Why is spawning threads in Java EE container discouraged?

Question

One of the first things I\'ve learned about Java EE development is that I shouldn\'t spawn my own threads inside a Java EE container. But when I come to think about it, I do

Answer 1

For EJBs, it's not only discouraged, it's expressly forbidden by the specification:

An enterprise bean must not use thread synchronization primitives to synchronize execution of multiple instances.

and

The enterprise bean must not attempt to manage threads. The enterprise bean must not attempt to start, stop, suspend, or resume a thread, or to change a thread’s priority or name. The enterprise bean must not attempt to manage thread groups.

The reason is that EJBs are meant to operate in a distributed environment. An EJB might be moved from one machine in a cluster to another. Threads (and sockets and other restricted facilities) are a significant barrier to this portability.