java programming: advanced topics 1 javabeans chapter 8

Java Programming: Advanced Topics

1

JavaBeans

Chapter 8


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Objectives

• Learn what the JavaBeans component model is

• Understand Bean Development environments

• Use the Sun BeanBox• Create a JavaBean class


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Objectives (Cont.)

• Explore JavaBean property types• Add custom event types to your beans• Create a JavaBean class with events• Supply additional information and

support classes for a JavaBean


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JavaBeans Component Model

• The JavaBeans component model is a framework for creating reusable Java classes

• Classes that conform to the JavaBeans specification can be loaded into development tools called beanboxes, with which developers can create applications by constructing them from parts

• JavaBeans are packaged in .jar files that include a manifest file


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What Makes a Class a Bean• There is no common superclass that all

JavaBean classes extend.• To define a JavaBean follow the programming

conventions• All beans should have a constructor that takes

no arguments because the Sun BeanBox call this constructor

• A JavaBean class must implement the marker interface Serializable, because beanboxes use serialization to save the state of beans


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Elements of a JavaBean Interface


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Bean Development Environments

• Sun created a demonstration development environment called the BeanBox

• The Bean Builder is a program that demonstrates new and emerging technologies within the Java platform that allow the construction of applications using component assembly mechanisms


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Using the Sun BeanBox• When a bean is instantiated in a beanbox, the

bean’s methods are called in the following order:– The constructor with no arguments is called to set

up the bean– The preferredSize method returns the display

dimensions of the bean– The paint method draws the bean on the BeanBox

window


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The BDK BeanBox Properties Window


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Creating a JavaBean Class

• Create a JavaBean class• Package your JavaBean classes into a .jar file• Load your .jar file into the BeanBox to connect

with other JavaBean components


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Exploring JavaBean Property Types

• Properties are the attributes of a bean, commonly implemented as the fields of a Java class

• There are four types of JavaBean Property Types:– Simple– Indexed– Bound– Constrained


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Indexed Properties

• Properties can be indexed under a single name with an integer index value

• Indexed properties are arrays of values• The mutator and accessor methods for an

indexed property must have the arguments, names, and return types


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Indexed Properties (Cont.)


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Bound Properties• Bound properties provide notification when they

change so that other JavaBeans can listen for these changes and act accordingly

• The package java.beans includes a class for use with bound properties, PropertyChangeSupport


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Bound Properties (Cont.)


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Constrained Properties• Constrained properties: bound properties with

the additional characteristics that other listeners can prevent a change in value from occurring

• To implement a constrained property, a JavaBean class should use an object of the VetoableChangeSupport class

• Each listener can veto a change and stop it from happening


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Constrained Properties (Cont.)


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Adding Custom Event Types

• To create and use a custom event:– Define the event class X that extends

java.util.EventObject or one of its subclasses– Define the interface, XListener, that the event

listeners must implement– Define the methods addXListener and

removeXListener for the JavaBean class that can fire the event

– The JavaBean class should define a fireX method


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Creating a JavaBean with Events• Event handling follows the same model as event

handling for components in the Swing and AWT APIs

• If a bean can generate an event Y, the class for the event is YEvent

• A listener class YListener should handle YEvent objects

• Changes to properties trigger events of PropertyChangeEvent objects


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Custom Event Class


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Custom Event Class (Cont.)


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Using the BeanInfo Classes• Beanboxes use the Reflection API to determine

what they need to know about a JavaBean• BeanInfo classes are used only when beans are

being connected in a beanbox• The BeanInfo classes implement the

java.beans.BeanInfo interface• If the Reflection API cannot provide all the

information that a beanbox needs about a bean, you can supply an additional information class that implements the interface BeanInfo


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JavaBean Information Classes


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Using the BeanInfo Classes(Cont.)

• Methods of the interface java.beans.BeanInfo:– BeanInfo[ ] getAdditionalBeanInfo()– BeanDescriptor getBeanDescriptor()– int getDefaultEventIndex()– int getDefaultPropertyIndex()– EventSetDescriptor[ ] getEventSetDescriptors()– Image getIcon( int iconKind )– MethodDescriptor[ ] getMethodDescriptors()– PropertyDescriptor[ ] getPropertyDescriptors()


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Providing a Custom Property Editor

• Beanboxes typically provide property editors for properties of types String, Font, and Color

• You can provide customized editors for other kinds of properties by defining a class that extends PropertyEditorSupport or implements the PropertyEditor interface

• If the property editor dialog box of the beanbox is not adequate, you can supply a customizer class for the bean


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A Customizer Class

• The customizer class must be a component that can be embedded in a dialog box

• The class must have a constructor that has no arguments

• When you create a customizer class, you must provide the method getBeanDescriptor in the BeanInfo class associated with the JavaBean


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JavaBean Class


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JavaBean Class (Cont.)


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Summary• The JavaBeans component model is a

framework for creating reusable Java classes• Classes that conform to the JavaBeans

specification can be loaded into development tools called beanboxes

• A bean must implement the interface java.io.Serializable, and must have a constructor that has no arguments

• Properties are the attributes of a bean, and can be single entities, indexed, bound or constrained


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Summary (Cont.)

• JavaBean objects use event-handling mechanism to notify other JavaBean objects that some event has occurred

• Changes to properties trigger events of PropertyChangeEvent objects

• Additional information classes, BeanInfo classes, can accompany a JavaBean class

• You can provide customized editors by defining a class that extends PropertyEditorSupport or implements the PropertyEditor interface

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