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)

2. 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

3. 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

7. 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

8. 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.

9. 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

11. 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 <bdk>\demo where <bdk> is the installation dir for BDK

12. 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);}}

13. 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]

14. 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

15. Design Pattern for Properties 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

16. 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

17. 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

18. 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

19. 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

20. 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);}

21. 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)

22. Registration For example: OurButton button = new OurButton(); button.addPropertyChangeListener(aButtonListener); …..

23. 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

24. 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.

25. 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

26. 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);}

27. 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); }

28. 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

29. 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

30. 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

31. 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);

32. 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;}}

33. 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);}

34. 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

35. 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();}

36. 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);}}}

37. 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

38. 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

39. 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

40. 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