Transcript PPT

ICS 123
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
Topic 12
JavaBeans
What Is JavaBeans?
ICS 123
• (One of) Sun Microsystems’s 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
bean’s capabilities
– But naming conventions have well-known problems
• Detailed documentation is available at
http://java.sun.com/products/javabeans/
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Topic 12
JavaBeans
Three Levels of Use
ICS 123
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”
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Topic 12
JavaBeans
What Is a Bean?
ICS 123
• 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
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• Any Java class can be a bean
– No predefined Bean class
• Many AWT components are beans
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Topic 12
JavaBeans
A Bare-Bones Bean
ICS 123
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
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JavaBeans
Elements of JavaBeans (I)
ICS 123
• Events signal a bean’s 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
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Topic 12
JavaBeans
Elements of JavaBeans (II)
ICS 123
• Customizers allow customization of a bean’s
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
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Topic 12
JavaBeans
Events
ICS 123
• 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
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So What’s So Special About
Java Events?
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JavaBeans
ICS 123
• 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 there’s still something missing ...
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Review:
Implicit Invocation
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JavaBeans
ICS 123
• 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
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JavaBeans
Listeners and Adapters
ICS 123
• 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 they’re 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
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Topic 12
JavaBeans
Predefined Events in Java
ICS 123
• 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)
ActionEvent Listener
ActionEventSource
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JavaBeans
ICS 123
ActionEvent Listener
addActionListener()
actionPerformed(ae)
addActionListener()
actionPerformed(ae)
actionPerformed(ae)
removeActionListener()
actionPerformed(ae)
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Action Event
Programming Example
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JavaBeans
ICS 123
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);
}
}
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public void ActionPerformed(ActionEvent buttonClick) {
...
}
Topic 12
JavaBeans
Properties
ICS 123
• 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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Topic 12
JavaBeans
Kinds of Properties (I)
ICS 123
• 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?
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JavaBeans
Example Simple Properties
ICS 123
public class Airplane extends Label {
protected String serialNumber = "N173UA";
public String getSerialNumber() {
return serialNumber;
}
public Airplane() {
setText(getSerialNumber());
}
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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JavaBeans
Kinds of Properties (II)
ICS 123
• 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(PropertyChangeListener l);
 public void removePropertyChangeListener(PropertyChangeListener l);
• How does a beanbox know that something is a
bound property?
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JavaBeans
Example Bound Property
import java.beans.*
ICS 123
// 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);
}
}
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public void removePropertyChangeListener(PropertyChangeListener l) {
changes.removePropertyChangeListener(l);
}
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JavaBeans
Kinds of Properties (III)
ICS 123
• 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(VetoableChangeListener l);
 public void removeVetoableChangeListener(VetoableChangeListener 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);
Topic 12
JavaBeans
ICS 123
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(VetoableChangeListener l) {
vetoes.removeVetoableChangeListener(l);
}
• This is also a bound property (using the setup for Altitude)
• PropertyVetoException would be raised within call at *****
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Topic 12
JavaBeans
JAR Files
ICS 123
• JAR archives contain files related to a bean
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.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
Topic 12
JavaBeans
Packaging the Airplane Bean

ICS 123
Create a MANIFEST file
Name: Airplane.class
Java-Bean: True
Name: AirplaneBeanInfo.class
Java-Bean: False
Name: 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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Topic 12
JavaBeans
BDK Directory Structure
ICS 123
<root-directory>
GNUmakefile
beanbox
docs
jars
…
• 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
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Topic 12
JavaBeans
The BDK Beanbox
ICS 123
• 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 doesn’t display
them in the property sheet for the bean
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The BDK Beanbox with the
Airplane Bean
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JavaBeans
ICS 123
Property Sheet
Beanbox
Toolbox
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Additional BDK Beanbox
Expectations
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JavaBeans
ICS 123
• 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
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Wiring Beans Together in the
BDK Beanbox
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JavaBeans
ICS 123
• 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 bean’s 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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JavaBeans
Wiring Up Two Beans (I)
ICS 123
• An Airplane bean and a
Juggler bean
• Airplane is currently selected
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JavaBeans
Wiring Up Two Beans (II)
ICS 123
• Select propertyChange
event in Airplane bean
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JavaBeans
Wiring Up Two Beans (III)
ICS 123
• Wire the propertyChange
event to the Juggler
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JavaBeans
Wiring Up Two Beans (IV)
ICS 123
• Have the juggler’s
startJuggling method called
upon receiving the
Airplane’s propertyChange
event
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JavaBeans
Wiring Up Two Beans (V)
ICS 123
• Juggler is now selected
• Select mouseClicked event
in Juggler bean
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JavaBeans
Wiring Up Two Beans (VI)
• Wire the Juggler’s
ICS 123
mouseClicked event to itself
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JavaBeans
Wiring Up Two Beans (VII)
ICS 123
• Have the juggler’s
stopJuggling method called
upon receiving its own
mouseClicked event
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JavaBeans
Wiring Up Two Beans (VIII)
ICS 123
• Everything is now wired up
– Click on the Juggler to stop its
juggling
– Change the altitude of the airplane
to start the Juggler juggling
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