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 EJB homes and helps these constructs to 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. The EntityBean, SessionBean , and MessageDrivenBean interfaces provide callback methods that notify the bean class of life-cycle events. At runtime, the container invokes these methods on the bean instance when relevant events occur. For example, when the container is about to write an entity bean instance’s state to the database, it first calls the bean instance’s ejbStore( ) method. This call gives the bean instance an opportunity to do cleanup on its state before it’s written to the database. The ejbLoad( ) method is called just after the bean’s fields are populated from the database, providing the bean developer with an opportunity to manage the bean’s state before the first business method is called.^[9] 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.

While the bean interfaces require implementations of all the callback methods, those implementations don’t have to be meaningful. The method body of any or all of the callback methods can be left empty, and often is. 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 implemented by the container and is also part of the bean-container contract. Entity beans use a subclass of javax.ejb.EJBContext called javax.ejb.EntityContext. Session beans use a subclass called javax.ejb.SessionContext. Message-driven beans use the subclass javax.ejb.MessageDrivenContext . 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 Chapter 10, Chapter 11, and Chapter 12.