JavaBeans

<h2 id="features">Features</h2>
Introspection
Introspection is a process of analyzing a Bean to determine its capabilities. This is an essential feature of the Java Beans specification because it allows another application, such as a design tool, to obtain information about a component.
Properties
A property is a subset of a Bean's state. The values assigned to the properties determine the behaviour and appearance of that component. They are set through a setter method and can be obtained by a getter method.
Customization
A customizer can provide a step-by-step guide that the process must follow to use the component in a specific context.
Events
Beans  may  interact with  the EventObject EventListener model.
Persistence
Persistence is the ability to save the current state of a Bean, including the values of a Bean's properties and instance variables, to nonvolatile storage and to retrieve them at a later time.
Methods
A Bean should use <a href="/facts/Mutator_method/sKfxYzFM">accessor methods</a> to <a href="/facts/Encapsulation_(computer_programming)/kzDnbHPe">encapsulate</a> the properties. A Bean can provide other methods for business logic not related to the access to the properties.
<h2 id="advantages">Advantages</h2>
<ul><li>The properties, events, and methods of a bean can be exposed to another application.</li>
<li>A bean may register to receive events from other objects and can generate events that are sent to those other objects. </li>
<li>Auxiliary software can be provided to help configure a bean. </li>
<li>The configuration settings of a bean can be saved to persistent storage and restored. </li></ul>
<h2 id="disadvantages">Disadvantages</h2>
<ul><li>A class with a <a href="/facts/Nullary_constructor/Yok4sanz">zero-argument constructor</a> is subject to being instantiated in an invalid state.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> 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 cannot detect such a problem, and even if it is documented, there is no guarantee that the developer will see the documentation.</li>
<li>JavaBeans are inherently mutable and so lack the advantages offered by <a href="/facts/Immutable_objects/Gsoq1bba">immutable objects</a>.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a></li>
<li>Having to create getters for every property and setters for many, most, or all of them can lead to an immense quantity of <a href="/facts/Boilerplate_code/xbYAfVbW">boilerplate code</a>.</li></ul>
<h2 id="javabeans-api">JavaBeans API</h2>
<p>The JavaBeans functionality is provided by a set of classes and interfaces in the java.beans package.
</p>
<table><tbody><tr><th>Interface</th><th>Description</th></tr><tr><td>AppletInitializer</td><td>Methods in this interface are used to initialize Beans that are also <a href="/facts/Java_applet/oulP2dyy">applets</a>.</td></tr><tr><td>BeanInfo</td><td>This interface allows the designer to specify information about the events, methods and properties of a Bean.</td></tr><tr><td>Customizer</td><td>This interface allows the designer to provide a graphical user interface through which a bean may be configured.</td></tr><tr><td>DesignMode</td><td>Methods in this interface determine if a bean is executing in design mode.</td></tr><tr><td>ExceptionListener</td><td>A method in this interface is invoked when an exception has occurred.</td></tr><tr><td>PropertyChangeListener</td><td>A method in this interface is invoked when a bound property is changed.</td></tr><tr><td>PropertyEditor</td><td>Objects that implement this interface allow the designer to change and display property values.</td></tr><tr><td>VetoableChangeListener</td><td>A method in this interface is invoked when a Constrained property is changed.</td></tr><tr><td>Visibility</td><td>Methods in this interface allow a bean to execute in environments where the GUI is not available.</td></tr></tbody></table>
<h2 id="javabean-conventions">JavaBean conventions</h2>
<p>In order to function as a JavaBean <a href="/facts/Class_(computer_science)/zvzkSAtE">class</a>, 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 Java Beans.
</p><p>The required conventions are as follows:
</p>
<ul><li>The class must have a public <a href="/facts/Default_constructor/UnyxDE4K">default constructor</a> (with no arguments). This allows easy instantiation within editing and activation frameworks.</li>
<li>The class <a href="/facts/Property_(programming)/MRIIAw5g">properties</a> must be accessible using <i>get</i>, <i>set</i>, <i>is</i> (can be used for boolean properties instead of get), <i>to</i>  and other methods (so-called <a href="/facts/Accessor/sKfxYzFM">accessor methods</a> and <a href="/facts/Mutator_method/sKfxYzFM">mutator methods</a>) according to a standard <a href="/facts/Naming_conventions_(programming)/4IVCWpSU">naming convention</a>. 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 arguments.</li>
<li>The class should be <a href="/facts/Serialization/T00auFvv">serializable</a>. (This allows applications and frameworks to reliably save, store, and restore the bean's state in a manner independent of the <a href="/facts/Virtual_machine/BLevU3sx">VM</a> and of the platform.)</li></ul>
<h3>Code example</h3>
package player;

public class PersonBean implements java.io.Serializable {

/** Properties **/
    private boolean deceased = false;

private List list;

/** Property "name", readable/writable. */
    private String name = null;

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

public List getList() {
        return list;
    }
	
    public void setList(final List list) {
        this.list = list;
    }

/**
     * Getter for property "name".
     */
    public String getName() {
        return name;
    }

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

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

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

<p>TestPersonBean.java:
</p>
import player.PersonBean;

/**
 * Class "TestPersonBean".
 */
public class TestPersonBean {
    /**
     * Tester method "main" for class "PersonBean".
     *
     * @param arguments
     */
    public static void main(final String[] arguments) {
        final PersonBean person = new PersonBean();

person.setName("Bob");
        person.setDeceased(false);
        person.setList(new ArrayList());

// Output: "Bob [alive]"
        System.out.print(person.getName());
        System.out.println(person.isDeceased() ? " [deceased]" : " [alive]");
    }
}

<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>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Software_package_(disambiguation)/hVpqN6NN">Software packaging</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://download.oracle.com/javase/tutorial/javabeans/">Oracle's JavaBeans tutorials</a></li>
<li><a href="http://www.oracle.com/technetwork/java/javase/documentation/spec-136004.html">JavaBeans specification</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Bloch, Joshua (2008). Effective Java (Second ed.). Addison-Wesley. p. 13. ISBN 978-0-321-35668-0. <a href="978-0-321-35668-0" target="_blank">978-0-321-35668-0</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Bloch, Joshua (2008). Effective Java (Second ed.). Addison-Wesley. p. 13. ISBN 978-0-321-35668-0. <a href="978-0-321-35668-0" target="_blank">978-0-321-35668-0</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

JavaBeans open-in-new

JavaBeans