The Bean-Container Contract
The environment that surrounds the beans on the EJB server is often called the container. The container is more a concept than a physical construct. It acts as an intermediary between the bean and the EJB server. It manages the EJB objects and helps these constructs manage bean resources and provide services such as transactions, security, concurrency, and naming at runtime. The distinction between the container and the server is not clearly defined, but the EJB specification defines the component model in terms of the container's responsibilities, so we will follow that convention here.
Enterprise bean components interact with the EJB container through a well-defined component model. All EJB types register for various life cycle events that the EJB container emits. They register for these events by annotating a method within their bean class that is interested in the specific event. At runtime, the container invokes these annotated methods on the bean instance when relevant events occur. For example, after the container allocates and injects referenced services into an EJB instance, it will call a method annotated with @javax.annotation.PostConstruct on the EJB's bean class if one is provided. This call gives the bean instance an opportunity to do any additional initialization before the EJB services any request. Other callback methods can be used by the bean class in a similar fashion. EJB defines when these various callback methods are invoked and what can be done within their contexts.
In EJB 3.0, your code does not have to implement empty callback methods like it does in older versions of the EJB specification. You write code only for those events that you are interested in. Beans that implement callback methods usually access resources that aren't managed by the EJB system. Enterprise beans that wrap legacy systems often fall into this category.
javax.ejb.EJBContext
is an interface that is implemented by the container and is also part of the bean-container contract. Session beans use a subclass called javax.ejb.SessionContext. Message-driven beans use the subclass javax.ejb.MessageDriven-Context. These EJBContext types provide the bean with information about its environment: its container, the client using the enterprise bean, and the bean itself. The bean can use this information while processing requests from clients and callback methods from the container.
An enterprise bean's interface with the container also includes a JNDI namespace, called the environment-naming context, which the bean can use to look up the resources it needs (including other beans). The JNDI environment-naming context and the EJBContext (and its subclasses) are described in more detail in Chapters 11, 12, and 14.
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