JavaBean Component

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15 Αυγ 2012 (πριν από 4 χρόνια και 8 μήνες)

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JavaBean Component

Java bean is a reusable software component that can be
manipulated visually in a builder tool

Graphic bean and Non
graphic bean

Javabean is not distributed component like EJB

Interface of javabean is provided by

Design pattern(implicitly)

2. Using a class to implement the
BeanInfo or Customizer


Javabean Component

It is a binary building block

Development and deployment of a javabean

Assembly javabeans to build a new javabean or a new
application, applet

Write glue codes to wire all beans together

javabean with CORBA as a CORBA client

Client side javabean

Javabean for business logic process in MVC on server

javabean on server is not visible

Advantage of Java Bean

Write once, run anywhere

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

They are the interface of the bean.

They are platform independent

Configuration setting of a bean can be saved in persistent
storage and restored later

Bean may register and receive events from other object and
can generate event sent to other objects

(Bean communication)









Design Pattern

All beans should implement the Serializable interface so
that the state can be saved and later restored

Methods must be made public

All exposed methods should be threadsafe, possibly
synchronized to prevent more than one thread from calling
method at a given time

Propertie X is exposed by public setX and getX methods

Boolean property may be exposed by isX method which
returns a boolean value

The bean which may trigger event must provide
addEventListener and removeEventListener mehods for
other bean to register with it to be notified

Deployment of Bean

All java classes can be converted to a bean

Bean is compressed and saved in the format of jar file
which contains manifest file, class files, gif files, and other
information customization files

Sun NetBeans, BDK, Visual Café, JBuilder, Visual Age
are the bean builder tools

Criteria to be a bean

Can this piece of code be used in more than one area?

Can you quickly think of ways that this piece of code
might be customized?

Is the purpose of this code easy to explain?

Does this code module contain all the info it needs to work

Does it have good encapsulation?

If you answer all “yes”, You should make the class a bean

JAR file

JAR file allows you to efficiently deploy a set of classes
and their associated resources.

JAR file makes it much easier to deliver, install, and
download. It is compressed

Manifest file


Name: SimpleBean.class

Bean: True


Creating and extract a jar file

Create a jar file

jar cfm simplebean.jar manifest.tmp *.class

Extracting files from a jar file

jar xf simplebean.jar

Develop a New Bean

Create a directory for the new bean

Create the java bean source file(s)

Compile the source file(s)

Create a manifest file

Generate a JAR file

Start BDK


dir can be at <bdk>
demo where <bdk> is the
installation dir for BDK

Create bean source file

package simplebean;

import java.awt.*;


public class SimpleBean extends Canvas implements

{ public SimpleBean(){



Compile and make jar file

d .

Edit a manifest file called manifest.tmp

Name: SimpleBean.class

Bean: True

jar cfm ..
simplebean.jar manifest.tmp

[SimpleBean and colorsbean demo]


Process of analyzing a bean to determine the

Allows application builder tool to present info
about a component to software designer

Naming convention

implicit method


class to explicitly infer info of a bean

Design Pattern for Properties

Property is a subset of a bean’s state which
determines the appearance and behavior of the

Simple property

Indexed Property

Bound Property

Constrained property

Simple Property

Simple property has a single value.

N is the name of the property and T is its type

public T getN();

public void setN(T arg)

For readonly property there is getN() method only

Indexed Property

One property may consists of multiple values stored in an

public T getN(int index);

public void setN(int index, T value);

public T[] getN();

public void setN(T values[]);

where N may be a double data[] and T is double

Bound Property

It can generate an event when the property
is changed

The event is of type
and is sent to objects that previously
registered an interest in receiving such

bean with bound property

Event source

Bean implementing listener

event target

Implement Bound Property in a Bean

1. Import java.beans package

2. Instantiate a PropertyChangeSupport object

private PropertyChangeSupport changes = new


3. Implement methods to maintain the property change
listener list:

public void
addPropertyChangeListener(PropertyChangeListener l)

{ changes.addPropertyChangeListener(l);}

also removePropertyChangeListener method is needed

Event Source Cont.

4. Modify a property’s setter method to fire a property change
event when the property is changed.

Public void setX(int newX){

int oldx = x;

x = newX;

changes.firePropertyChange(“x”, oldX, newX);}

Implement Bound Property Listener

1. Listener bean must implement PropertyChangeListner

public class MyClass implements PropertyChangeListener,

2. It must override this method:

public abstract void propertyChange(PropertyChangeevent evt)


For example:

OurButton button = new OurButton();



Constrained Property

It generates an event when an attempt is made to change it

The event type is

The event is sent to objects that previously registered an
interest in receiving an such notification

Those other objects have the ability to veto the proposed

This allows a bean to operate differently according to the
runtime environment

Three Parts in Implementation of

Constrained Property

1. Source bean containing one or more constrained properties

2. Listener objects that implement the
VetoableChangeListener interface. This object either
accepts or rejects the proposed change.

3. PropertyChangeEvent object containing property name, old
value, new value.

Implement Constrained Property in a Bean

Bean with constrained property must

1. Allow VetoableChangeListener object to register and
unregister its interest in receiving notifications

2. Fire property change at those registered listeners. The event
is fired before the actual property change takes place

Implementation of Constrained Property

in a Bean

1. Import java.beans package

2. Instantiate a VetoableChangeSupport object:

private VetoableChangeSupport vetos=new


3. Implement methods to maintain the property change
listener list:

public void

addVetoableChangelistener(VetoableChangelistener l)

{ vetos.addVetoableChangeListener(l);}


4. Write a property’s setter method to fire a property change

public void setX(int newX)

{ int oldX=X;

vetos.fireVetoableChange(“X”, oldX, newX);

//if no veto there


changes.firePropertyChange(“X”, oldX, newX); }

Implementing Constrained Property Listeners

1. Implements the VetoableChangeListener interface which
has an abstract method

Void vetoChange(PropertyChangeEvent evt)

2. Overide this abstract method. This is the method that will
be called by the source bean on each object in the listener
list kept by vetoableChangeSupport object


It has the ability to save a bean to storage and retrieve it at
a later time

Configuration settings are saved

It is implemented by Java serialization

If a bean inherits directly or indirectly from Component
class it is automatically Serializable.

Transient keyword can be used to designate data members
not be saved ex. Thread reference member


Property sheet may not be the best user interface for a
complex component

It can provide step
step wizard guide to use component

It can provide a GUI frame with image which visually tells
what is changed such as radio button, check box, ...

It can customize the appearance and behavior of the

Design Pattern for User Defined Events

Bean can generate events and send them to other objects in
delegation event model

Listener registration and unregistration

public void addTListener(TListener x);

public void removeTListener(TListener x);

User defined listener and event

Timer class will fire off timeout event:

public interface TimerListener extends EventListener{
public void timeOut(TimeEvent e);}

public class TimerEvent extends EventObject{

int count=0;

public TimerEvent(Object obj){super(obj);}

public int getCount(){ return count;}

public void setCount(int count){this.count=count;}}

Timer class fires off timeout event to all
registered objects

public void startTimer(){

if (t==null){t=new Thread(this); t.start();}

public void run(){for(;;){


catch(Exception e){}


void fireOff(){TimerEvent te=new TimeEvent(this);


Vector listeners = (Vector)listeners.clone();

for(int I=0;I<listeners.size(); I++){


Any class that implements TimerListener can
register itself by calling addTimerListener()

Vector listeners = new Vector();

public void addTimerListener(TimerListener l){


public void removeTimerListener(TimerListener l){


Any class that implements TimerListener interface must
override the timeOut method of interface TimerListener to
respond the notification

package colors;

import java.awt.*;

import java.awt.event.*;

public class Colors extends Canvas{

transient private Color color;

private boolean rect;

public Colors(){

addMouseListener(new MouseAdapter(){

public void mousePressed(MouseEvent me){change();}});

rect=false; setSize(200,100); change();}

public boolean getRect(){ return rect;}

public void setRect(boolean flag){this.rect=flag; repaint();}

public void change(){color=randomColor(); repaint();}

private Color randomColor(){

int r=(int)(255*Math.random());

int g =(int)(255*Math.random()); int b=(int)(255*Math.random());

return new Color(r,g,b);}

public void paint(Graphics g){

Dimension d = getSize(); int h=d.height; int w=d.width;




JavaBean is a platform
neutral component
architecture for reusable software

It is a black box component to be used to
build large component or application

Property,method,event, introspector,
customizer are parts of javabean interface


True of False

1. One JavaBean can fire off an event taken by more than one

2. Javabean can’t be inherited by other bean.

3. Javabean can be distributed.

4. Javabean is in binary format and deployed in JAR file

5. Javabean can only be introspected by property sheet.

6. Javabean is a language independent architecture


Javabean can only be developed by BDK.

Every java class can be turned into a Javabean.

Javabean must be a graphic component.

Javabean can be used on web server.

The event a Javabean can trigger is a subclass of class

The bound property bean can prevent its property change
by other bean.

JAR file is compressed file


Event source must implement listener and override
the method provided by the listener

Event Target must provide the addTListener and
removeTListener methods

BeanInfo class is used to explicitly introspect the
bean property

Customizer class can be used to provide winzard
for bean configuration