Spring boot 2 @Transactional annotation makes Autowired fields null

Question

I'm trying to use @Transactional annotation in a method on my service to lazily load a field. However using @Transactional on my Implementation class makes all autowired fields null.

Here is my implementation :

@Service
public class UserServiceImpl implements UserService {

 /**
  * DefaultMapper.
  */
 @Autowired
 private DefaultMapper defaultMapper;

 /**
  * Resource service injection.
  */
 @Autowired
 private ResourceService resourceService;

 /**
  * UserRepository.
  */
 @Autowired
 private UserRepository userRepository;

 /**
  * Jwt Factory.
  */
 @Autowired
 private JwtService jwtService;

 @Override
 @Transactional
 public final UserDto findByLogin(final String login) throws ResourceNotFoundException {
 // user repository is null here when using @Transactional
  User user = this.userRepository.findByLogin(login)
   .orElseThrow(() -> new ResourceNotFoundException(
    resourceService.getMessage(MessageBundle.EXCEPTION, "resource.notfound.user.login")
   ));
  UserDto userDto = defaultMapper.asUserDtoWithRoles(user);
  return userDto;
 }

Thank you in advance.

Answer 1

Transaction, amongst others, are applied using AOP, the default AOP mechanism in Spring is to use proxies. When using Spring Boot the proxy mode is set the class based proxies.

You can fix this in 1 of 2 ways.

1. Remove final from your method
2. Disable class based proxies by adding spring.aop.proxy-target-class=false to your application.properties

Now when you aded @Transactional this will lead to a proxy of your UserServiceImpl to be created, a class-based proxy to be exact. What happens is that a subclass is created for your UserServiceImpl and all methods are overriden to apply the TransactionInterceptor. However as your method is marked final the dynamically created class cannot override this method. As a result the method looks at field instances in the dynamically created proxy class which always will be null.

When removing final the method can be overridden, the behavior applied and it will look at the proper field instances (of the actual UserServiceImpl instead of the proxy).

When disabling class based proxies, you will get a JDK Dynamic Proxy which is basically a thin wrapper which implements all the interfaces your service implements. It applies the added behavior (transactions) and calls the actual service. There is no extension of the actual class needed and as such you can proxy final methods (as long as it is part of your interface).