问题2、3和4 – Spring不会注意不是Spring Bean的任何类.因此，Spring不会关注您的MyServlet类. 因此，答案是

• 2)是
• 3)仅Spring托管Beans
• 4)这是Springs的责任，因为Spring是容器

对于问题1).它以这种方式工作，因此使用Spring Injected Entity Manager可以有效地节省线程.

EntityManager is not thread-safe by definition. Servlets specs says that in non-distributed environment and without implementing SingleThreadModel, there is only one servlet instance per definition.

Therefore, in Java EE when you inject an EntityManager through the @PersistenceContext into Servlet's field - it's not thread safe:

public class MyServlet extends HttpServlet {

    // Not thread-safe, should be using EMF instead.
    @PersistenceContext
    private EntityManager em;
}

1. Is this correct to say that even though the default scope of Spring beans is singleton, the EntityManager is thread-safe as the Spring uses ThreadLocal to bind its transaction and EntityManager to it?

2. Is the above Servlets example still valid in Spring? Is it still not thread-safe?

3. Does the ThreadLocal approach works only for Spring managed beans and plain servlet is not one of those?

4. As far as I remember, it's the container responsibility to inject the EntityManager. In Glassfish Java EE implementation, it was the application server who discovers the @PersistenceContext as injection point.
   How does it look like in Spring? Is the Spring Framework responsible for discovering those annotations or it's responsibility of the JPA implementor?

Question 2, 3, and 4 -- Spring does not pay attention to any class that is not a Spring Bean. Therefor Spring does not pay attention to you MyServlet class. Therefore the answer for

• 2) is no
• 3) only Spring managed Beans
• 4) it is Springs responsibility, because Spring is the Container

For Question 1). It works this way, so the usage of an Spring Injected Entity Manager is effective thread save.