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Java in a Nutshell

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4.9 Java Beans

JavaBeans is a "software component model" for Java that has generated quite a lot of interest from many quarters. The JavaBeans API specification defines "beans" as follows: "A Java Bean is a reusable software component that can be manipulated visually in a builder tool." The java.beans package defines classes and interfaces designed to work with beans at three distinct levels, described below.

Much of the JavaBeans API is intended for use only by those few people who are writing interface builder tools that manipulate beans. The main thing that a builder tool needs to be able to do with beans is "introspect" on them--i.e., to determine what properties are exposed by a bean, what methods it exports, and what events it can generate. This is information that a builder tool must be able to display to the programmer who is using the tool. The JavaBeans API defines a set of naming conventions for the methods that a bean defines. If a bean follows these conventions, a builder tool can use the new Reflection API to determine what properties, methods, and events the bean supports. The Introspector class uses reflection to obtain information about a bean and presents it to the builder tool in the form of a BeanInfo object, which itself contains various FeatureDescriptor objects describing the properties, methods, and events of the bean.

At the second level, the JavaBeans API contains classes and interfaces intended for use by programmers who are creating beans for others to use. Chapter 10, Java Beans describes how to use the classes in java.beans in this manner. One of the surprising features of the JavaBeans API is that there is no Bean class that all beans must extend. A bean can be of any class; however, as we've seen, beans should follow certain naming conventions. The java.beans classes that a bean creator uses are generally auxiliary classes, used not by the bean, but by the builder tool that manipulates the bean. These auxiliary classes are shipped with a bean, and provide additional information or methods that a builder tool may use with the bean. These classes are not included in finished software built with the bean.

For example, one of the auxiliary classes a bean may define is a custom BeanInfo class to provide information to the builder tool that is not available through the Reflection API. This information might include a human-readable description of the bean's properties, methods, and events, for example. Or, if a bean does not follow the standard naming conventions, this custom BeanInfo class must also provide more basic information about the bean's properties, methods, and events.

Besides a BeanInfo class, complex beans may also provide a Customizer class and one or more PropertyEditor classes. A Customizer class is a kind of configuration tool or "wizard" for a bean. It is instantiated by the builder tool in order to guide the user through bean customization. A PropertyEditor class is used to allow the user to edit the value of bean properties of a particular class. Builder tools have built-in property editors for common types such as strings, colors, and fonts, but a bean that has properties of some unusual or custom type may want to provide a PropertyEditor subclass to allow the user to easily specify values for those properties.

The third level at which the JavaBeans API can be used is by programmers who are assembling an application using beans. Some programmers may do this through a builder tool, while others may do it "by hand", the old-fashioned way. Programmers using beans do not typically have to use the java.beans package. At this level, it is more a matter of reading the documentation for the particular beans being used and following those instructions. Nevertheless, a programmer using beans does need to be familiar with the event model used by beans, which is the same as the Java 1.1 event model for AWT. Also, programmers using beans "by hand" should be familiar with the naming conventions for bean properties, methods, and events, in order to more easily understand how a given bean can be used. In Java 1.1, all AWT components are beans and follow these naming conventions.

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