An entity Bean Provider can specify that an entity bean is non-reentrant. If an instance of a non-reentrant entity bean executes a client request in a given transaction context, and another request with the same transaction context arrives for the same entity object, the container will throw an exception to the second request. This rule allows the Bean Provider to program the entity bean as single-threaded, non-reentrant code. The functionality of entity beans with container-managed persistence may require loopbacks in the same transaction context. An example of a loopback is when the client calls entity object A, A calls entity object B, and B calls back A in the same transaction context. The entity bean’s method invoked by the loopback shares the current execution context (which includes the transaction and security contexts) with the Bean’s method invoked by the client. If the entity bean is specified as non-reentrant in the deployment descriptor, the container must reject an attempt to re-enter the instance via the entity bean’s component interface while the instance is executing a business method. (This can happen, for example, if the instance has invoked another enterprise bean, and the other enterprise bean tries to make a loopback call.) If the attempt is made to reenter the instance through the remote interface, the container must throw the java.rmi.RemoteException to the caller. If the attempt is made to reenter the instance through the local interface, the container must throw the javax.ejb.EJBException to the caller. The container must allow the call if the Bean’s deployment descriptor specifies that the entity bean is re-entrant.
Re-entrant entity beans must be programmed and used with caution. First, the Bean Provider must code the entity bean with the anticipation of a loopback call. Second, since the container cannot, in general, tell a loopback from a concurrent call from a different client, the client programmer must be careful to avoid code that could lead to a concurrent call in the same transaction context.
Concurrent calls in the same transaction context targeted at the same entity object are illegal and may lead to unpredictable results. Since the container cannot, in general, distinguish between an illegal concurrent call and a legal loopback, application programmers are encouraged to avoid using loopbacks. Entity beans that do not need callbacks should be marked as non-reentrant in the deployment descriptor, allowing the container to detect and prevent illegal concurrent calls from clients.
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