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JavaBeans: Introspection and
Customization
Umer Farooq
January 30, 2002
CS6704: Design Patterns & Component Frameworks
Deploying Beans to the Enterprise
 How do we treat beans as independent software components?
 How do you examine a bean and expose its features:
properties, events, and methods?
 How do you create global international beans?
Reflection and Introspection
 A Bean has to reveal it’s properties, methods and events to
the builder tool.
 Reflection is the ability to obtain information about the
fields,constructors and methods of any class.
 Introspection is the ability to obtain information about the
properties, events and methods of a Bean.
Reflection
 Is Java a true Object-Oriented language?
 For reflection, all data types must be objects
 Classes and interfaces exist for reflection:


class Array, Class, Constructor, Field, Method,
Modifier
interface Member
Introspection
 class Introspector uses core reflection API to discover a




Bean’s methods
All java.beans.Introspector class methods are static
Call Introspector.getBeanInfo(…) method to get the
BeanInfo object
Explicitly expose features
Rely on BeanInfo to expose some Bean features while
relying on low-level reflection to expose others.
BeanInfo interface
 BeanInfo defines methods that return descriptors for each
property, method, or event that you want exposed:
 PropertyDescriptor[] getPropertyDescriptors();
 MethodDescriptor[] getMethodDescriptors();
 EventSetDescriptor[] getEventSetDescriptors();
Creating a BeanInfo class (1)
1. Name your BeanInfo class
Target class: ExplicitButton
Bean information class: ExplicitButtonBeanInfo
2. Subclass SimpleBeanInfo
public class ExplicitButtonBeanInfo extends
SimpleBeanInfo
Creating a BeanInfo class (2)
3. Override methods
public PropertyDescriptor[] getPropertyDescriptors() {
try {
PropertyDescriptor background = new PropertyDescriptor("background",
beanClass);
PropertyDescriptor foreground = new
PropertyDescriptor("foreground", beanClass);
PropertyDescriptor font = new
PropertyDescriptor("font", beanClass);
PropertyDescriptor label = new PropertyDescriptor("label", beanClass);
}
background.setBound(true);
foreground.setBound(true);
font.setBound(true);
label.setBound(true);
PropertyDescriptor rv[] = {background, foreground, font, label};
return rv;
} catch (IntrospectionException e) {
throw new Error(e.toString());
}
Creating a BeanInfo class (3)
4. Specify the target Bean class
public BeanDescriptor getBeanDescriptor() {
return new BeanDescriptor(beanClass);
}
...
private final static Class beanClass = ExplicitButton.class
Bean Customization
 A Bean's appearance and behavior can be customized at
design time:
1. By using a property editor:
Property sheet
Associated with a property
2. By using customizers:
Complete GUI control
Associated with a bean
Property Editors
 Explicit association via a BeanInfo object using
the method setPropertyEditorClass(…)
 Explicit registration via
java.Beans.PropertyEditorManager.registerE
ditor
 Naming convention
Customizers
 Extend java.awt.Component or one of its subclasses.
 Implement the java.beans.Customizer interface This
means implementing methods to register
PropertyChangeListener objects, and firing property
change events at those listeners when a change to the target
Bean has occurred.
 Implement a default constructor.
 Associate the customizer with its target class via
BeanInfo.getBeanDescriptor.
International Beans
 Programs are written with one language and culture
 What about localization?
 Locale is a geographic, political, or culturally cohesive
region sharing the same language and customs
 Localizing objects:
 Use Java’s locale class
 Encapsulate locale-specific data in resource bundles
 Locale localeUS = new Locale(“en”, “US”);
 public Locale getLocale( )
References

http://java.sun.com/docs/books/tutorial/javabeans/

JavaBeans Unleased, Dr. Don Doherty, Rick Leinecker

ftp://ftp.javasoft.com/docs/beans/beans.101.pdf
JavaBeans 1.01 Specification

Beginning Java 2, Ivor Horton
Discussion
 When should the reflection API not be used?
 When to use BeanInfo and when to use low-level
Reflection API?
 What kind of applications might want to use the Reflection
API?