Component Based Technology

Presentation on:Java Bean Component Model

By:

Gitansh chopra – 2K11/SE/30

Prashant Thakur – 2K11/SE/50

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)

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

Interface of a component

• This model defines four types of port: – methods, – properties, – event sources and – event sinks called listeners.

Read-only property

Write-only property

Property

Method

Event source

Event sink (listener)

Bounded property

v Vetoable property

ro

wo

1 Unicast event source

Ports

Implementations of a Bean Components

Object

Method

Method call

Binding

A simple implementation A more complex implementation

Components Assembly• Assembly is one of the key features of Java

Bean though no not specific solution is provided.– Different ways of assembling components are

supplied.

Component-based assembly Heterogeneous assembly

Advantages 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)

JavaBeanComponent

BeanInfo

EventsMethods

Properties

JARCustomizer

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

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

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

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.

Manifest file

Manifest.tmp

Name: SimpleBean.class

Java-Bean: True

...

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

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

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);}}

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]

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

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

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

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

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

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

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);}

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)

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

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.

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

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);}

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); }

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

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

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