PropertyChangeListener

PropertyChangeListener is an interface in the JavaBeans framework that enables one object to listen for changes to bound properties of another bean. Implementations receive notifications when a bound property changes, allowing components to react to state updates in a decoupled way.

The interface defines a single method: propertyChange(PropertyChangeEvent evt). The PropertyChangeEvent object carries information about the change, including the source bean, the name of the changed property, the old value, and the new value. This event is delivered to all registered listeners when a bean fires a property change.

Beans that expose bound properties provide methods to manage listeners, typically addPropertyChangeListener and removePropertyChangeListener, with optional overloads that accept a specific property name. When a bound property changes, the bean should fire a PropertyChangeEvent, commonly via a helper like PropertyChangeSupport, which handles listener management and event dispatch.

A typical usage pattern involves creating or obtaining a bean that supports bound properties, registering a PropertyChangeListener, and implementing the propertyChange method to respond to updates. Conversely, the bean’s setter methods update the property value and then invoke firePropertyChange to notify listeners.

PropertyChangeSupport is a utility class in java.beans that simplifies listener management, offering addPropertyChangeListener, removePropertyChangeListener, and firePropertyChange methods. This pattern is widely used in Java Swing models and other JavaBeans-based architectures to enable GUI components and other collaborators to react to state changes.

Notes: PropertyChangeListener is part of the JavaBeans event model. It is distinct from Constrained properties (which use VetoableChangeListener) and is meant for observing changes rather than enforcing constraints.