As mentioned in the IoC chapter, at the core of Spring IoC is the bean concept which defines the way in which an object is instantiated, assembled and managed by the Spring container. XML is the most popular way of describing beans configuration, though Spring itself can read from virtually any type of metadata that can be translated into Java code. Annotations, available in JDK 5+, constitute such a type as they allow source code components to provide additional metadata which can affect the runtime semantics, making them a great configuration candidate.