To minimize the resources used by a bean, the classes used by bean editors are loaded only when the bean is being edited. They are not needed while the bean is running in an application and therefore not loaded. This information is kept in what's called a bean-info (see java.beans.BeanInfo).

Unless explicitly stated, null values or empty Strings are not valid parameters for the methods in this package. You may expect to see exceptions if these parameters are used.

Long-Term Persistence

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Note: The persistence scheme cannot automatically instantiate custom inner classes, such as you might use for event handlers. By using the java.beans.EventHandler class instead of inner classes for custom event handlers, you can avoid this problem.

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You read and write beans in XML format using the java.beans.XMLDecoder and java.beans.XMLEncoder classes, respectively. One notable feature of the persistence scheme is that reading in a bean requires no special knowledge of the bean.

Writing out a bean, on the other hand, sometimes requires special knowledge of the bean's type. If the bean's state can be expressed using only the no-argument constructor and public getter and setter methods for properties, no special knowledge is required. Otherwise, the bean requires a custom persistence delegate -- an object that is in charge of writing out beans of a particular type. All classes provided in the JDK that descend from java.awt.Component, as well as all their properties, automatically have persistence delegates.

If you need (or choose) to provide a persistence delegate for a bean, you can do so either by using a java.beans.DefaultPersistenceDelegate instance or by creating your own subclass of PersistenceDelegate. If the only reason a bean needs a persistence delegate is because you want to invoke the bean's constructor with property values as arguments, you can create the bean's persistence delegate with the one-argument DefaultPersistenceDelegate constructor. Otherwise, you need to implement your own persistence delegate, for which you're likely to need the following classes:

java.beans.PersistenceDelegate

The abstract class from which all persistence delegates descend. Your subclass should use its knowledge of the bean's type to provide whatever Statements and Expressions are necessary to create the bean and restore its state.

java.beans.Statement

Represents the invocation of a single method on an object. Includes a set of arguments to the method.

java.beans.Expression

A subclass of Statement used for methods that return a value.

Once you create a persistence delegate, you register it using the setPersistenceDelegate method of XMLEncoder.

Related Documentation

• JavaBeans, a trail in The Java Tutorial.
• Long-Term Persistence, an article in The Swing Connection.

Subpackages
java.beans.beancontext

Interfaces
AppletInitializer
BeanInfo
Customizer
DesignMode
ExceptionListener
PropertyChangeListener
PropertyEditor
VetoableChangeListener
Visibility

Exceptions
IntrospectionException
PropertyVetoException

Classes
BeanDescriptor
Beans
DefaultPersistenceDelegate
Encoder
EventHandler
EventSetDescriptor
Expression
FeatureDescriptor
IndexedPropertyChangeEvent