# Package java.beans

package java.beans
Contains classes related to developing beans -- components based on the JavaBeans architecture. A few of the classes are used by beans while they run in an application. For example, the event classes are used by beans that fire property and vetoable change events (see `PropertyChangeEvent`). However, most of the classes in this package are meant to be used by a bean editor (that is, a development environment for customizing and putting together beans to create an application). In particular, these classes help the bean editor create a user interface that the user can use to customize the bean. For example, a bean may contain a property of a special type that a bean editor may not know how to handle. By using the `PropertyEditor` interface, a bean developer can provide an editor for this special type.

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 `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

As of v1.4, the `java.beans` package provides support for long-term persistence -- reading and writing a bean as a textual representation of its property values. The property values are treated as beans, and are recursively read or written to capture their publicly available state. This approach is suitable for long-term storage because it relies only on public API, rather than the likely-to-change private implementation.

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

You read and write beans in XML format using the `XMLDecoder` and `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 `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:

`PersistenceDelegate`
  The abstract class from which all persistence delegates descend. Your subclass should use its knowledge of the bean's type to provide whatever `Statement`s and `Expression`s are necessary to create the bean and restore its state.
`Statement`
  Represents the invocation of a single method on an object. Includes a set of arguments to the method.
`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

For overview, architecture, and tutorial documentation, please see:

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

Since:
  1.1

Related Packages
Package
Description
java.beans.beancontext
Provides classes and interfaces relating to bean context.
Class
Description
AppletInitializer
Deprecated, for removal: This API element is subject to removal in a future version.
The Applet API is deprecated.
BeanDescriptor
A BeanDescriptor provides global information about a "bean", including its Java class, its displayName, etc.
BeanInfo
Use the `BeanInfo` interface to create a `BeanInfo` class and provide explicit information about the methods, properties, events, and other features of your beans.
BeanProperty
An annotation used to specify some property-related information for the automatically generated `BeanInfo` classes.
Beans
This class provides some general purpose beans control methods.
ConstructorProperties
An annotation on a constructor that shows how the parameters of that constructor correspond to the constructed object's getter methods.
Customizer
A customizer class provides a complete custom GUI for customizing a target Java Bean.
DefaultPersistenceDelegate
The `DefaultPersistenceDelegate` is a concrete implementation of the abstract `PersistenceDelegate` class and is the delegate used by default for classes about which no information is available.
DesignMode
This interface is intended to be implemented by, or delegated from, instances of java.beans.beancontext.BeanContext, in order to propagate to its nested hierarchy of java.beans.beancontext.BeanContextChild instances, the current "designTime" property.
Encoder
An `Encoder` is a class which can be used to create files or streams that encode the state of a collection of JavaBeans in terms of their public APIs.
EventHandler
The `EventHandler` class provides support for dynamically generating event listeners whose methods execute a simple statement involving an incoming event object and a target object.
EventSetDescriptor
An EventSetDescriptor describes a group of events that a given Java bean fires.
ExceptionListener
An ExceptionListener is notified of internal exceptions.
Expression
An `Expression` object represents a primitive expression in which a single method is applied to a target and a set of arguments to return a result - as in `"a.getFoo()"`.
FeatureDescriptor
The FeatureDescriptor class is the common baseclass for PropertyDescriptor, EventSetDescriptor, and MethodDescriptor, etc.
IndexedPropertyChangeEvent
An "IndexedPropertyChange" event gets delivered whenever a component that conforms to the JavaBeans specification (a "bean") changes a bound indexed property.
IndexedPropertyDescriptor
An IndexedPropertyDescriptor describes a property that acts like an array and has an indexed read and/or indexed write method to access specific elements of the array.
IntrospectionException
Thrown when an exception happens during Introspection.
Introspector
The Introspector class provides a standard way for tools to learn about the properties, events, and methods supported by a target Java Bean.
JavaBean
An annotation used to specify some class-related information for the automatically generated `BeanInfo` classes.
MethodDescriptor
A MethodDescriptor describes a particular method that a Java Bean supports for external access from other components.
ParameterDescriptor
The ParameterDescriptor class allows bean implementors to provide additional information on each of their parameters, beyond the low level type information provided by the java.lang.reflect.Method class.
PersistenceDelegate
The PersistenceDelegate class takes the responsibility for expressing the state of an instance of a given class in terms of the methods in the class's public API.
PropertyChangeEvent
A "PropertyChange" event gets delivered whenever a bean changes a "bound" or "constrained" property.
PropertyChangeListener
A "PropertyChange" event gets fired whenever a bean changes a "bound" property.
PropertyChangeListenerProxy
A class which extends the `EventListenerProxy` specifically for adding a `PropertyChangeListener` with a "bound" property.
PropertyChangeSupport
This is a utility class that can be used by beans that support bound properties.
PropertyDescriptor
A PropertyDescriptor describes one property that a Java Bean exports via a pair of accessor methods.
PropertyEditor
A PropertyEditor class provides support for GUIs that want to allow users to edit a property value of a given type.
PropertyEditorManager
The PropertyEditorManager can be used to locate a property editor for any given type name.
PropertyEditorSupport
This is a support class to help build property editors.
PropertyVetoException
A PropertyVetoException is thrown when a proposed change to a property represents an unacceptable value.
SimpleBeanInfo
This is a support class to make it easier for people to provide BeanInfo classes.
Statement
A `Statement` object represents a primitive statement in which a single method is applied to a target and a set of arguments - as in `"a.setFoo(b)"`.
Transient
Indicates that an attribute called "transient" should be declared with the given `value` when the `Introspector` constructs a `PropertyDescriptor` or `EventSetDescriptor` classes associated with the annotated code element.
VetoableChangeListener
A VetoableChange event gets fired whenever a bean changes a "constrained" property.
VetoableChangeListenerProxy
A class which extends the `EventListenerProxy` specifically for adding a `VetoableChangeListener` with a "constrained" property.
VetoableChangeSupport
This is a utility class that can be used by beans that support constrained properties.
Visibility
Under some circumstances a bean may be run on servers where a GUI is not available.
XMLDecoder
The `XMLDecoder` class is used to read XML documents created using the `XMLEncoder` and is used just like the `ObjectInputStream`.
XMLEncoder
The `XMLEncoder` class is a complementary alternative to the `ObjectOutputStream` and can used to generate a textual representation of a JavaBean in the same way that the `ObjectOutputStream` can be used to create binary representation of `Serializable` objects.