JavaBeans

JavaBeans is heavily dependent upon reflection and thus documentation of the JavaBeans specification is always accompanied with a little note about reflection although the connection is not made clear all the time. In summary, reflection is useful for JavaBeans because it allows software components to be manipulated by builder tools (usually visual-based).

Java Beans are not required to inherit from any particular base class or interface although visible beans must inherit from java.awt.Component. The three most important features of a bean class are:

• Properties - named attributes (ie. member fields) associated with a bean that can be read or written using accessor methods, for example setXXX()/getXXX() methods.
• Methods - normal Java (public) methods which can be called by other components or from a scripting environment.
• Events - provide ways for notification using the AWT event model and registering listeners.

Bean programmers can explicitly provide an implementation of a BeanInfo class which describes the structure of a bean. That is, the BeanInfo class has methods, getEventDescrips()/getMethodDescrips()/... which return arrays of EventDescriptors, MethodDescriptors, etc. A PropertyDescriptor, for example, has methods which return the Method objects corresponding to the get and set methods of that property of the bean class.

By default, bean programmers do not have to provide BeanInfo classes, however. Instead, reflection is used to generate a BeanInfo class for a given bean. Reflection does things like looking at the fields of the bean classes to obtain a list of properties and then looking for the getXXX()/setXXX() methods, which it takes to be the get and set accessor methods of the property. There are a set of syntactic rules (called ``Design Patterns'' in the beans spec.) which are used to extract the features of the bean class. For example, if a java.awt.Color color field is found in the class, representing a ``color'' property, then there should also be public java.awt.Color getColor() and public void setColor(Color c) methods in the class.

Java beans are indeed useful to application programmers and the Reflection API is crucial in order for the entire JavaBeans API to be 100% pure Java (platform independence being one of the main goals of JavaBeans).