Question about thread safe of EntityManager

"If you're using the EntityManager API directly from a component in the web tier, we recommend that you avoid injecting entity managers because the EntityManager interface is not thread-safe. Instead, you should use a JNDI lookup to grab an instance of a container-managed EntityManager."

The above paragraph is come from EJB 3 in Action, I cannot understand it very much. Is there any difference between the injected EntityManager and the JNDI lookuped EntityManager? Or it just means if injecting entity manager to a servlet, the entity manager should be an instance variable of that servlet so it is not thread safe?

The injected EntityManager is an instance of a servlet, so as you said, this is not thread safe. On the contrary, declaring an EntityManager locally and doing a JNDI lookup manually is thread safe.