@Autowired vs @PersistenceContext for EntityManager bean

Question

What is the difference between:

@Autowired private EntityManager em; 

versus:

@PersistenceContext private EntityManager em; 

Both options work in my application, but can I break something by using the @Autowired annotation?

Answer 1

@PersistenceContext allows you to specify which persistence unit you want to use. Your project might have multiple data sources connected to different DBs and @PersistenceContext allows you to say which one you want to operate on

check the explanation here: http://www.coderanch.com/t/481448/java-EJB-SCBCD/certification/unitName-PersistenceContext

Answer 2

You shouldn't use @Autowired. @PersistenceContext takes care to create a unique EntityManager for every thread. In a production application you can have multiple clients calling your application in the same time. For each call, the application creates a thread. Each thread should use its own EntityManager. Imagine what would happen if they share the same EntityManager: different users would access the same entities.

usually the EntityManager or Session are bound to the thread (implemented as a ThreadLocal variable). 

Source: https://stackoverflow.com/a/42074452/2623162

EntityManager instances are not thread-safe.  

Source: https://docs.oracle.com/cd/E19798-01/821-1841/bnbqy/index.html

Please notice that @PersistenceContext annotation comes from javax.persistence package, not from spring framework. In JavaEE it is used by the JavaEE container (aka the application server) to inject the EntityManager. Spring borrowed the PersistenceContext annotation to do the same: to inject an application-managed (= not container-managed) EntityManager bean per thread, exactly as the JavaEE container does.