CHAPTER
37 Java Beans
This chapter provides an overview of Java Beans. Beans are important because they allow you to build complex systems from software components. These components may be provided by you or supplied by one or more different vendors. Java Beans defines an architecture that specifies how these building blocks can operate together.
To better understand the value of Beans, consider the following. Hardware designers have a wide variety of components that can be integrated together to construct a system. Resistors, capacitors, and inductors are examples of simple building blocks. Integrated circuits provide more advanced functionality. All of these different parts can be reused. It is not necessary or possible to rebuild these capabilities each time a new system is needed. Also, the same pieces can be used in different types of circuits. This is possible because the behavior of these components is understood and documented.
The software industry also sought the benefits of reusability and interoperability of a component-based approach. To realize these benefits, a component architecture is needed that allows programs to be assembled from software building blocks, perhaps provided by different vendors. It must also be possible for a designer to select a component, understand its capabilities, and incorporate it into an application. When a new version of a component becomes available, it should be easy to incorporate this functionality into existing code. Java Beans provides just such an architecture.
What Is a Java Bean?
A Java Bean is a software component that has been designed to be reusable in a variety of different environments. There is no restriction on the capability of a Bean. It may perform a simple function, such as obtaining an inventory value, or a complex function, such as forecasting the performance of a stock portfolio. A Bean may be visible to an end user. One example of this is a button on a graphical user interface. A Bean may also be invisible to a user. Software to decode a stream of multimedia information in real time is an example of this type of building block. Finally, a Bean may be designed to work autonomously on a user’s workstation or to work in cooperation with a set of other distributed components. Software to generate a pie chart from a set of data points is an example of a Bean that can execute locally. However, a Bean that provides real-time price information from a stock or commodities exchange would need to work in cooperation with other distributed software to obtain its data.
Advantages of Java Beans
The following list enumerates some of the benefits that Java Bean technology provides for a component developer:
• A Bean obtains all the benefits of Java’s “write-once, run-anywhere” paradigm.
• The properties, events, and methods of a Bean that are exposed to another application can be controlled.
• Auxiliary software can be provided to help configure a Bean. This software is only needed when the design-time parameters for that component are being set. It does not need to be included in the run-time environment.
• The state of a Bean can be saved in persistent storage and restored at a later time.
• A Bean may register to receive events from other objects and can generate events that are sent to other objects.
Introspection
At the core of Java Beans is introspection. This is the process of analyzing a Bean to determine its capabilities. This is an essential feature of the Java Beans API because it allows another application, such as a design tool, to obtain information about a component. Without introspection, the Java Beans technology could not operate.
There are two ways in which the developer of a Bean can indicate which of its properties, events, and methods should be exposed. With the first method, simple naming conventions are used. These allow the introspection mechanisms to infer information about a Bean. In the second way, an additional class that extends the BeanInfo interface is provided that explicitly supplies this information. Both approaches are examined here.
Design Patterns for Properties
A property is a subset of a Bean’s state. The values assigned to the properties determine the behavior and appearance of that component. A property is set through a setter method. A property is obtained by a getter method. There are two types of properties: simple and indexed.
Simple Properties
A simple property has a single value. It can be identified by the following design patterns, where N is the name of the property and T is its type:
public T getN( )
public void setN(T arg)
A read/write property has both of these methods to access its values. A read-only property has only a get method. A write-only property has only a set method.
Here are three read/write simple properties along with their getter and setter methods:
NOTE For a boolean property, a method of the form isPropertyName( ) can also be used as an accessor.
Indexed Properties
An indexed property consists of multiple values. It can be identified by the following design patterns, where N is the name of the property and T is its type:
public T getN(int index);
public void setN(int index, T value);
public T[ ] getN( );
public void setN(T values[ ]);
Here is an indexed property called data along with its getter and setter methods:
Design Patterns for Events
Beans use the delegation event model that was discussed earlier in this book. Beans can generate events and send them to other objects. These can be identified by the following design patterns, where T is the type of the event:
public void addTListener(TListener
eventListener)
public void addTListener(TListener eventListener) throws java.util.TooManyListenersException
public void removeTListener(TListener eventListener)
These methods are used to add or remove a listener for the specified event. The version of addTListener( ) that does not throw an exception can be used to multicast an event, which means that more than one listener can register for the event notification. The version that throws TooManyListenersException unicasts the event, which means that the number of listeners can be restricted to one. In either case, removeTListener( ) is used to remove the listener. For example, assuming an event interface type called TemperatureListener, a Bean that monitors temperature might supply the following methods:
Methods and Design Patterns
Design patterns are not used for naming nonproperty methods. The introspection mechanism finds all of the public methods of a Bean. Protected and private methods are not presented.
Using the BeanInfo Interface
As the preceding discussion shows, design patterns implicitly determine what information is available to the user of a Bean. The BeanInfo interface enables you to explicitly control what information is available. The BeanInfo interface defines several methods, including these:
PropertyDescriptor[ ] getPropertyDescriptors( )
EventSetDescriptor[ ] getEventSetDescriptors( )
MethodDescriptor[ ] getMethodDescriptors( )
They return arrays of objects that provide information about the properties, events, and methods of a Bean. The classes PropertyDescriptor, EventSetDescriptor, and MethodDescriptor are defined within the java.beans package, and they describe the indicated elements. By implementing these methods, a developer can designate exactly what is presented to a user, bypassing introspection based on design patterns.
When creating a class that implements BeanInfo, you must call that class bnameBeanInfo, where bname is the name of the Bean. For example, if the Bean is called MyBean, then the information class must be called MyBeanBeanInfo.
To simplify the use of BeanInfo, JavaBeans supplies the SimpleBeanInfo class. It provides default implementations of the BeanInfo interface, including the three methods just shown. You can extend this class and override one or more of the methods to explicitly control what aspects of a Bean are exposed. If you don’t override a method, then design-pattern introspection will be used. For example, if you don’t override getPropertyDescriptors( ), then design patterns are used to discover a Bean’s properties. You will see SimpleBeanInfo in action later in this chapter.
Bound and Constrained Properties
A Bean that has a bound property generates an event when the property is changed. The event is of type PropertyChangeEvent and is sent to objects that previously registered an interest in receiving such notifications. A class that handles this event must implement the PropertyChangeListener interface.
A Bean that has a constrained property generates an event when an attempt is made to change its value. It also generates an event of type PropertyChangeEvent. It too is sent to objects that previously registered an interest in receiving such notifications. However, those other objects have the ability to veto the proposed change by throwing a PropertyVetoException. This capability allows a Bean to operate differently according to its run-time environment. A class that handles this event must implement the VetoableChangeListener interface.
Persistence
Persistence is the ability to save the current state of a Bean, including the values of a Bean’s properties and instance variables, to nonvolatile storage and to retrieve them at a later time. The object serialization capabilities provided by the Java class libraries are used to provide persistence for Beans.
The easiest way to serialize a Bean is to have it implement the java.io.Serializable interface, which is simply a marker interface. Implementing java.io.Serializable makes serialization automatic. Your Bean need take no other action. Automatic serialization can also be inherited. Therefore, if any superclass of a Bean implements java.io.Serializable, then automatic serialization is obtained.
When using automatic serialization, you can selectively prevent a field from being saved through the use of the transient keyword. Thus, data members of a Bean specified as transient will not be serialized.
If a Bean does not implement java.io.Serializable, you must provide serialization yourself, such as by implementing java.io.Externalizable. Otherwise, containers cannot save the configuration of your component.
Customizers
A Bean developer can provide a customizer that helps another developer configure the Bean. A customizer can provide a step-by-step guide through the process that must be followed to use the component in a specific context. Online documentation can also be provided. A Bean developer has great flexibility to develop a customizer that can differentiate his or her product in the marketplace.
Table 37-1 The Interfaces in java.beans
The Java Beans API
The Java Beans functionality is provided by a set of classes and interfaces in the java.beans package. Beginning with JDK 9, this package is in the java.desktop module. This section provides a brief overview of its contents. Table 37-1 lists the interfaces in java.beans and provides a brief description of their functionality. Table 37-2 lists the classes in java.beans.
Table 37-2 The Classes in java.beans
Although it is beyond the scope of this chapter to discuss all of the classes, four are of particular interest: Introspector, PropertyDescriptor, EventSetDescriptor, and MethodDescriptor. Each is briefly examined here.
Introspector
The Introspector class provides several static methods that support introspection. Of most interest is getBeanInfo( ). This method returns a BeanInfo object that can be used to obtain information about the Bean. The getBeanInfo( ) method has several forms, including the one shown here:
static BeanInfo getBeanInfo(Class<?> bean) throws IntrospectionException
The returned object contains information about the Bean specified by bean.
PropertyDescriptor
The PropertyDescriptor class describes the characteristics of a Bean property. It supports several methods that manage and describe properties. For example, you can determine if a property is bound by calling isBound( ). To determine if a property is constrained, call isConstrained( ). You can obtain the name of a property by calling getName( ).
EventSetDescriptor
The EventSetDescriptor class represents a set of Bean events. It supports several methods that obtain the methods that a Bean uses to add or remove event listeners, and to otherwise manage events. For example, to obtain the method used to add listeners, call getAddListenerMethod( ). To obtain the method used to remove listeners, call getRemoveListenerMethod( ). To obtain the type of a listener, call getListenerType( ). You can obtain the name of an event set by calling getName( ).
MethodDescriptor
The MethodDescriptor class represents a Bean method. To obtain the name of the method, call getName( ). You can obtain information about the method by calling getMethod( ), shown here:
Method getMethod( )
An object of type Method that describes the method is returned.
A Bean Example
This chapter concludes with an example that illustrates various aspects of Bean programming, including introspection and using a BeanInfo class. It also makes use of the Introspector, PropertyDescriptor, and EventSetDescriptor classes. The example uses three classes. The first is a Bean called Colors, shown here:
The Colors Bean displays a colored object within a frame. The color of the component is determined by the private Color variable color, and its shape is determined by the private boolean variable rectangular. The constructor defines an anonymous inner class that extends MouseAdapter and overrides its mousePressed( ) method. The change( ) method is invoked in response to mouse presses. It selects a random color and then repaints the component. The getRectangular( ) and setRectangular( ) methods provide access to the one property of this Bean. The change( ) method calls randomColor( ) to choose a color and then calls repaint( ) to make the change visible. Notice that the paint( ) method uses the rectangular and color variables to determine how to present the Bean.
The next class is ColorsBeanInfo. It is a subclass of SimpleBeanInfo that provides explicit information about Colors. It overrides getPropertyDescriptors( ) in order to designate which properties are presented to a Bean user. In this case, the only property exposed is rectangular. The method creates and returns a PropertyDescriptor object for the rectangular property. The PropertyDescriptor constructor that is used is shown here:
PropertyDescriptor(String property, Class<?> beanCls) throws IntrospectionException
Here, the first argument is the name of the property, and the second argument is the class of the Bean.
The final class is called IntrospectorDemo. It uses introspection to display the properties and events that are available within the Colors Bean.
The output from this program is the following:
Notice two things in the output. First, because ColorsBeanInfo overrides getPropertyDescriptors( ) such that the only property returned is rectangular, only the rectangular property is displayed. However, because getEventSetDescriptors( ) is not overridden by ColorsBeanInfo, design-pattern introspection is used, and all events are found, including those in Colors’ superclass, Canvas. Remember, if you don’t override one of the “get” methods defined by SimpleBeanInfo, then the default, design-pattern introspection is used. To observe the difference that ColorsBeanInfo makes, erase its class file and then run IntrospectorDemo again. This time it will report more properties.