JavaBeans: Introspection and Customization Umer Farooq CS6704: Design Patterns & Component FrameworksJanuary 30, 2002

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

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 (2)

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

 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 Bean Customization

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   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

 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? Discussion