The exceptions thrown by enterprise beans fall into two categories: system and application.
A system exception indicates a problem with the services that support an application. Examples of these problems include the following: a connection to an external resource cannot be obtained or an injected resource cannot be found. If your enterprise bean encounters a system-level problem, it should throw a javax.ejb.EJBException. Because the EJBException is a subclass of the RuntimeException, you do not have to specify it in the throws clause of the method declaration. If a system exception is thrown, the EJB container might destroy the bean instance. Therefore, a system exception cannot be handled by the bean's client program; it requires intervention by a system administrator.
An application exception signals an error in the business logic of an enterprise bean. Application exceptions are typically exceptions that you've coded yourself, such as the BookException thrown by the business methods of the CartBean example. When an enterprise bean throws an application exception, the container does not wrap it in another exception. The client should be able to handle any application exception it receives.
If a system exception occurs within a transaction, the EJB container rolls back the transaction. However, if an application exception is thrown within a transaction, the container does not roll back the transaction.