EJB containers
An Enterprise JavaBeans (EJB) container provides a run-time environment for enterprise beans within the appserver. The container handles all aspects of an enterprise bean's operation within the application server and acts as an intermediary between the user-written business logic within the bean and the rest of the appserver environment.
One or more EJB modules, each containing one or more enterprise beans, can be installed in a single container.
The EJB container provides many services to the enterprise bean, including the following:
- Beginning, committing, and rolling back transactions as necessary.
- Maintaining pools of enterprise bean instances ready for incoming requests and moving these instances between the inactive pools and an active state, ensuring that threading conditions within the bean are satisfied.
- Most importantly, automatically synchronizing data in an entity bean's instance variables with corresponding data items stored in persistent storage.
By dynamically maintaining a set of active bean instances and synchronizing bean state with persistent storage when beans are moved into and out of active state, the container makes it possible for an application to manage many more bean instances than could otherwise simultaneously be held in the appserver's memory. In this respect, an EJB container provides services similar to virtual memory within an operating system.
Between transactions, the state of an entity bean can be cached. The EJB container supports option A, B, and C caching.
- With option A caching, the appserver assumes that the entity bean is used within a single container. Clients of that bean must direct their requests to the bean instance within that container. The entity bean has exclusive access to the underlying database, which means that the bean cannot be cloned or participate in workload management if option A caching is used.
- With option B caching, the entity bean remains active in the cache throughout the transaction but is reloaded at the start of each method call.
- With option C caching (the default), the entity bean is always reloaded from the database at the beginning of each transaction. A client can attempt to access the bean and start a new transaction on any container that has been configured to host that bean. This is similar to the session clustering facility described for HTTP sessions in that the entity bean's state is maintained in a shared database that can be accessed from any server when required.
This product supports the cloning of stateful session bean home objects among multiple appservers. However, it does not support the cloning of a specific instance of a stateful session bean. Each instance of a particular stateful session bean can exist in just one application server and can be accessed only by directing requests to that particular application server. State information for a stateful session bean cannot be maintained across multiple members of a server cluster.
Enterprise beans
Managing EJB containers
Enterprise beans: Resources for learning