30-03-2011, 09:46 AM
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JavaBeans
An introduction to component-based development in general
Introduction to JavaBeans
– Java components
– client-side
Working with the BDK
The beans development life cycle
Writing simple and advanced beans
Software Components
All engineering discplines use components to build systems. In SE we rely on line-by-line SD.
We have class libraries
– create objects from class libraries
– we still need to write a large amount of code
– objects are not enough
They are like Integrated Circuit (IC) components
Over 20 years ago, hardware vendors learned how to package transistors
Hardware Engineers integrate ICs to make a board of chips
In SE, we are where hardware engineers were 20 years ago
We are building software routines
Java Components
Instead of worrying about rotuines, we can buy routines and use/reuse them in our applications (assemble applications)
JavaBeans -- portable, platform-independent component model
Java components are known as beans
A bean: a reusable software component that can be manipulated visually in a builder tool
JavaBeans vs. Class Libraries
Beans are appropriate for software components that can be visually manipulated
Class libraries are good for providing functionality that is useful to programmers, and doesn’t benefit from visual manipulation
JavaBeans Concepts
A component is a self-contained reusable software unit
Components expose their features (public methods and events) to builder tools
A builder tool maintains Beans in a palette or toolbox.
You can select a bean from the toolbox, drop it in a form, and modify its appearance and behavior.
Also, you can define its interaction with other beans
ALL this without a line of code.
JavaBean Characteristics
a public class with 0-argument constuctor
it has properties with accessory methods
it has events
it can customized
its state can be saved
it can be analyzed by a builder tool
Key Concepts
A builder tool discover a bean’s features by a process known as introspection.
– Adhering to specific rules (design pattern) when naming Bean features.
– Providing property, method, and event information with a related Bean Information class.
Properties (bean’s appearance and behavior characteristics) can be changed at design-time.
Properties can be customized at design-time. Customization can be done:
– using property editor
– using bean customizers
Events are used when beans want to intercommunicate
Persistence: for saving and restoring the state
Bean’s methods are regular Java methods.
Security Issues
JavaBeans are sbject to the standard Java security model
The security model has neither extended nor relaxed.
If a bean runs as an untrusted applet then it will be subject to applet security
If a bean runs as a stand-alone application then it will be treated as a normal Java application.
JavaBeans and Threads
Assume your beans will be running in a multi-threaded environment
It is your responsibility (the developer) to make sure that their beans behave properly under multi-threaded access
For simple beans, this can be handled by simply making all methods …...
Beans Development Kit (BDK)
To start the BeanBox:
– run.bat (Windows)
– run.sh (Unix)
BDK
ToolBox contains the beans available
BeanBox window is the form where you visually wire beans together.
Properties sheet: displays the properties for the Bean currently selected within the BeanBox window.
MyFirstBean
import java.awt.*;
import java.io.Serializable;
public class FirstBean extends Canvas implements Serializable {
public FirstBean() {
setSize(50,30);
setBackground(Color.blue);
}
}
First Bean
Compile: javac FirstBean.java
Create a manifest file:
manifest.txt
– Name: FirstBean.class
– Java-Bean: True
Create a jar file:
jar cfm FirstBean.jar mani.txt FirstBean.class
Using Beans in hand-written app
Use Beans.instantiate
Frame f;
f = new Frame("Testing Beans");
try {
ClassLoader cl = this.getClass().getClassLoader();
fb =(FirstBean)Beans.instantiate(cl,"FirstBean");
} catch(Exception e) {
e.printStackTrace();
}
f.add(fb);
Properties
Bean’s appearance and behavior -- changeable at design time.
They are private values
Can be accessed through getter and setter methods
getter and setter methods must follow some rules -- design patterns (documenting experience)
A builder tool can:
– discover a bean’s properties
– determine the properties’ read/write attribute
– locate an appropriate “property editor” for each type
– display the properties (in a sheet)
– alter the properties at design-time
Types of Properties
Simple
Index: multiple-value properties
Bound: provide event notification when value changes
Constrained: how proposed changes can be okayed or vetoed by other object
Simple Properties
When a builder tool introspect your bean it discovers two methods:
– public Color getColor()
– public void setColor(Color c)
The builder tool knows that a property named “Color” exists -- of type Color.
It tries to locate a property editor for that type to display the properties in a sheet.
Adding a Color property
– Create and initialize a private instance variable
private Color color = Color.blue;
– Write public getter & setter methods
public Color getColor() {
– return color;
}
public void setColor(Color c) {
– color = c;
– repaint();
}
Events “Introspection”
For a bean to be the source of an event, it must implement methods that add and remove listener objects for the type of the event:
– public void add<EventListenerType>(<EventListenerType> elt);
– same thing for remove
These methods help a source Bean know where to fire events.
Source Bean fires events at the listeners using method of those interfaces.
Example: if a source Bean register ActionListsener objects, it will fire events at those objects by calling the actionPerformed method on those listeners
Events “using BeanInfo”
Implementing the BeanInfo interface allows you to explicitly publish the events a Bean fires
BeanInfo interface
Question: how does a Bean exposes its features in a property sheet?
Answer: using java.beans.Introspector class (which uses Core Reflection API)
The discovery process is named “introspection”
OR you can associate a class that implements the BeanInfo with your bean
BeanInfo interface….
Why use BeanInfo then?
Using BeanInfo you can:
– Expose features that you want to expose
Bean Customization
The appearance and behavior of a bean can be customized at design time.
Two ways to customize a bean:
– using a property editor
each bean property has its own editor
a bean’s property is displayed in a property sheet
– using customizers
gives you complete GUI control over bean customization
used when property editors are not practical
Property Editors
A property editor is a user interface for editing a bean property. The property must have both, read/write accessor methods.
A property editor must implement the PropertyEditor interface.
PropertyEditorSupport does that already, so you can extend it.
If you provide a custom property editor class, then you must refer to this class by calling PropertyDescriptor.setPropertyEditorClass in a BeanInfo class.
Each bean may have a BeanInfo class which customizes how the bean is to appear. SimpleBeanInfo implements that interface
How to be a good bean?
JavaBeans are just the start of the Software Components industry.
This market is growing in both, quantity and quality.
To promote commercial quality java beans components and tools, we should strive to make our beans as reusable as possible.
Here are a few guidelines...
Creating beans
– Your bean class must provide a zero-argument constructor. So, objects can be created using Bean.instantiate();
– The bean must support persistence
implement Serializable or Externalizable