foreword

Under @Componentthe annotated class, @Resourcethe or @Autowiredannotation is used again. Doing so will cause dependency injection to fail.

This is because the order in which spring loads them is different. When using @Componentannotations to instantiate beans into the spring container, because @Autowiredit is in this bean, @Autowiredthe automatic loading has not been completed at this time, so the dependency injection service is null

@Component and @Autowired or @Resource

In Spring applications, @Componentannotations are used to mark classes as autoscannable components. When the Spring container starts, it scans for @Componentannotated classes and instantiates them as beans. These beans will be added to the bean factory of the Spring container for use in the application.

@AutowiredAnnotations are used for dependency injection in Spring applications. When the Spring container creates an @Autowiredannotated bean, it will automatically find the matching type for injection. If more than one matching type is found, an exception is thrown.

@ResourceAnnotations can also be used for dependency injection in Spring applications. When the Spring container creates an @Resourceannotated bean, it will preferentially use name matching for injection. If no matching name is found, type matching is used for injection.

Therefore, in a Spring application, @Componentthe annotated class will be loaded before @Autowiredor @Resourcebefore the annotated class. @AutowiredAnnotations will prefer type matching for dependency injection, while @Resourceannotations will prefer name matching for dependency injection.

When using @Component, @Autowiredor @Resourceannotations for dependency injection, you need to pay attention to the following points:

If you want to use @Autowiredannotations to inject multiple matching types, you can use @Qualifierannotations to specify specific bean names.

If you wish to inject non-essential dependencies using @Autowiredthe or annotation, you can use or [email protected]@Autowired(required=false)@Resource(required=false)

If the dependency that you want to use @Autowiredor @Resourceannotation injection does not exist, you can use @Autowired(required=false)or @Resource(required=false), and handle it in the code accordingly.

If you want to use @Autowiredor @Resourceannotation to inject dependencies that can be null, you can use @Autowired(required=false)or @Resource(required=false)and handle them accordingly in the code.

If you want to use @Autowiredor @Resourceannotation to inject dependencies that can be null, you can use @Autowired(required=false)or @Resource(required=false)and handle them accordingly in the code.

Solution

@Component
public class Test  {

    private final Logger logger = LoggerFactory.getLogger(this.getClass());

//    @Autowired
//    private UserServer userServer;

    private static UserServer userServer;

    @Autowired
    public void setUserServer(UserServer userServer) {
        Test.userServer = userServer;
    }    
}

Putting @Autowiredthe annotation on the method will automatically inject the parameters of this method after the class is loaded , and execute the method once.