JavaBeans*

1
JavaBeans

• ICS 123
• Richard N. Taylor Eric M. Dashofy
• UC Irvine
• http//www.isr.uci.edu/classes/ics123s02/
• with the usual thanks to David Rosenblum

2
What Is JavaBeans?

• (One of) Sun Microsystemss entry in the
  component sweepstakes
• Currently realized in Beans Development Kit (BDK)
  1.1 spec (1999)
• Component model for Java
• Platform-independent
• Language-dependent
• Implemented as language extensions
• Additional classes and interfaces in the Java
  Class Library
• Basically a design pattern
• Based on standard naming conventions
• The naming conventions allow beanboxes and other
  beans to dynamically discover a beans
  capabilities
• But naming conventions have well-known problems
• Detailed documentation is available at
• http//java.sun.com/products/javabeans/

3
Three Levels of Use

• Building applications from beans written by bean
  developers
• Developing individual beans
• Developing beanboxes and other tools for
  graphically manipulating beans
• A more neutral term for such tools is sandboxes

4
What Is a Bean?

• A bean is a Java object
• Exports properties, events and methods
• Interface conforms to certain naming rules
• Conforms to certain packaging rules
• Usually (but not always) has a visual form for
  manipulation in beanboxes
• Supported by package java.beans
• Any Java class can be a bean
• No predefined Bean class
• Many AWT components are beans

5
A Bare-Bones Bean
public class Airplane extends Canvas public
Airplane() setBackground(Color.white) //
Plus other visual settings and
setForeground(Color.blue) // other
initialization public Dimension
getPreferredSize() return new
Dimension(50,50) // Or whatever is appropriate
public Dimension getMinimumSize()
return getPreferredSize() // Or return an
actual Dimension

• Canvas is subclassed to make the bean a GUI
  object having its own window
• Can subclass Component instead to create a
  lightweight component without its own window

6
Elements of JavaBeans (I)

• Events signal a beans response to changes in its
  properties and other phenomena
• Properties represent attributes of a bean
• BeanInfo customizes the information about a bean
  that is made available within a beanbox

7
Elements of JavaBeans (II)

• Customizers allow customization of a beans
  appearance and behavior in beanboxes
• PropertyEditors define custom editors for a
  specific property
• JAR files are archive files containing all the
  artifacts related to a bean
• Manifest files describe the content of JAR files

8
Events

• Java events are used heavily in JavaBeans
• An event in Java is an object
• Events are instances of (subclasses of)
  java.util.EventObject
• An event is sent by an object to a specific
  object
• The identity of the sender is associated with the
  event
• An event is sent as an argument in a method call
• An object that generates events must keep track
  of its own listeners

9
So Whats So Special About Java Events?

• How do they differ from regular method calls?
• They define a protocol for asynchronous
  interaction
• They are generated by many predefined Java
  components
• They are the primary mechanism for wiring
  interoperation paths between beans
• But theres still something missing ...

10
ReviewImplicit Invocation

• Advantages
• Allows for decoupling and autonomy of components
• Java events require tight coupling
• Enhances reuse and evolution
• Easy to introduce new components without
  affecting existing ones
• Disadvantages
• Components announcing events have no guarantee of
  getting a response
• Components announcing events have no control over
  the order of responses
• Event abstraction does not cleanly lend itself to
  data exchange
• Difficult to reason about the behavior of an
  announcing component independently of the
  components that register for its events

11
Listeners and Adapters

• Listeners are interfaces
• They define methods that an object must implement
  in order to receive event notifications
• Listeners add and remove themselves (i.e.,
  register and unregister) with the source of
  events theyre interested in
• Event sources must provide methods to add and
  remove listeners
• Adapters are classes
• They are bare-bones implementations of listener
  interfaces
• They provide a convenient way of quickly creating
  event listeners
• They are usually subclassed to override a single
  listener method of interest

12
Predefined Events in Java

• ActionEvent high-level user event
• AdjustmentEvent event from an Adjustable object
  (such as a scrollbar)
• ComponentEvent event from a GUI Component
  object
• Container, Focus, Input (Key or Mouse), Paint,
  Window
• ItemEvent event from an ItemSelectable object
• TextEvent event from the editing of a
  TextComponent
• Many of these have associated Adapters and/or
  Listeners

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Event Handling Scenario(involving ActionEvents)
ActionEventSource
ActionEvent Listener
ActionEvent Listener
addActionListener()
actionPerformed(ae)
addActionListener()
actionPerformed(ae)
actionPerformed(ae)
removeActionListener()
actionPerformed(ae)
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Action EventProgramming Example
public class Button extends Component //
defined in java.awt public synchronized void
addActionListener(ActionListener l) ...
protected void processEvent(ActionEvent ae)
// called internally ...
l.ActionPerformed(ae) public class
ButtonListener implements ActionListener
Button button public ButtonListener()
button new Button("Press Me")
button.addActionListener(this) public
void ActionPerformed(ActionEvent buttonClick)
...
15
Properties

• A property of a bean is a public attribute,
  accessed via Get and Set methods
• Read-only properties have only a Get method
• Write-only properties have only a Set method
• Read/Write properties have both

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Kinds of Properties (I)

• Simple properties a single value P of some type
  T
• public T getP()
• public void setP(T newP)
• public boolean isP() // same as getP when T is
  boolean
• Indexed properties an array of values
• public T getP()
• public void setP(T newP)
• public T getP(int index)
• public void setP(int index, T newP)
• Not supported in BDK 1.0
• How does a beanbox know that something is a
  simple or indexed property?

17
Example Simple Properties
public class Airplane extends Label protected
String serialNumber "N173UA" public String
getSerialNumber() return serialNumber
public Airplane() setText(getSerialNumb
er()) protected int flightNumber 0
public int getFlightNumber() return
flightNumber public void
setFlightNumber(int n) flightNumber n

• FlightNumber is a read/write property
• SerialNumber is a read-only property
• Attributes storing the property values are made
  protected

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Kinds of Properties (II)

• Bound properties signal a PropertyChange event
  when changed
• One event listener list for all bound properties
• Supported by classes and interfaces in package
  java.beans
• public class PropertyChangeEvent extends
  java.util.EventObject
• public interface PropertyChangeListener extends
  EventListener
• class PropertyChangeSupport implements
  Serializable
• public void addPropertyChangeListener(PropertyChan
  geListener l)
• public void removePropertyChangeListener(PropertyC
  hangeListener l)
• How does a beanbox know that something is a bound
  property?

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Example Bound Property
import java.beans. // Needed for PropertyChange
stuff public class Airplane extends Label
protected int altitude 0 protected
PropertyChangeSupport changes new
PropertyChangeSupport(this) public int
getAltitude() return altitude public void
setAltitude(int a) int oldAltitude
altitude altitude a
changes.firePropertyChange("Altitude", new
Integer(oldAltitude),
new Integer(a)) public void
addPropertyChangeListener(PropertyChangeListener
l) changes.addPropertyChangeListener(l)
public void removePropertyChangeListener(Prop
ertyChangeListener l) changes.removeProperty
ChangeListener(l)
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Kinds of Properties (III)

• Constrained properties changes can be vetoed by
  other objects
• One event listener list for all constrained
  properties
• Supported by classes and interfaces in package
  java.beans
• exception PropertyVetoException
• public interface VetoableChangeListener extends
  EventListener
• class VetoableChangeSupport implements
  Serializable
• public void setP(T newP) throws
  PropertyVetoException
• public void addVetoableChangeListener(VetoableChan
  geListener l)
• public void removeVetoableChangeListener(VetoableC
  hangeListener l)
• Usually also implemented as bound properties
• Bound property listeners also get notified of
  changes to constrained properties
• Only registered vetoers are given the opportunity
  to veto a change

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Example Constrained Property
public class Airplane extends Label protected
int heading 0 protected VetoableChangeSupport
vetoes new VetoableChangeSupport(this)
public int getHeading() return heading
public void setHeading(int h) throws
PropertyVetoException int oldHeading
heading vetoes.fireVetoableChange("Heading",
new Integer(oldHeading),
new Integer(h)) // heading h
changes.firePropertyChange("Heading", new
Integer(oldHeading),
new Integer(h)) public void
addVetoableChangeListener(VetoableChangeListener
l) vetoes.addVetoableChangeListener(l)
public void removeVetoableChangeListener(Veto
ableChangeListener l) vetoes.removeVetoableC
hangeListener(l)

• This is also a bound property (using the setup
  for Altitude)
• PropertyVetoException would be raised within call
  at

22
JAR Files

• JAR archives contain files related to a bean
• .class files
• .ser files (serialized Beans)
• .html files
• image files
• audio files
• Manifest file
• Manifest entries can include several fields,
  including an optional indication of whether or
  not a file is a bean
• In the absence of a manifest file, all .class and
  .ser files are treated as beans

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Packaging the Airplane Bean

• Create a MANIFEST file
• Name Airplane.classJava-Bean TrueName
  AirplaneBeanInfo.classJava-Bean FalseName
  Airplane.gif
• Create the JAR file
• jar cfm Airplane.jar MANIFEST Airplane.class \
  AirplaneBeanInfo.class Airplane.gif
• The jar command creates a file called
  META-INF/MANIFEST in the .jar file it creates,
  using the information contained in MANIFEST above

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BDK Directory Structure
ltroot-directorygt
beanbox
jars

GNUmakefile
docs

• Run beanbox from beanbox directory
• run.sh in UNIX, run.bat in Windows
• Beanbox looks in jars directory for .jar files
• Look at GNUmakefile for ideas on how to automate
  creation of beans
• Lots of documentation in docs directory

25
The BDK Beanbox

• A simple beanbox for graphically composing beans
  and testing bean interactions
• Three windows
• Beanbox A canvas where beans are placed and
  wired together
• Toolbox A palette of available beans
• Property sheet Properties of the selected bean
• The beanbox finds read-only and write-only
  properties for a bean but by default doesnt
  display them in the property sheet for the bean

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The BDK Beanbox with the Airplane Bean
Property Sheet
Beanbox
Toolbox
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Additional BDK Beanbox Expectations

• Each bean class needs a zero-argument constructor
  method
• The beanbox calls this method when you drop a
  bean in the beanbox
• Helpful to define a package for all classes
  related to a bean

28
Wiring Beans Together in the BDK Beanbox

• Beanbox customizes Edit menu according to
  capabilities of currently selected bean
• Wiring events
• Event from generated bean passed as argument to
  method of target bean
• Predefined events for the beans class
• Component, container, key, mouse, focus,
  mouseMotion events
• PropertyChange events
• VetoableChange events
• Any other events generated by the bean
• Wiring bound properties
• Change in bound property in one bean can be bound
  to a property in another bean

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Wiring Up Two Beans (I)

• An Airplane bean and a Juggler bean
• Airplane is currently selected

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Wiring Up Two Beans (II)

• Select propertyChange event in Airplane bean

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Wiring Up Two Beans (III)

• Wire the propertyChange event to the Juggler

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Wiring Up Two Beans (IV)

• Have the jugglers startJuggling method called
  upon receiving the Airplanes propertyChange event

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Wiring Up Two Beans (V)

• Juggler is now selected
• Select mouseClicked event in Juggler bean

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Wiring Up Two Beans (VI)

• Wire the Jugglers mouseClicked event to itself

35
Wiring Up Two Beans (VII)

• Have the jugglers stopJuggling method called
  upon receiving its own mouseClicked event

36
Wiring Up Two Beans (VIII)

• Everything is now wired up
• Click on the Juggler to stop its juggling
• Change the altitude of the airplane to start the
  Juggler juggling