Spring-使用@Autowired字段基于@Service创建@Bean会导致@Autowired字段出现NullPointerException

Question

I have following Context:

@Configuration
@ComponentScan
public class MyContext {

    @Bean
    public Logger getLogger() {
        return LoggerFactory.getLogger(MyContext.class);
    }

    @Bean
    @Autowired
    public RequestSpecification getRequestSpecification(RestServiceImpl restService) {
        return restService.getService();
    }
}

My inside my service I have @Autowired field (Logger):

@Service
public class RestServiceImpl {

      @Autowired
      Logger logger;
}

And now the problem comes. When I am trying to reuse my RequestSpecification in other service:

@Service
public class RequestWrapperForMyEndpoint {
    @Value("${rest.service.url}")
    private String SERVICE_URL;

    @Autowired
    private RequestSpecification restService;
}

I am running into NullPointerException . Because Logger is not injected into RestServiceImpl class.

What is the reason?

Answer 1

My very simple answer to your specific problem would be: Do NOT inject a logger.

@Service
public class RestServiceImpl {
      private static Logger logger = LoggerFactory.getLogger(RestServiceImpl.class);
}

Argument #1: Whatever you log inside a class, should be done with a logger of that class. You should NOT log from within RestServiceImpl with a logger which was initialized from MyContext. There is no point in reducing the number of loggers. There should be one logger per class and it would not affect your performance, since creating loggers is very cheap.

Argument #2: What's not quite as cheap as instantiating a logger is autowiring a bean. Autowiring / and maybe having an auto-generated proxy bean in between is quite expensive.

Argument #3: One might argue that it is good design to autowire EVERYTHING. Then you could still argue - YES, everything but objects which do not depend on anything else. And a logger is one of those exceptions. It does not depend on anything else by definition.

Sorry - but I don't have any good idea for your actual autowiring problem.

Answer 2

This would rather be a comment, but since I do not have enough points I only can post answers. Here we go:

As such, this code should work, this is how autowiring should be done as such.

Although there is no point in autowiring loggers (as Mick explained), since every log entry will be linked to MyContext you won't be able to tell where it actually comes from (not helpful when debugging), ere are possible solutions for the autowiring issue:

1. Run the debugger and check if the return value of LoggerFactory.getLogger(MyContext.class) is not null in the first place.
2. Autowiring only works when the Spring application is actually run and the context is built. Ergo it does not work in unit tests (if this is when it occurs). For testing, you need to run integration tests.
3. Check if the bean is not overwritten somewhere else in the code.
4. You did not show what your restService.getService() method looks like. There is a chance, this is where your autowired logger gets lost, since you obviously are not "keeping" the entire autowired restService bean.