Java Beans

Introduction to Java Beans

A Java Beans is software component that has been designed to be reusable in a variety of different environments. There is no restriction on the capability of a Bean. It may perform simple function, such as checking the spelling of a document, or complex function, such as forecasting the performance of a stock portfolio. A bean may be visible to an end user. One example of this is a button on a graphical user interface. A bean may be designed to work autonomously on a user’s workstation or to work in cooperation with a set of other distributed components.  

Advantages of Java Beans

·        A bean obtains all the benefits of Java’s “write once, run-anywhere” paradigm. 

·        The properties, events and methods of a bean that are exposed to an application builder tool can be controlled. 

·        A bean may be designed to operate correctly in different locales, which makes it useful in global markets. 

·        Auxiliary software can be provided to help a person configure a bean. 

·        The configuration settings of a bean can be saved in persistent storage and 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. 

BDK Introspection

Introspection is the process of analyzing a bean to determine its capabilities. This is a very important feature of Java Bean API, because it allows an application builder tool to present information about a component to a software designer. Without introspection, the java beans technology could not operate. One way exposed the properties, events and methods of bean to application builder tool is using simple naming conventions.  

Design pattern for properties

Property is a subset of a bean’s state. The values that are assigned to the properties determine the behavior and appearance of that component. 

Simple properties: 

               A simple property has a single value. It can be identified by the following 

               design patterns, where N is the name of the property and T is its type. 

Public T getN( ); 

Public void setN( ); 

Boolean properties:

A Boolean property has a value of true or false. It can be identified by the following design patterns, where N is name of the property. 

Public Boolean isN ( ); 

Public Boolean getN( ); 

Public void setN(Boolean value); 

Indexed properties:

An indexed property consists of multiple values. It can be identified by the following design patterns, where N is the name of the property and T is  its type

               Public T getN(int index); 

               Public T[ ] getN( ); 

               Public void setN(T values[ ]); 

Using Bound Properties

A bean that has a bound property generates an event when the property is changed. The event is of type PropertyChangeEvent and is sent to objects that previously registered an interest in receiving such notifications. 

Example 17: Application that uses TickTock bean to automatically control the Color bean 

Steps: 

1.      Go to menu bar of the bean box and select Edit | Events | propetyChange. We can now see a line extending from the button to the cursor 

2.      Move the cursor so that it is inside the Colors bean display area , and click the left mouse button.  See the Event Target Dialog  dialog box. 

3.      the dialog box allows your to choose a method that should be invoked when this event occurs.  Select the entry labeled “change” and click the Ok button. 

Using BeanInfo Interface

This interface defines several methods, including these: 

               PropertyDescription[ ] getPropertyDescriptors( )

EventSetDescriptor[ ] getEventSetDescriptors( ) 

MethodDescriptor[ ] getMethodDescriptors( ) 

The above methods will return array of objects that provide information about the properties, events, and methods of bean. SimpleBeanInfo is a class that provides default implementations of the BeanInfo interface, including the three methods just shown. We can extend this class and override on or more of them. 

Constrained Properties

A bean that has a constrained property generates an event when an attempt is made to change its value. The event is of type PropertyChangeEvent. It is sent to objects that previously registered an interest in receiving such notifications. This capability allows a Bean to operate differently according to its run-time environment.  A 

Persistence

Persistence is the ability to save a Bean to nonvolatile storage and retrieve it at a later time. The information that is particularly important are the configuration settings. 

Interface	Description
AppletInitializer	Methods present in this interface are used to initialize Beans that are also applets
BeanInfowww.jntuworld.com	This interface allows a designer to specify information about the properties, events and methods of a Bean.
Customizer	This interface allows a 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.
PropertyChangeListener	A method in this interface is invoked when a bound property is changed.
Visibility	Methods in this interface allow a bean to execute in environments where graphical user interface is not available.
Class	Description
BeanDescriptor	This class provides information about a Bean.
Beans	This class is used to obtain information about a Bean
IntrospectionException	An exception of this type is generated if a problem occurs when analyzing a bean.
PropertyChangeEvent	This event is generated when bound or constrained properties are changed.
PropertyDescriptor	Instances of this class describe a property of a Bean