Finding deadlock problems and resolving them is the topic of this section. We will describe what deadlocking MBeans, how you can detect it within your application, and how you can resolve deadlocks. Deadlock can occur when two or more threads have locks on shared resources. Figure 11.8, “Deadlock definition example” illustrates a simple deadlock scenario. Here, Thread 1 has the lock for Bean A, and Thread 2 has the lock for Bean B. At a later time, Thread 1 tries to lock Bean B and blocks because Thread 2 has it. Likewise, as Thread 2 tries to lock A it also blocks because Thread 1 has the lock. At this point both threads are deadlocked waiting for access to the resource already locked by the other thread.
The default locking policy of JBoss is to lock an Entity bean when an invocation occurs in the context of a transaction until the transaction completes. Because of this, it is very easy to encounter deadlock if you have long running transactions that access many entity beans, or if you are not careful about ordering the access to them. Various techniques and advanced configurations can be used to avoid deadlocking problems. They are discussed later in this section.