Spring transactions, EntityManager lifecycle

Question

I am developing a project with spring framework. I have following architecture: all database code is in dao classes, all application logic is in service classes, http requests are processed using spring mvc rest controllers.

@Repository
public class UserDao {

    @PersistenceContext
    private EntityManager em;

    public void editUser(User user) {
        ...
    }
}

@Service
public class UserService {

    @Autowired
    private UserDao userDao;

    @Transactional
    public void editUser(User user) {
        ...
    }
}

@RestController
@RequestMapping(value = "/UserCtrl", produces = "application/json;charset=UTF-8")
public class UserCtrl {

    private static final Logger logger = LogManager.getLogger(AppConfig.LOGGER_NAME);

    @Autowired
    private ApplicationContext appContext;

    @RequestMapping(value = "/editUser")
    public ActionResult editUser(@ModelAttribute User user) {
        ActionResult rslt = new ActionResult();

        try {
            UserService userService = appContext.getBean(UserService.class);
            userService.editUser(user);

            rslt.setSuccess(true);
        } catch (Exception ex) {
            logger.error("editUser", ex);
            rslt.setSuccess(false);
        }

        return rslt;
    }
}

This code works correctly. One thing I cannot understand is how aop proxy of UserService bean starts transaction, when EntityManager is injected only in UserDao class. Can somebody explain me how this works and how EntityManager lifecycle is managed by spring framework ?

Any help appreciated.

Answer 1

The TransactionManager is responsible for managing the transaction boundaries in Spring Framework. When you annotate the business method with @Transactional you are instrumenting the method with an aspect. Before executing the method, this aspect interact with TransactionManager which decides if it is necessary to create a new Transaction or use a preexisting one. In the case of create a new Transaction, a new Entity Manager is created from EntityManagerFactory and is associated to the current thread alonside with the Transaction.

The EntityManager is responsible to iterate with the database. When you inject it with @PersistenceContext, Spring injects an proxy to the EntityManager. Whenever an operation is executed in the EntityManager proxy it looks the EntityManager associated with the thread.