Spring自动装配失败

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService': Injection of autowired dependencies failed; nested exception is java.lang.NullPointerException

错误原因：
注释@Autowired必须写在需要装配的对象的上方

@Service
public class UserService {
    private Parent parent;
    @Autowired
    public void start(){
        parent.begin();
    }
}

正确写法：

@Service
public class UserService {
    @Autowired
    private Parent parent;
    public void start(){
        parent.begin();
    }
}