Transcript JavaBean Component

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
1. Design pattern(implicitly)
2. Using a class to implement the BeanInfo or Customizer
interface(explicitly)
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)
BeanInfo
Methods
JavaBean
Component
Properties
Customizer
Events
JAR
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
itself?
• 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
Manifest.tmp
Name: SimpleBean.class
Java-Bean: True
...
Creating and extract a jar file
• Create a jar file
jar cfm simplebean.jar manifest.tmp
• Extracting files from a jar file
jar xf simplebean.jar
*.class
Develop a New Bean
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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
Test
Working-dir can be at <bdk>\demo where <bdk> is the
installation dir for BDK
Create bean source file - SimpleBean.java
package simplebean;
import java.awt.*;
import java.io.Serializable;
public class SimpleBean extends Canvas implements
Serializable
{ public SimpleBean(){
setSize(60,40);
setBackground(Color.red);}}
Compile and make jar file
• Javac -d . SimpleBean.java
• Edit a manifest file called manifest.tmp
Name: SimpleBean.class
Java-Bean: True
• jar cfm ..\jars\simplebean.jar manifest.tmp
simplebean\*.class
[SimpleBean and colorsbean demo]
Introspection
• Process of analyzing a bean to determine the
capability
• Allows application builder tool to present info
about a component to software designer
• Naming convention implicit method
• BeanInfo class to explicitly infer info of a bean
Design Pattern for Properties
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Property is a subset of a bean’s state which
determines the appearance and behavior of the
component
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
array
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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 PropertyChangeEvent
and is sent to objects that previously
registered an interest in receiving such
notifications
• 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
PropertyChangeSupport(this);
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
interface
public class MyClass implements PropertyChangeListener,
Serializable
2. It must override this method:
public abstract void propertyChange(PropertyChangeevent evt)
Registration
For example:
OurButton button = new OurButton();
button.addPropertyChangeListener(aButtonListener);
…..
Constrained Property
• It generates an event when an attempt is made to change it
value
• The event type is PropertyChangeEvent
• 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
change
• 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
VetoableChangeSupport(this);
3. Implement methods to maintain the property change
listener list:
public void
addVetoableChangelistener(VetoableChangelistener l)
{ vetos.addVetoableChangeListener(l);}
Cont.
4. Write a property’s setter method to fire a property change
event:
public void setX(int newX)
{ int oldX=X;
vetos.fireVetoableChange(“X”, oldX, newX);
//if no veto there
X=newX;
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
Persistence
• 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
Customizers
• Property sheet may not be the best user interface for a
complex component
• It can provide step-by-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
properties
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(;;){
try{t.sleep(timeout);}
catch(Exception e){}
fireOff();}}
void fireOff(){TimerEvent te=new TimeEvent(this);
te.setCount(number);
Vector listeners = (Vector)listeners.clone();
for(int I=0;I<listeners.size(); I++){
((TimerListener)listeners.elementAt(I).timeOut(te);}
Any class that implements TimerListener can
register itself by calling addTimerListener()
Vector listeners = new Vector();
public void addTimerListener(TimerListener l){
listeners.addElement(l);}
public void removeTimerListener(TimerListener l){
listeners.removeElement(l);}
Any class that implements TimerListener interface must
override the timeOut method of interface TimerListener to
respond the notification
Colors.java
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();}
Colors.java(Cont.)
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;
g.setColor(color);
if(rect){g.fillRect(0,0,w-1,h-1);}
else{g.fillOval(0,0,w-1,h-1);}}}
Summary
• JavaBean is a platform-neutral component
architecture for reusable software
component
• 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
Quiz
True of False
1. One JavaBean can fire off an event taken by more than one
targets.
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
Quiz
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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
EventObject
• The bound property bean can prevent its property change
by other bean.
• JAR file is compressed file
Quiz
• 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