What is an EJB, and what does it do?

Question

Been trying to learn what EJB beans are, what does it mean their instances are managed in a pool, blah blah. Really can\'t get a good grip of them.

Can you

Answer 1

An EJB is a Java component, containing business logic, that you deploy in a container, and that benefits from technical services provided by the container, usually in a declarative way, thanks to annotations:

• transaction management: a transaction can be started automatically before a method of the EJB is invoked, and committed or rollbacked once this method returns. This transactional context is propagated to calls to other EJBs.
• security management: a check can be made that the caller has the necessary roles to execute the method.
• dependency injection: other EJBs, or resources like a JPA entity manager, a JDBC datasource, etc. can be injected into the EJB.
• concurrency: the container makes sure that only one thread at a time invokes a method of your EJB instance.
• distribution: some EJBs can be called remotely, from another JVM.
• failover and load-balancing: remote clients of your EJBs can automatically have their call redirected to another server if necessary.
• resource management: stateful beans can automatically be passivated to disk in order to limit the memory consumption of your server.
• ... I probably have forgotten some points.