JavaBean Component

1
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)

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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

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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)

4
BeanInfo
Methods
JavaBean Component
Events
Properties
JAR
Customizer
5
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

6
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

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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

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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.

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Manifest file

• Manifest.tmp
• Name SimpleBean.class
• Java-Bean True
• ...

10
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

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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
• Test
• Working-dir can be at ltbdkgt\demo where ltbdkgt
  is the installation dir for BDK

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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)

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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

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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

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Design Pattern for Properties

• Property is a subset of a beans state which
  determines the appearance and behavior of the
  component
• Simple property
• Indexed Property
• Bound Property
• Constrained property

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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

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Indexed Property

• One property may consists of multiple values
  stored in an array
• 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

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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

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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(Property
  ChangeListener l)
• changes.addPropertyChangeListener(l)
• also removePropertyChangeListener method is
  needed

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Event Source Cont.

• 4. Modify a propertys 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)

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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(PropertyChange
  event evt)

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Registration

• For example
• OurButton button new OurButton()
• button.addPropertyChangeListener(aButtonListener)
  
• ..

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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

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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.

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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

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Implementation of Constrained Property in a Bean

• 1. Import java.beans package
• 2. Instantiate a VetoableChangeSupport object
• private VetoableChangeSupport vetosnew
• VetoableChangeSupport(this)
• 3. Implement methods to maintain the property
  change listener list
• public void
• addVetoableChangelistener(VetoableChangelis
  tener l)
• vetos.addVetoableChangeListener(l)

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Cont.

• 4. Write a propertys setter method to fire a
  property change event
• public void setX(int newX)
• int oldXX
• vetos.fireVetoableChange(X, oldX,
  newX)
• //if no veto there
• XnewX
• changes.firePropertyChange(X, oldX,
  newX)

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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

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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

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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

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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)

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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 count0
• public TimerEvent(Object obj)super(obj)
• public int getCount() return count
• public void setCount(int
  count)this.countcount

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Timer class fires off timeout event to all
registered objects

• public void startTimer()
• if (tnull)tnew Thread(this) t.start()
• public void run()for()
• tryt.sleep(timeout)
• catch(Exception e)
• fireOff()
• void fireOff()TimerEvent tenew TimeEvent(this)
• te.setCount(number)
• Vector listeners (Vector)listeners.clone()
• for(int I0Iltlisteners.size() I)
• ((TimerListener)listeners.elementAt(I).timeOut(
  te)

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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

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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())
• rectfalse setSize(200,100) change()

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Colors.java(Cont.)

• public boolean getRect() return rect
• public void setRect(boolean flag)this.rectflag
  repaint()
• public void change()colorrandomColor()
  repaint()
• private Color randomColor()
• int r(int)(255Math.random())
• int g (int)(255Math.random()) int
  b(int)(255Math.random())
• return new Color(r,g,b)
• public void paint(Graphics g)
• Dimension d getSize() int hd.height int
  wd.width g.setColor(color)
• if(rect)g.fillRect(0,0,w-1,h-1)
• elseg.fillOval(0,0,w-1,h-1)

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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

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Quiz

• True of False
• 1. One JavaBean can fire off an event taken by
  more than one targets.
• 2. Javabean cant 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

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Quiz

• 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

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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