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JAVA BEANS

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Component Framework Component

A reusable software module that is supplied in abinary form and can be used to composeapplications

Container Provides the context that components can be

assembled and interact with one another

Component Model Defines the architecture of how components can

interact in a dynamic environment

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Component Model Features

Property Management

Event Handling

Persistence and Versioning Portability and Interoperability

Distributed Computing Support

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What is a JavaBean?

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JavaBeans are reusablesoftwarecomponents

a reusable software component that can be visuallymanipulated in builder tools

They are classes written in theJava programming

language conforming to a particular convention

They are used to encapsulate many objects into a

single object (the bean)

So that they can be passed around as a single beanobject instead of as multiple individual objects

http://en.wikipedia.org/wiki/Code_reusehttp://en.wikipedia.org/wiki/Component-based_software_engineeringhttp://en.wikipedia.org/wiki/Component-based_software_engineeringhttp://en.wikipedia.org/wiki/Java_(programming_language)http://en.wikipedia.org/wiki/Java_(programming_language)http://en.wikipedia.org/wiki/Java_(programming_language)http://en.wikipedia.org/wiki/Java_(programming_language)http://en.wikipedia.org/wiki/Component-based_software_engineeringhttp://en.wikipedia.org/wiki/Component-based_software_engineeringhttp://en.wikipedia.org/wiki/Code_reuse

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Serialization

serialization is the process of converting adata structure or object state into a format that canbe stored

"resurrected" later in the same or another computerenvironment

When the resulting series of bits is reread accordingto the serialization format, it can be used to create asemantically identical clone of the original object.

A JavaBean is a Java Object that is serializable, has-

http://en.wikipedia.org/wiki/Data_structurehttp://en.wikipedia.org/wiki/Object_(computer_science)http://en.wikipedia.org/wiki/Serializationhttp://en.wikipedia.org/wiki/Nullary_constructorhttp://en.wikipedia.org/wiki/Nullary_constructorhttp://en.wikipedia.org/wiki/Nullary_constructorhttp://en.wikipedia.org/wiki/Serializationhttp://en.wikipedia.org/wiki/Object_(computer_science)http://en.wikipedia.org/wiki/Data_structure

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Advantages A Bean obtains all of the benefits of Java's "write

once, run anywhere" paradigm.

The properties, events, and methods of a Bean that

are exposed to another application can be controlled.

Auxiliary software can be provided to help configurea Bean.

The configuration settings of a Bean can be saved ina persistent storage and can be restored at a latertime.

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Dis-Advantages 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 offramework), the developer might not realize that hehas instantiated the class in an invalid state.

Having to create a getter for every property and asetter for many, most, or all of them, creates animmense amount ofboilerplate code.

http://en.wikipedia.org/wiki/Nullary_constructorhttp://en.wikipedia.org/wiki/Boilerplate_codehttp://en.wikipedia.org/wiki/Boilerplate_codehttp://en.wikipedia.org/wiki/Nullary_constructor

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JavaBean Conventions In order to function as a JavaBean class, an object

class must obey certain conventions about methodnaming, construction, and behaviour. The requiredconventions are as follows:

The class must have a public default constructor(no-argument). This allows easy instantiation within

editing and activation frameworks.

The class properties must be accessibleusing get, set, is (used for boolean propertiesinstead of get) and other methods (so-calledaccessor methods ) following a standard namingconvention.

http://en.wikipedia.org/wiki/Naming_conventions_(programming)http://en.wikipedia.org/wiki/Class_(computer_science)http://en.wikipedia.org/wiki/Default_constructorhttp://en.wikipedia.org/wiki/Property_(programming)http://en.wikipedia.org/wiki/Accessorhttp://en.wikipedia.org/wiki/Naming_conventions_(programming)http://en.wikipedia.org/wiki/Naming_conventions_(programming)http://en.wikipedia.org/wiki/Naming_conventions_(programming)http://en.wikipedia.org/wiki/Naming_conventions_(programming)http://en.wikipedia.org/wiki/Accessorhttp://en.wikipedia.org/wiki/Property_(programming)http://en.wikipedia.org/wiki/Default_constructorhttp://en.wikipedia.org/wiki/Class_(computer_science)

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JavaBeans Objectives

Provide a platform neutralcomponent architecture

Simple to develop Leverage existing Java technologies

Provide design-time support for

visual builder tools

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JavaBeans Characteristics

Properties

Event

Persistence Introspection

Customization

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Properties

Discrete, named attributes thatdetermine the appearance andbehavior and state of a component

Accessible programmatically throughaccessor methods

Accessible visually through propertysheets

Exposed as object fields in a scriptingenvironment

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Property Types

Simple Properties

Bound Properties

Constrained Properties

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Simple Properties

Represent a single valueThe accessor methods should follow

standard naming conventions

public get();

public void set( value);

Example:

public String getHostName();

public void setHostName( String hostName );

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Boolean Properties

They are simple properties

The getter methods follow anoptional design pattern

public boolean is();

Example:

public boolean isConnected();

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Indexed Properties

Represent an array of valuespublic get(int index);

public void set(int index, value);

public [] get();

public void set([] values);

Example:

public Color setPalette(int index);

public void setPalette(int index,Colorvalue);

public Color[] getPalette();

public void setPalette(Color[] values);

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Bound Properties

Registered listeners object arenotified when the value of theproperty changes

Listeners must implement thejava.beans.PropertyChangeListenerinterface

propertyChange(PropertyChangeEvent event);

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Bound Properties SupportFramework

TerminalBeanhostNameportNumb

eraddPropertyChangeListener()removePropertyChangeListene

r()

PropertyChangeEvent

source

newValue

propertyNameoldVal

ue

generates

uses

notifies

PropertyChangeListener

propertyChange()

PropertyChangeSupport

addPropertyChangeListener()removePropertyChangeListene

r() firePropertyChange()

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Bound PropertiesNotification Mechanism

:PropertyEditor

:Bean

:PropertyChangeListener

:PropertyChangeSupport

1.

addPropertyChangeListener

2.addPropertyChangeListener

3.SetHostName(hostName)

4.firePropertyChange(event)

5.propertyChange(eve

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Constrained Properties

Allow registered listeners to validatea proposed change

Listeners must implement thejava.beans.VetoablecChangeListenerinterface

vetoableChange( PropertyChangeEvent event )

throws PropertyVetoException;

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Constrained PropertiesSupport Framework

Bean

hostNameportNumb

eraddVetoableChangeListener()removeVetoableChangeListene

r()

PropertyChangeEve

nt source

newValue

propertyNameoldVal

ue

VetoableChangeSuppo

rtaddVetoableChangeListener()removeVetoableChangeListene

r()

generates

uses

notifies

VetoableChangeL

istener

vetoableChange()

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Constrained Properties -Example

public void setHostName( String newHostName )throws java.beans.PropertyVetoException

{ String oldHostName = this.hostName;

// First tell the vetoers about the change. If anyone objects, we// don't catch the exception but just let if pass on to our caller.vetoableChangeSupport.fireVetoableChange( "hostName",

oldHostName, newHostName ); // change accepted; update state this.hostName = newHostName;

// notify property change listenerspropertyChangeSupport.firePropertyChange("hostName",

oldHostName, newHostName );}

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Demo

Terminal Bean

Bound Properties

connected

connectedColor

disconnectedColor

Constrained Properties hostName

portNumber

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Overview of Event Model

Provides a notification mechanismbetween a source object and one ormore listener objects

Source or listener objects does notneed to be graphical components

Supports introspection

Extensible

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Event Delegation Model

Listener

Event Source

Event

3. Notify

2. Generate

1.Register

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Event Objects

Encapsulate notification properties

Inherit from thejava.util.EventObjectclass

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Event Object - Example

publicclass CursorEvent extends java.util.EventObject

{

public CursorEvent(Object source,Point location)

{

super( source );

this.location = location;

}

Point getLocation() { return location; }

private Point location;

}

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Event Listeners

Implement an interface that is definedby the source

Register with source using publishedregistration method

Listener interfaces

Inherit from java.util.EventListener

Have an individual method for each eventtype

Related event handling methods are

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Event Listeners - Example

publicinterface CursorListenerextendsjava.util.EventListener

{

public void cursorMoved( CursorEvent cursorEvent);

}

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Event Sources

Provide methods for registering andde-registering event listeners

Singlecast support

public void add( listener)

throws java.util.TooManyListenersException;

public void remove( listener)

Multicast support

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Event Source - Example

publicclass TerminalBean{

public void addCursorListener( CursorListener listener ){}

public void removeCursorListener( CursorListener listener ){}...

}

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Persistence

Java object serialization facilitiesprovide an automatic mechanism tostore out and restore the internal

state of an object to/from a streamJava externalization mechanism allow

a Java object to have complete

control over the format of the stream

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Serialization Example

Saving and Retrieving componentstate:

publicclass TerminalBean implements java.io.Serializable{

private String hostName;privatetransient boolean isConnected;staticfinal long serialVersionUID = -3283293045227786848L;

}

ObjectOutputStream objectOutputStream=new ObjectOutputStream(outStream );outputStream.writeObject( new TerminaBean() );...ObjectInputStream objectInputStream = new ObjectInputStream( inStream );TerminalBean terminal = (TerminalBean) objectInputStream.readObject( );

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Versioning

Java Serialization support:

Reading streams written by orderversion of the same class

Writing stream intended to be read byorder version of the same class

Identify and load streams that match the

exact class used to write the stream

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Introspection

Discovering the properties, events,and method that a componentsupports

Two methods to performintrospection

Use low level reflection mechanism-

Explicit specification using the BeanInfoclass

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Reflection Mechanism

Uses reflection facilities of the JavaAPI to determine the public methods,fields of the Java Bean

Apply design patterns to determineproperties, methods, and events thatthe JavaBean supports

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BeanInfo

Provides explicit information about aJavaBean

Defined by the creator of the bean

Does not need to provide completeinformation

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Customization

Customize the appearance andbehavior of a component in a designtime environment

Customization is supported through:

Property Editors

Property Sheets Customizers

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Property Editors

Visual element for editing a specificJava Type

For standard types editors areprovided

Located using thePropertyEditorManger

Displayed on Property sheets

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Property Sheets

Consist of all editors necessary toedit the properties of a component

Keep track of which editors valueschange

Update bean after each change

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Customizers

More elaborate visual interfaces toedit the properties of a bean in amultiple step process

Act like wizards or experts

Builder tools register with thecustomizers for property changesand update bean after each change

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Packaging and Deployment

JAR (Java ARchive) files are theprimary method of packaging anddistributing JavaBeans

A JAR file contains:

Class files

Serialized prototypes of a bean

Documentation files

Resources