API Enhancements to the JavaBeansTM Component API in v1.4

This document describes changes to the java.beans package that were introduced in version 1.4 of the Java 2 Platform, Standard Edition:

New API for Long-Term Persistence

To support long-term persistence, the following classes were added:

Class Description
Statement An object that represents a method call, possibly with arguments, upon an object. For example: a.setFoo(b).
     Expression A statement that returns a result. For example: a.getFoo().
XMLDecoder Reads XML documents that were created using XMLEncoder.
Encoder Uses persistence delegates to break an object graph down into a series of Statements and Expressions that can be used to recreate it.
     XMLEncoder An Encoder that produces statements and expressions in an XML encoding.
PersistenceDelegate An abstract class that defines objects that can express the state of another object using the public methods of that object's class.
    DefaultPersistenceDelegate The persistence delegate used, by default, for beans.

See JavaBeans Component API for links to where you can find more information about long-term persistence.

Other API Additions

The following classes were also added in v1.4:

Class Description
EventHandler Provides support for dynamically generating event listeners that have a small footprint and can be saved automatically by the persistence scheme.
ExceptionListener Defines a listener to be notified when a exception was thrown but then recovered from. You can register an exception listener on an XMLEncoder or XMLDecoder object to be notified when the object encounters a recoverable problem while writing or reading a bean.
PropertyChangeListenerProxy A proxy that implements PropertyChangeListener and serves to group another PropertyChangeListener (the real event handler) with a specific property; the proxy forwards property change events to the real event handler.
VetoableChangeListenerProxy A proxy that implements VetoableChangeListener and serves to group another VetoableChangeListener (the real event handler) with a specific constrained property; the proxy forwards vetoable property change events to the real event handler.

The following classes have additional methods:


Changes to the Introspector

The Introspector class has been reimplemented, and its performance has improved. The new implementation has caused the following changes in the behavior of the introspector:

Garbage Collection and Custom Attributes

As of v 1.4, a BeanInfo can be garbage collected when no direct references to it exist and the system is low on memory. This is implemented in the Introspector by wrapping BeanInfos in SoftReferences.

The possibility of garbage collection affects how you store custom attributes inside BeanInfos. If a particular BeanInfo is garbage collected, then the next time you invoke Introspector.getBeanInfo to get the BeanInfo, the returned object won't contain any custom attributes.

To avoid this problem, if you store custom attributes inside a BeanInfo you must ensure that the BeanInfo is correctly initialized with those attributes every time you retrieve the BeanInfo. The following is an example of initializing a BeanInfo class with a custom property in the bean descriptor. The code would be similar for a custom attribute in a property descriptor, method descriptor, or event set descriptor.

BeanInfo beanInfo = getBeanInfo(SomeBean.class);
BeanDescriptor beanDescriptor = beanInfo.getBeanDescriptor();

/*
 * Before using the BeanInfo, check to see if our custom
 * property has been initialized.  (Even if we initialized
 * it before, if the BeanInfo has been garbage collected,
 * then we need to initialize it again.)  Since our custom
 * property's value could be null, we define another property
 * to tell us if the custom property is initialized.
 */
if (beanDescriptor.getValue("myProperty_init") == null) {
    beanDescriptor.setValue("myProperty", someValue);
    beanDescriptor.setValue("myProperty_init", Boolean.TRUE);
}

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