98-09-28 1

JavaBeans introductionJavaBeans introduction

Klaus-Peter Heidrich

for the University of Karlstad

Inst. for Information Technology

Dept. of Computer Science

98-09-28 Computer Science, University of

Karlstad2

IntroductionIntroduction

What is a Java Bean? „A Java Bean is a reusable software component

that can be visually manipulated in builder tools.“

Java Beans are Java classes that conform to the Java Beans specification, can be visible (button, slider) or invisible (network component)

98-09-28 Computer Science, University of

Karlstad3

Example for BuilderExample for Builder

98-09-28 Computer Science, University of

Karlstad4

MotivationMotivation

Build an application out of prebuild parts like in industrial production of for example cars or computers

Cross platform component model Interoperability (ActiveX, OpenDoc...) Constructing instead of coding Visual building of applications

98-09-28 Computer Science, University of

Karlstad5

Example InteroperabilityExample Interoperability

98-09-28 Computer Science, University of

Karlstad6

The Converter in DelphiThe Converter in Delphi

98-09-28 Computer Science, University of

Karlstad7

How is it done?How is it done?

A Bean is delivered in binary form It exposes its attributes (methods, instance

variables, properties, and interfaces) through a default mechanism included in the Java API or explicit Information Classes

Bridges translate the attributes to other Component Models

98-09-28 Computer Science, University of

Karlstad8

Basic ConceptsBasic Concepts

A Java Bean supports Introspection Customization Properties Events Persistence

98-09-28 Computer Science, University of

Karlstad9

Requirements for a BeanRequirements for a Bean

A Bean is represented by one ore more Java classes which follow the Java Beans specifications, it must not inherit from any special class

A JavaBean must Have a default Constructor without no arguments be Customizable allowing access to its internal state be Serializable, must be able to save its state in a stream

It should be Packaged in JAR-File

all Classes, Pictures, Documentation... in one file provide BeanInfo and Customizer Classes

98-09-28 Computer Science, University of

Karlstad10

Design vs. RuntimeDesign vs. Runtime

A Java Bean has two lives: In the design environment, where it should

provide design information to the application builder to let the user customize its behaviour and appearance

In the application to do the task it was build for

98-09-28 Computer Science, University of

Karlstad11

Features of a Java BeanFeatures of a Java Bean

A Java Bean has certain attributes Properties

To represent the internal state Methods

To be called from outside Events

To inform about changes

98-09-28 Computer Science, University of

Karlstad12

The Bean Development KitThe Bean Development Kit

Available for free at http://www.javasoft.com/products/javabeans/software/

Reference implementation of a builder application to test own Java Beans

Allows customization of Beans and simple linking of Beans with adapters

98-09-28 Computer Science, University of

Karlstad13

The BeanboxThe Beanbox

98-09-28 Computer Science, University of

Karlstad14

ExampleExample•ProgressBean

•TimerBean

•ButtonBean

The TimerBean calls the ProgressBar in an editable intervall after being started by the Button.

98-09-28 Computer Science, University of

Karlstad15

Class DiagramClass Diagram

startstoprunaddTimerListenerremoveTimerListenergetTimeOutsetTimeOut

timeOutlisteners

TimerBean

Thread Component

getValuesetValuegetMaxsetMaxaddPropertyChangeListenerremovePropertyChangeListenerprogress

maxvaluelisteners

ProgressBar

addActionListenerremoveActionListener...

Listeners...

Button

98-09-28 Computer Science, University of

Karlstad16

Example Interaction DiagramExample Interaction Diagram

Button TimerBean ProgressBarAdapter1

actionperfomedstart

Adapter2

timedOutprogress

timedOutprogress

98-09-28 Computer Science, University of

Karlstad17

PropertiesProperties

•represent the internal state of a component

•control behaviour or visual representation

•can be read or written•the process of setting up properties at design- or runtime is called Customization

98-09-28 Computer Science, University of

Karlstad18

PropertiesProperties

Properties can be: Simple

(simple Java type or Class) Indexed

(an array of the above) Bound

an event is created when the property changes to inform the attached Listeners

Constrainedlike Bound, but the Listeners can revert the change

98-09-28 Computer Science, University of

Karlstad19

Property ExampleProperty Example

A Property is defined by two accessor methods (setter and getter) allowing other classes or the Builder to manipulate their state

public void setTimeout (int t) {

timeout = t;

}

public int getTimeout () {

return timeout;

}

98-09-28 Computer Science, University of

Karlstad20

Bound Properties ExampleBound Properties Example

protected PropertyChangeSupport listeners =

new PropertyChangeSupport(this);

public void addPropertyChangeListener (PropertyChangeListener l) {

listeners.addPropertyChangeListener(l);

}

public void removePropertyChangeListener (PropertyChangeListener l) {

listeners.removePropertyChangeListener(l);

}

public synchronized void setValue(int v) {

Integer oldValue = new Integer(value);

value = v;

repaint();

listeners.firePropertyChange("value", oldValue, new Integer(value));

}

98-09-28 Computer Science, University of

Karlstad21

EventsEvents

Mechanism to plug beans together Event is a notification between source and

one or more listeners Listener registers itself in being interested

in events Beans may be plugged together by adapters

generated by builder tool

98-09-28 Computer Science, University of

Karlstad22

TimerEventTimerEvent

public class TimerEvent extends EventObject {

long time;

public TimerEvent(Object source, long time) {

super(source);

this.time = time;

}

public long getTime() {

return time;

}

}

public interface TimerListener extends EventListener {

public abstract void timedOut(TimerEvent te);

}

98-09-28 Computer Science, University of

Karlstad23

Handling ListenersHandling Listeners

protected Vector listeners = new Vector ();

public void addTimerListener (TimerListener listener) {

listeners.addElement (listener);

}

public void removeTimerListener (TimerListener listener) {

listeners.removeElement (listener);

}

protected void fireTimeOut() {

TimerEvent event = new TimerEvent (this, System.currentTimeMillis());

Vector listeners = (Vector) this.listeners.clone ();

for (int i = 0; i < listeners.size (); ++ i)

((TimerListener) listeners.elementAt (i)).timedOut (event);

}