Colossal Java Bean

7/28/2019 Colossal Java Bean

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Java Bean

Mubarak,

Solution Architect,

Colossal Software Technologies.

What is JavaBeanReusable Java components

Visual and non-visual components possible

any object that conforms to a few basic rules can be a bean

originally developed to provide java with a componenttechnology like vbx/ocx of VB

originally used for user interface widgets

Visual manipulation in builder tools

Beans support separate design-time and runtime interfaces


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What makes a Bean a Bean

Must have a zero-argument (empty) constructor

Should have no public instance variables

Persistent values should be accessed through methods

called getXxx and setXxx

Must support persistence (the bean is serializable)

Properties : Named attributes associated with a bean

that can be read or manipulated through methods

Event-Handling: Ability for beans to communicate with

themselves and with their outside world asynchronously

Introspection: Ability of seeing inside a bean and analyzing

what services it provides

Customization: Ability of manipulating the appearance or

behavior of a bean while designing the application

Persistence: Capturing the customized state of a bean forlater (re) use

Security: Access control based Java security

Characteristics of Java Beans


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Bean PropertiesSimple

represents a single property and can be defined with a

pair of get/set methods.

Indexed

represents an array of properties.

Bound

a bound property notifies other objects when its value

changes generates a PropertyChange event with

property name, old value and new value.

Constrained

an object with constrained properties allows otherobjects to veto a constrained property value change

Constrained property listeners can veto a change by

throwing a PropertyVetoException.

Simple Property To be a bean, you cannot have public fields instead you

should expose fields using simple property.

So, you should replace

public double speed;

with

private double speed;

public double getSpeed() {

return(speed);

}

public void setSpeed(double newSpeed) {speed = newSpeed;

}


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Simple Property

You can put constraints on values.

public void setSpeed(double newSpeed) {

if (newSpeed < 0) {

sendErrorMessage(...);

newSpeed = Math.abs(newSpeed);

}

speed = newSpeed;

}

If users of your class accessed the fields directly, then

they would each be responsible for checking constraints.

Simple Property You can change your internal representation without

changing interface

// Now using metric units (kph, not mph)

public void setSpeed(double newSpeed)

{

setSpeedInKPH = convert(newSpeed);

}

public void setSpeedInKPH(double newSpeed)

{

speedInKPH = newSpeed;

}


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Simple Property

You can perform arbitrary side effects.

public double setSpeed(double newSpeed)

{

speed = newSpeed;

updateSpeedometerDisplay();

}

If users of your class accessed the fields directly, then theywould

each be responsible for executing side effects.

Too much work and runs huge risk of having display beinconsistent from actual values.

Indexed Property Collections of properties accessed by an index (like an

array)

Methods to set & get array of values

public [] get();

public void set([]value);

Methods to set & get individual valuespublic get(int pos);

public void set(intpos,[]

value);


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Property binding If a beans properties are changed it is possible another bean

want to act on that change

private PropertyChangeSupport changes = newPropertyChangeSupport( this );

public void addPropertyChangeListener(PropertyChangeListener l ) {

changes.addPropertyChangeListener( l );

};

public void removeProperyChangeListener(

PropertyChangeListener l){

changes.removePropertyChangeListener( l );

};

Implementation firePropertyChange predefined method calls all registeredchange listeners

public void firePropertyChange( string propertyName,

Object oldValue, Object newValue);

Example

public void setBackgroundColor( java.awt.Color ColornewColor ) {

java.awt.Color oldColor = backgroundColor;

backgroundColor = newColor;

changes.firePropertyChange(backgroundColor,

oldColor, newColor );

};


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Property Listeners firePropertyChange predefined method calls all registered

change listeners which implements the interface:PropertyChangeListener

All listeners implements the interface:

PropertyChangeListener which has only 1 method:

public abstract void propertyChange( PropertyChangeEvent

evt );

Example

public class someButtonThatAlsoWantsToChangeColor

implements java.beans.PropertyChangeListener

{public void propertyChange( PropertyChangeEvent evt )

{

setBackgroundColor( inverse(evt.Color) );

};

Constrained Property Bean property is constrained when any change to it can be

vetoed.

Before a bean property changes: other beans might want to

stop that

change.

A bean which wants to stop the changes throws

PropertyVetoException

private VetoableChangeSupport vetos = newVetoableChangeSupport(this);

public void addVetoableChangeListener( VetoableChangeListenerl )

{ vetos.addVetoableChangeListener( l ); };

public void removeVetoableChangeListener(VetoableChan eListener l


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Property veto implementation

fireVetoableChange calls all registered veto listeners

public void setBackgroundColor( java.awt.Color ColornewColor ) {

java.awt.Color oldColor = backgroundColor;

vetos.fireVetoableChange( backgroundColor,

new java.awt.Color( oldColor ),

new java.awt.Color( newColor ) );

backgroundColor = newColor;

changes.firePropertyChange(backgroundColor,

oldColor, newColor );

};

Property veto Listeners fireVetoableChange calls all registered veto listeners Those listeners implement the vetoableListener interface

This interface has the method:

public abstract void vetoableChange(VetoableChangeEvent evt )

throws PropertyVetoException

Example:

public class ChangeColor implementsjava.beans.PropertyChangeListener

implements java.beans.VetoableChangeListener

{

public void propertyChange( PropertyChangeEvent evt );public void vetoableChange( VetoableChangeEvent evt )

throws PropertyVetoException

{

if( evt.id = BackgroundColor && evt.newColor == RED )

throw PropertyVetoExeption;


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Primary communication mechanism for Java Beans.

For plugging together components in application builders,Some

components act as sources and others as listeners ofevents.

Usually, events propagate state changes from the sourceObject to all its listeners.

Examples for events are mouse actions and widget updates.

Listener components need to register to receive events.

Event notifications are propagated by the event source viaimplicit method invocation.

Each Java Bean acts as an event server.

Event Handling

Event Mechanism


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Introspection

The process by which a builder tool finds out which

properties, methods, and events a bean supports.

searching for classes and methods that follow certain

naming conventions

by querying the BeanInfo of a class

BeanInfo is an interface whose methods allow the

interrogation of a beans properties, methods and

events


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Beans need to support a range of storage behavior for: the use of existing data formats (e.g. OpenDoc)

the storage of its internal state

Beans support two mechanisms:

use Java Serialization Mechanism which provides an

automatic way of storing and restoring the internalstate of

Java objects.

use Java Externalization Mechanism (extension ofabove)

which gives a class complete control over the writing ofits

state.

Besides properties (run-time state), beans also need to store

customized information (e.g. how to deal with references to

other beans).

Persistence

Saving an Objectpublic void SaveState()

{

FileOutputStream fos=null;

ObjectOutputStream oos=null;

try

{

fos=new FileOutputStream(State.ser);

oos=new ObjectOutputStream(fos);

oos.writeObject(currentState);

}catch(IOException e)

{

System.out.println(e.String());

}

}


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Loading an Object

public void loadState(){

FileInputStream fos=null;

ObjectInputStream oos=null;

try

{

fis=new FileInputStream(State.ser);

ois=new ObjectOutputStream(fis);

State currentState=(State)ois.readObject();

}

catch(ClassNotFoundException e)

{System.out.println(e.String());

}

}

Beans are packaged in a java archive (jar) file

the jar utility comes with the JDK

the jar file must be explicitly in your class path

in JDK 1.2.x all AWT and Swing classes are beans

Jarring the Bean

Colossal Java Bean

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