问题
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?
回答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
回答2:
@PersistenceContext:
does not return entity manager instance
it returns container-managed proxy that acquires and releases presistence context on behalf of the application code
回答3:
@PersistenceContext is a JPA standard annotation designed for that specific purpose. Whereas @Autowired is used for any dependency injection in Spring. Using @PersistenceContext gives you greater control over your context as it provides you with ability to specify optional elements e.g. name, properties
回答4:
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.
来源：https://stackoverflow.com/questions/31335211/autowired-vs-persistencecontext-for-entitymanager-bean