@Autowired

@Autowired - Official Documents
构造器注入，只有一个构造函数的时候@Autowired可省略

As of Spring Framework 4.3, an @Autowired annotation on such a constructor is no longer necessary if the target bean only defines one constructor to begin with. However, if several constructors are available, at least one must be annotated to teach the container which one to use.

只因多看了一眼提示，又一次刷新了@Autowired注释的认知

1、构造器注入适用具有强依赖和不变性的依赖； 2、Setter注入适用于具有可选性和可变性的依赖注入； 3、Field注入，尽量少使用，如果需要则使用@Resource进行替代，以降低耦合性。

构造器注入

@SpringBootTest
public class SimpleTest {
    private final Svc svc;
    // 省略了@Autowired
    public SimpleTest(@Qualifier(value = "svcB") Svc svc) {
        this.svc = svc;
    }
    @Test
    void rc() {
        svc.sayHello();
    }
}

Setter注入

@SpringBootTest
public class SimpleTest {
    private Svc svc;
    // 不能省略@Autowired
    @Autowired
    public void setSvc(@Qualifier(value = "svcB") Svc svc) {
        this.svc = svc;
    }
    @Test
    void rc() {
        svc.sayHello();
    }
}

// Setter注入例子，有可选性和可变现
@Service
public class ActivityService extends EntityManager<UserActivity, Long> {
    private HibernateDao<UserActivityType, Long> userActivityTypeDao;
    private HibernateDao<ActivityRewardRecord, Long> activityRewardRecordDao;
    @Autowired
    public void setDao(final SessionFactory sessionFactory) {
        userActivityTypeDao = new HibernateDao<UserActivityType, Long>(sessionFactory, UserActivityType.class);
        activityRewardRecordDao = new HibernateDao<ActivityRewardRecord, Long>(sessionFactory, ActivityRewardRecord.class);
    }
}