EntityManagers and Sessions are not thread safe. See https://docs.jboss.org/hibernate/orm/4.3/manual/en-US/html/ch13.html

A Session is an inexpensive, non-threadsafe object that should be used once and then discarded for: a single request, a conversation or a single unit of work.

A Session is not thread-safe. Things that work concurrently, like HTTP requests, session beans, or Swing workers, will cause race conditions if a Session instance is shared. If you keep your Hibernate Session in your HttpSession (this is discussed later in the chapter), you should consider synchronizing access to your Http session. Otherwise, a user that clicks reload fast enough can use the same Session in two concurrently running threads.