Thread deadlock when using ThreadlocalPool for stateless EJBs and Spring on JBoss

Issue

We have many mbean services running concurrently. Each mbean service have upto 5 worker threads. Today some of the mbeans were not working and when we logged onto jmx console and clicked these mbean services, the system was not responding. It almost felt like that the worker threads for some of these mbean services were dead.

Our question is that why? Could it be too many threads caused the problem? But the cpu and memory usage were normal. The cpu usage was less than 10%.