JavaBeans: Difference between revisions

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{{Short description|Computing technology developer by Sun Microsystems}}

{{Distinguish|Enterprise JavaBeans}}

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{{refimprovemore citations needed|date=June 2016}}

{{manualhow-to|date=October 2012}}

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In computing based on the [[Java (programming language)|Java]] Platform, '''JavaBeans''' is a technology developed by [[Sun Microsystems]] and released in 1996, as part of [[Java Development Kit|JDK]] 1.1.

In computing based on the Java Platform, '''JavaBeans''' are classes that encapsulate many [[Object (computer science)|objects]] into a single object (the bean). They are [[Serialization|serializable]], have a [[nullary constructor|zero-argument constructor]], and allow access to properties using [[Mutator method|getter and setter methods]]. The name "Bean" was given to encompass this standard, which aims to create [[Code reuse|reusable]] [[Component-based software engineering|software components]] for [[Java (programming language)|Java]].

The 'beans' of JavaBeans are classes that encapsulate one or more [[Object (computer science)|objects]] into a single standardized object (the bean). This standardization allows the beans to be handled in a more generic fashion, allowing easier [[code reuse]] and [[Type introspection|introspection]]. This in turn allows the beans to be treated as [[Component-based software engineering|software components]], and to be manipulated visually by [[Integrated development environment|editors and IDEs]] without needing any initial configuration, or to know any internal implementation details.

It is a reusable software component written in Java that can be manipulated visually in an application builder tool.

As part of the standardization, all beans must be [[Serialization|serializable]], have a [[nullary constructor|zero-argument constructor]], and allow access to properties using [[Mutator method|getter and setter methods]].

==Features==

*;Introspection

:Introspection is a process of analyzing a Bean to determine its capabilities. This is an essential feature of the Java Beans APIspecification 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. ItThey isare 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 be followedfollow to use the component in a specific context.

*;Events

:Beans ( may / can / should ) ( support / interact towith ) the EventObject EventListener model.{{clarify|date=May ''-- please help to ( correct / improve ) this! --''2020}}

*;Persistence

It: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 beanBean should use [[Mutator_methodMutator method#Java_exampleJava example|accessor methods]] to [[Encapsulation_Encapsulation (computer_programmingcomputer programming)|encapsulate]] the properties. A beanBean can provide other methods for business logic not related to the access to the properties.

== Advantages ==

* The properties, events, and methods of a bean can be exposed to another application.

* A bean may register to receive events from other objects and can generate events that are sent to those other objects. {{citation needed|date=January 2023}}

* Auxiliary software can be provided to help configure a bean. {{citation needed|date=January 2023}}

*The configuration settings of a bean can be saved to persistent storage and restored. {{citation needed|date=January 2023}}

== Disadvantages ==

* A class with a [[nullary constructor|zero-argument constructor]] is subject to being instantiated in an invalid state.<ref name="Bloch">{{cite book|last1=Bloch|first1=Joshua|authorlink1=Joshua Bloch|title=Effective Java|date=2008|publisher=Addison-Wesley|isbn=978-0-321-35668-0|page=[https://archive.org/details/effectivejava00bloc_0/page/13 13]|edition=Second|url-access=registration|url=https://archive.org/details/effectivejava00bloc_0/page/13}}</ref> 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.

* JavaBeans are inherently mutable and so lack the advantages offered by [[immutable objects]].<ref name="Bloch"/>

* Having to create getters for every property and setters for many, most, or all of them can lead to an immense quantity of [[boilerplate code]].

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== JavaBeans API ==

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

{| class="wikitable sortable"

|-

! Interface !! Description

|-

| <tt>AppletInitializer</tt> || Methods in this interface are used to initialize Beans that are also [[Java applet|applet]]s.

|-java

| <tt>BeanInfo</tt> || This interface allows the designer to specify information about the events, methods and properties of a Bean.

|-

| <tt>Customizer</tt> || This interface allows the designer to provide a graphical user interface through which a bean may be configured.

|-

| <tt>DesignMode</tt> || Methods in this interface determine if a bean is executing in design mode.

|-

| <tt>ExceptionListener</tt> || A method in this interface is invoked when an exception has occurred.

|-

| <tt>PropertyChangeListener</tt> || A method in this interface is invoked when a bound property is changed.

|-

| <tt>PropertyEditor</tt> || Objects that implement this interface allow the designer to change and display property values.

|-

| <tt>VetoableChangeListener</tt> || A method in this interface is invoked when a Constrained property is changed.

|-

| <tt>Visibility</tt> || Methods in this interface allow a bean to execute in environments where the GUI is not available.

|}

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* The class must have a public [[default constructor]] (with no arguments). This allows easy instantiation within editing and activation frameworks.

* The class [[property (programming)|properties]] must be accessible using ''get'', ''set'', ''is'' (can be used for boolean properties instead of get), ''to'' and other methods (so-called [[Accessor|accessor methods]] and [[mutator method]]s) according to a standard [[naming conventions (programming)|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 argumentarguments.

* The class should be [[Serialization#JavaProgramming language support|serializable]]. (This allows applications and frameworks to reliably save, store, and restore the bean's state in a manner independent of the [[Virtual machine|VM]] and of the platform.)

===Code example===

<sourcesyntaxhighlight lang=Java>

package player;

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/**

* Getter for property "deceased"

* Different syntax for a boolean field (is v.s.vs get)

*/

public boolean isDeceased() {

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}

}

</syntaxhighlight>

</source>

'''<u><code>TestPersonBean.java</code></u>''':

<sourcesyntaxhighlight lang=Java>

import player.PersonBean;

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}

}

</syntaxhighlight>

</source>

<syntaxhighlight lang="xml">

====Use in a [[JavaServer Pages|JavaServer Page]]====

'''<u><code>testPersonBean.jsp</code></u>''';

<source lang=HTML4Strict>

<% // Use of PersonBean in a JSP. %>

<jsp:useBean id="person" class="player.PersonBean" scope="page"/>

<jsp:setProperty name="person" property="*"/>

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</body>

</html>

</syntaxhighlight>

</source>

== See also ==

* [[Software package (disambiguation)|Software packaging]]

==References==

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[[Category:Java platform]]

[[Category:Articles with example Java code]]

[[Category:Architectural pattern (computer science)]]

[[Category:Software design patterns]]