Chương 5 Java Beans

 Builder tools e.g JBuilder, Bean Builder know a bean's features by looking at its public methods, properties and events  called introspection  implemented using Java's reflection cla

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Chương 5: Java Beans

 Introduction

 The Bean-Writing process

 Using Bean to build an Application

 Naming Patterns for Bean Properties and Events

 Bean Property Types

 Adding Custom Bean Events

 Property Editors

 Going Beyond Naming Patterns

 Customizers

 a self-contained, reusable software unit

 it can be visually composed with other beans to

make bigger components, applets, applications, etc

1 Introduction (cont.)

 A bean is designed to expose its features:

 e.g public methods, properties (data), events

 Exposure allows beans to be used inside visual

building tools (e.g JBuilder)

 Exposure is based on coding rules that a programmer must follow when writing a bean

 these rules are mostly standard naming

conventions for methods

Object Oriented Programming 2 - Chapter 6: Java Beans

1 Introduction (cont.)

 Beans are persistent

 their data is automatically stored when the program using them finishes

 the data is restored when the program next starts

 persistence is implemented in JavaBeans by using Java's object serialization features

1 Introduction (cont.)

 Simple Java object must support the following five

features for it to become a JavaBean:

2 The Bean-Writing process

How Builder Tools Use Beans?

 Builder tools (e.g JBuilder, Bean Builder) know a

bean's features by looking at its public methods, properties and events

 called introspection

 implemented using Java's reflection classes

 A builder can also examine a bean's information object (BeanInfo), if the programmer has included one

Object Oriented Programming 2 - Chapter 6: Java Beans

2 The Bean-Writing process (cont.)

How Builder Tools Use Beans?

 A builder will have visual support for setting/reading a bean's properties, and for accessing its methods

 A builder will be able to 'link' beans based on events

 a source bean can send an event to a listener bean

2 The Bean-Writing process (cont.)

Using Java's Bean Builder

 Bean Builder Website:

 https://bean-builder.dev.java.net/

 there's a "readme" link, and a good tutorial

 Download Bean Builder v.1.0 beta

 from the "Documents and Files" link under "Project Tools"

 also at our Website on fivedots

Object Oriented Programming 2 - Chapter 6: Java Beans

2 The Bean-Writing process (cont.)

JavaBeans – Development Phases:

 Involves execution of the container application in

which the JavaBean is placed

2 The Bean-Writing process (cont.)

JavaBeans – Development Phases (cont.):

 Construction Phase involves creation of the following:

 class files

 mft file

 jar file

Object Oriented Programming 2 - Chapter 6: Java Beans

2 The Bean-Writing process (cont.)

JavaBeans – Development Phases (cont.):

 The Manifest file:

 Is used by the target application to recognize a

JavaBean

 Is saved with a mft extension

 Contains a list of all class files that make up a

JavaBean

2 The Bean-Writing process (cont.)

JavaBeans – Development Phases (cont.):

 The JAR file:

 Is similar to a zip file

 Is used to package a JavaBean for distribution

 Contains the manifest file and all other files such as the class files and picture files of the bean

 Has a jar extension

Object Oriented Programming 2 - Chapter 6: Java Beans

3 Using Bean to build an

Application

 Packaging Beans in JAR Files

 Composing Beans in a Builder Environment

3 Using Bean to build an

Application (cont.)

From Package to JAR

 A JAR file must contain a manifest

 it lists the contents of the JAR file

 a programmer writes the manifest in the

manifest.tmp file

 When the JAR file is being created, the jar utility

converts manifest.tmp into a MANIFEST.MF file stored

in the META_INF directory of the JAR file

Object Oriented Programming 2 - Chapter 6: Java Beans

3 Using Bean to build an

3 Using Bean to build an

Application (cont.)

 There may be many classes (and other resources) in

a JAR file

 each has its own Name and Java-Bean line

 Not all the resources in a JAR file need to be beans; if

a class is a bean then the Java-Bean line is set to true

(otherwise false)

 beans will appear in the Toolbox window

Object Oriented Programming 2 - Chapter 6: Java Beans

3 Using Bean to build an

Application (cont.)

Create the JAR file

c:> jar cfm LogoAnimator.jar manifest.tmp

myPackage\*.*

 Options

 c - create a JAR file

 f - indicates that next argument is a filename

 m - next argument is a manifest.tmp file

 used to create MANIFEST.MF

3 Using Bean to build an

Application (cont.)

List the JAR file

 To confirm files were stored properly:

Object Oriented Programming 2 - Chapter 6: Java Beans

3 Using Bean to build an

Application (cont.)

Run the JAR file

c:> java -jar LogoAnimator.jar

 run the application in the bean

 java uses the manifest file to execute main() of

Main-Class

 Or give java the name of the class to execute:

c:> java -cp myJar.jar

myPackage.myClass

3 Using Bean to build an

Application (cont.)

Adding Bean to the Bean Builder

 Load the JAR file into Bean Builder from the "File/Load Jar File" menu item.

 Then select it in the Design Mode window, then click

in the design panel

Object Oriented Programming 2 - Chapter 6: Java Beans

4 Naming Patterns for Bean

Properties and Events

5 Bean Property Types

 JavaBeans have 4 kinds of properties, which are

supported in different ways:

 simple properties

 indexed properties

 bound properties

 constrained properties

Object Oriented Programming 2 - Chapter 6: Java Beans

5 Bean Property Types (cont.)

A Simple Property = two methods

 A read/write bean property is implemented as a

set/get method pair:

 public void setPropertyName(

DataType value)

 public DataType getPropertyName()

 If the property is boolean, use isPropertyName()

instead of getPropertyName()

5 Bean Property Types (cont.)

Example

 A fileName property (a String) is represented by:

 public String getFileName();

 public void setFileName(String fn);

 A running property (a boolean) is:

 public boolean isRunning();

 public void setRunning(boolean b);

Object Oriented Programming 2 - Chapter 6: Java Beans

5 Bean Property Types (cont.)

Indexed Properties

 An indexed property is one that gets or sets an array

 The programmer must supply two pairs of get and set methods:

 one pair for setting/getting the entire array

 one pair for setting/getting an array element

5 Bean Property Types (cont.)

Object Oriented Programming 2 - Chapter 6: Java Beans

5 Bean Property Types (cont.)

5 Bean Property Types (cont.)

Object Oriented Programming 2 - Chapter 6: Java Beans

5 Bean Property Types (cont.)

 When a change occurs in the bean, an event listener will be called

 it must notify the PropertyChangeListeners by

calling firePropertyChange()

bean

<listener_method>() {

firePropertyChange( );

}

Property Change Listeners

listening objects

change

events

5 Bean Property Types (cont.)

Example SliderFieldPanel Bean

 A JSlider and a JTextField

 A change to either should affect the other, and any

listener objects should be notified

Object Oriented Programming 2 - Chapter 6: Java Beans

5 Bean Property Types (cont.)

Listening Beans Format

 Beans that are registered listeners for property

changes must implement the PropertyChangeListener interface

 The interface has one method:

 void propertyChange(PropertyChangeEvent e)

 it is called when the property value changes, and ewill contain the old and new values

5 Bean Property Types (cont.)

Listening Beans Format (cont.)

class Listener implements

PropertyChangeListener {

void propertyChange(PropertyChangeEvent ev) {

Object newVal = ev.getNewValue();

Object oldVal = ev.getOldValue();

}

Object Oriented Programming 2 - Chapter 6: Java Beans

5 Bean Property Types (cont.)

5 Bean Property Types (cont.)

 Example

0

from IntText bean

10

to IntText bean

range bean GUI

0

from

veto if from > to

range bean IntText bean change events

Object Oriented Programming 2 - Chapter 6: Java Beans

6 Updating Steps for a Bean

 1 Notify all change listeners of the wish to

change the property value

 2 If none of the listeners throws an exception,

then

 3 Notify all the listeners that the change has now been carried out

6 Updating in Code

 // method in bean with property

public void setValue(int v)

{

Integer newVal = new Integer(v);

Integer oldVal = new Integer(v-1);

try {

vetoSupport.fireVetoableChange(

"value", oldVal, newVal);

// if here then no exception thrown

Object Oriented Programming 2 - Chapter 6: Java Beans

7 Notes

 Constrained properties use bound property techniques

to do the final update e.g

 changeSupport.firePropertyChange( )

 vetoSupport is used in a similar way to changeSupport:

 vetoable listeners must be registered with it via

public add/remove methods

8 Vetoable Listener Code

 Beans that are registered vetoable listeners for

property changes must implement the VetoableChangeLi stener interface

 The interface has one method:

 void vetoableChange

(PropertyChangeEvent e)

 it is called when the property value wishes to

change, and e will contain the old and new values

Object Oriented Programming 2 - Chapter 6: Java Beans

class Listener implements

VetoableChangeListener {

void

vetoableChange(PropertyChangeEvent ev) {

Object newVal = ev.getNewValue(); Object oldVal = ev.getOldValue();

if ( /* some test */ ) throw new PropertyVetoException(

9 Bean Information

 A builder uses introspection to create lists of all

the methods, events, and properties in a bean

 This can be confusing, especially if the bean

inherits a class with many properties

 e.g LogoAnimator inherits JPanel

Object Oriented Programming 2 - Chapter 6: Java Beans

 It is possible to specify which

methods/events/properties will be visible for a bean inside the builder

 The bean must include an extra class which

implements the BeanInfo interface

 it lists the visible attributes of the bean

9 Bean Information (cont.)

 Use a BeanInfo class to restrict the display to 4

properties and 1 event

Any Bound Property?

 The builder must be told about any bound properties

in the bean:

currentValue.setBound(true);

Object Oriented Programming 2 - Chapter 6: Java Beans

Introspection Exceptions

 Both getPropertyDescriptors() and

getEventSetDescriptors()must have try/catch blocks which can catch IntrospectionExceptions

 these occur if the builder detects an error while examining the property or event information at run time

Specifying an Event

 Each EventSetDescriptor object has 4 values:

 event source class, event set name, event

listener interface class, listener method name

 e.g EventSetDescriptor changed =

new EventSetDescriptor(

beanClass, "propertyChange", java.beans.PropertyChangeListener.class,

"propertyChange");

Object Oriented Programming 2 - Chapter 6: Java Beans

Other Descriptor Classes

Using Descriptor Objects

 All the descriptor objects of a given kind (e.g

PropertyDescriptor objects) must be collected into

an array and be returned by the get method for th

eir class

 e.g getPropertyDescriptors(),

getEventSetDescriptors()

 a Customizer class allows several properties to

be edited at the same time

 The Bean context

 the environment in which the bean is running

 it can be examined at run time to find other

beans, so that beans can communicate

JavaBeans Persistence

 JavaBeans persistence uses JavaBeans

properties to save beans to a stream and to read

them back at a later time or in a different virtual

Example (cont.)

Object Oriented Programming 2 - Chapter 6: Java Beans

Example (cont.)

 To save an object to a stream, use an XMLEncoder:

XMLEncoder out = new XMLEncoder(new

FileOutputStream( .));

out.writeObject(frame);

out.close();

 To read it back, use an XMLDecoder:

XMLDecoder in = new XMLDecoder(new

FileInputStream( .));

JFrame newFrame = (JFrame) in.readObject();

in.close();