Field injection is not recommended using @ Autowired

Use @Autowired There is a warning in the code ：

as a result of Spring Field based dependency injection is not recommended

It can be changed to dependency injection based on constructor ,idea Can automatically generate , Too many injected classes will lead to too many parameters of the constructor （ It's a signal , Indicates that there are too many dependencies ）

You can also use @Resource, Just no warning

Make a distinction @Autowired and @Resource The difference between the two notes ：
(1)、@Autowired The default in accordance with the byType In a way that bean matching ,@Resource The default in accordance with the byName In a way that bean matching
(2)、@Autowired yes Spring Annotations ,@Resource yes J2EE Annotations

Field injection is not recommended（Spring IOC Attribute injection is no longer recommended ）

Use @Autowired Note warning Field injection is not recommended

@Autowired Note tips Field injection Warning

@Autowire The injected object may be null

Example ：

@Component
@Transactional
public class XXService {
    
    
    private Logger logger = LoggerFactory.getLogger(getClass());
    
    @Autowired
    private XXDao xxDao;  
    
    public void add(XX xxl) {
            
        logger.debug(" Get into XXService.add(XX xx) Method ");
        xxDao.save(xx); // Null pointer is reported here , as a result of xxDao by null
}

Tracing to the source ：

@RestController
@RequestMapping(value = "/xxx/xx")
public class XXController {
    

    Logger logger = LoggerFactory.getLogger(getClass());
    /*  Here through new To get an instance that leads to service Injected dao by null;  Must be used in all dao The place of , Including calling it servcie To carry out @Autowired Inject , Otherwise, the subsequent injection will fail  */
    private XXService xxService = new XXService();
      .....

}

reason ：

@Autowired When injecting, the class is handed over to Springboot management , and new The examples are separated from Springboot Management of , Two things are not managed by one manager , So it can't be connected ,@Autowired Injection will be null.

@Autowire The object of injection is null

Springboot in new The resulting example contains @Autowired At the time of Injection Spring Bean by NULL

ps：Mapper No need to comment @Component It can also work ,spring Will be able to @MapperSacn Scan all configured packages ;@EnableFeignClients By scanning the path @FeignClient Annotated interfaces are injected IOC Containers .