javabeans-4

8/7/2019 Javabeans-4

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Object is an runtime instance of a class.

Software component is an reusable

object that can be plugged into anysoftware application.

Ex:

Spell check utility, calculator, find and

replace utility. Reusable software components can be

simple, such as buttons, text fields, listboxes, scrollbars, and dialogs.


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JavaBeans is a portable, platform-independent component model written

in theJava programming language.

Java Bean is a reusable softwarecomponent that can be used in varietyof different environment and it can be

manipulated visually in a builder tool. With the JavaBeans API you can create

reusable, platform-independentcomponents.


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Beans are dynamic in that they can bechanged or customized through thedesign mode of a builder tool.

It posses properties, methods, and

events.

These can be developed using a toolcalled Bean Developer Kit(BDK)


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A Bean obtains all the benefits of Java's "write-once, run-anywhere" paradigm.

The properties, events, and methods of a Beanthat are exposed to an application

builder tool can be controlled. A Bean may be designed to operate correctly

in different locales, which makes ituseful in global markets.

Auxiliary software can be provided to help a

person configure a Bean. This software isonly needed when the design-time parametersfor that component are being set. Itdoes not need to be included in the run-timeenvironment.


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The configuration settings of a Bean canbe saved in persistent storage and

restored at a later time. A Bean may register to receive events

from other objects and can generateevents that are sent to other objects.


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Support forintrospection

x so that a builder tool can analyze how a bean works

Support forcustomization

x so that when using an application builder a user cancustomize the appearance and behavior of a bean

Support foreventsx as a simple communication metaphor than can be used

to connect up beans

Support forproperties

x both for customization and for programmatic use

Support forpersistence

x so that a bean can be customized in an applicationbuilder and then have its customized state saved awayand reloaded later


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A mechanism that allows the builder toolto analyze a bean.

Two ways to analyze a bean: low-level reflection APIs.

vendor provided explicit information(Customization).

Application builder will provide defaultBeanInfo class BeanInfo.


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package acme.beans;

import java.awt.*;

import java.io.Serializable;

public class Acme04Bean extends Canvas implements Serializable {

public Acme04Bean() {

resize(60,40);

this.label="Bean";

setFont(new Font("Dialog", Font.PLAIN, 12));

}


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public void paint(Graphics g) {

g.setColor(beanColor);

g.setColor(Color.blue);

int width = size().width;int height = size().height;

FontMetrics fm = g.getFontMetrics();

g.drawString(label, (width - fm.stringWidth(label)) / 2,

(height + fm.getMaxAscent() - fm.getMaxDescent()) / 2);

}

public Color getColor() {

return beanColor;

}


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Purpose:

To use existing data formats and plug into OLE orOpenDoc documents (e.g., Excel doc inside a Word doc)

To be trivial for the common case of a tiny Bean (bysaving its internal state)

Solutions

Externalization: provides a Bean with full control over theresulting data layout.

Serialization: provides an automatic way of storing out andrestoring the internal state of a collection of Java objects

All bean must support either Serialization or Externalization

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