Bean

Bean is a reusable software component. Beans can be combined to create an application.

Individual Java Beans will vary in the functionality they support, but the typical unifying features that distinguish a Java Bean are:
• Support for “introspection” so that a builder tool can analyze how a bean works
• Support for “customization” so that when using an application builder a user can customize the appearance and behaviour of a bean.
• Support for “events” as a simple communication metaphor than can be used to connect up beans.
• Support for “properties”, both for customization and for programmatic use.
• Support for persistence, so that a bean can be customized in an application builder and then have its customized state saved away and reloaded later.

Beans v. Class Libraries
Not all useful software modules should necessarily turn into beans. Beans are appropriate for software components that can be visually manipulated and customized to achieve some effect. Class libraries are an appropriate way of providing functionality that is useful to programmers, but which doesn't benefit from visual manipulation. So for example it makes sense to provide the JDBC database access API as a class library rather than as a bean, because JDBC is essentially a programmatic API and not something that can be directly presented for visual manipulation. However it does make sense to write database access beans on top of JDBC. So for example you might write a “select” bean that at customization time helped a user to compose a select statement, and then when the application is run uses JDBC to run the select statement and display the results.