API Enhancements to the JavaBeans Component API in v1.4

This document describes changes to the java.beans package that were introduced in Java SE 1.4:

New API for Long-Term Persistence
Other API Additions
Changes to the Introspector
Garbage Collection and Custom Attributes

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 `Statement`s and `Expression`s 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:

The PropertyChangeSupport class now contains a no-argument method to get all the registered property change listeners: getPropertyChangeListeners.
Similarly, the VetoableChangeSupport class now contains a no-argument method to get all the registered vetoable change listeners: getVetoableChangeListeners.

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:

FeatureDescriptors are now shared on a per-BeanInfo basis, rather than copied when the BeanInfo is returned. This change improves the performance of the getBeanInfo method. It also means that when an attribute/value in a FeatureDescriptor changes, that change is persistent among getBeanInfo invocations as long as the BeanInfo is not garbage collected. (Garbage collection of BeanInfos is described in Garbage Collection and Custom Attributes.)
The getBeanInfo methods no longer create a copy of the requested BeanInfo. Instead, they cache the BeanInfo and return it. If necessary, you can get the old behavior by storing a reference to the returned BeanInfo and using the flushFromCaches method to flush the bean's class from the introspector's cache.
If the instantiate method can't instantiate a particular class using the sibling or bootstrap class loader, it now tries to load the class using the current thread's class loader.

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);
}