interface allow the designer to change and display property values.
VetoableChangeListener	A method in this interface is invoked when a Constrained property is changed.
Visibility	Methods in this interface allow a bean to execute in environments where GUI is not available.

JavaBean conventions

In order to function as a JavaBean class, an object class must obey certain conventions about method naming, construction, and behaviour. These conventions make it possible to have tools that can use, reuse, replace, and connect JavaBeans.

The required conventions are as follows:

The class must have a public default constructor (no-argument). This allows easy instantiation within editing and activation frameworks.
The class properties must be accessible using get, set, is (used for boolean properties instead of get) and other methods (so-called accessor methods and mutator methods), following a standard naming convention. This allows easy automated inspection and updating of bean state within frameworks, many of which include custom editors for various types of properties. Setters must receive only one argument.
The class should be serializable. It allows applications and frameworks to reliably save, store, and restore the bean's state in a fashion independent of the VM and of the platform.

JavaBean Example

PersonBean.java:

package beans;
 
/**
 * Class <code>PersonBean</code>.
 */
public class PersonBean implements java.io.Serializable {
 
    private String name;
 
    private boolean deceased;
 
    /** No-arg constructor (takes no arguments). */
    public PersonBean() {
    }
 
    /**
     * Property <code>name</code> (note capitalization) readable/writable.
     */
    public String getName() {
        return this.name;
    }
 
    /**
     * Setter for property <code>name</code>.
     * @param name
     */
    public void setName(final String name) {
        this.name = name;
    }
 
    /**
     * Getter for property "deceased"
     * Different syntax for a boolean field (is vs. get)
     */
    public boolean isDeceased() {
        return this.deceased;
    }
 
    /**
     * Setter for property <code>deceased</code>.
     * @param deceased
     */
    public void setDeceased(final boolean deceased) {
        this.deceased = deceased;
    }
}

TestPersonBean.java:

import beans.PersonBean;
 
/**
 * Class <code>TestPersonBean</code>.
 */
public class TestPersonBean {
    /**
     * Tester method <code>main</code> for class <code>PersonBean</code>.
     * @param args
     */
    public static void main(String[] args) {
        PersonBean person = new PersonBean();
        person.setName("Bob");
        person.setDeceased(false);
 
        // Output: "Bob [alive]"
        System.out.print(person.getName());
        System.out.println(person.isDeceased() ? " [deceased]" : " [alive]");
    }
}

testPersonBean.jsp;

<% // Use of PersonBean in a JSP. %>
<jsp:useBean id="person" class="beans.PersonBean" scope="page"/>
<jsp:setProperty name="person" property="*"/>
 
<html>
    <body>
        Name: <jsp:getProperty name="person" property="name"/><br/>
        Deceased? <jsp:getProperty name="person" property="deceased"/><br/>
        <br/>
        <form name="beanTest" method="POST" action="testPersonBean.jsp">
            Enter a name: <input type="text" name="name" size="50"><br/>
            Choose an option:
            <select name="deceased">
                <option value="false">Alive</option>
                <option value="true">Dead</option>
            </select>
            <input type="submit" value="Test the Bean">
        </form>
    </body>
</html>

External links

Oracle's JavaBeans tutorials