Is making the methods thread safe, allowed in EJB

Is making the methods thread safe (probably by synchonizing), allowed in Session/Entity beans by the bean developer? Or is that taken care by the Container? As a developer , is there any information needs to provide to the container through deployment descriptor? What are the attributes used in that?

[B]Question asked by visitor Ravi[/b]

If you use synchronize in EJB methods then that program violates the following EJB guideline:

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