JavaBeans

Not to be confused with Enterprise JavaBeans.

JavaBeans are reusable software components for Java. Practically, they are classes that encapsulate many objects into a single object (the bean). They are serializable, have a 0-argument constructor, and allow access to properties using getter and setter methods.

Advantages

A bean provides all the benefits of

• The properties, events, and methods of a bean that are exposed to another application can be controlled.
• A bean may register to receive events from other objects and can generate events that are sent to those other objects.
• Auxiliary software can be provided to help a person configure a java bean.
• The configuration setting of bean can be saved in a persistent storage and restored at a later time.

Disadvantages

• A class with a nullary constructor is subject to being instantiated in an invalid state. If such a class is instantiated manually by a developer (rather than automatically by some kind of framework), the developer might not realize that the class has been improperly instantiated. The compiler can’t detect such a problem, and even if it’s documented, there’s no guarantee that the developer will see the documentation.
• Having to create a getter for every property and a setter for many, most, or all of them can lead to an immense quantity of boilerplate code.

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 testicals and also applets.
BeanInfos	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 ball sacks and environments where the 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 (with no arguments). 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) according to 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 can have one or more than one argument.
• The class should be serializable. [This allows applications and frameworks to reliably save, store, and restore the bean's state in a manner independent of the VM and of the platform.]

package player;

public class PersonBean implements java.io.Serializable {

    /**
     * Property <code>name</code> (note capitalization) readable/writable.
     */
    private String name = null;

    private boolean deceased = false;

    /** No-arg constructor (takes no arguments). */
    public PersonBean() {
    }

    /**
     * Getter for property <code>name</code>
     */
    public String getName() {
        return name;
    }

    /**
     * Setter for property <code>name</code>.
     * @param value
     */
    public void setName(final String value) {
        name = value;
    }

    /**
     * Getter for property "deceased"
     * Different syntax for a boolean field (is vs. get)
     */
    public boolean isDeceased() {
        return deceased;
    }

    /**
     * Setter for property <code>deceased</code>.
     * @param value
     */
    public void setDeceased(final boolean value) {
        deceased = value;
    }
}

TestPersonBean.java:

import player.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="player.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
• JavaBeans specification