Spring DI注入规则

1.Spring怎么知道注入哪个实现?
As long as there is only a single implementation of the interface and that implementation is annotated with @Component with Spring’s component scan enabled, S
 spring framework can find out the (interface, implementation) pair. If component scan is not enabled, then you have to define the bean explicitly in your application-config.xml (or equivalent spring configuration file).
如果Spring配置了component scan，并且要注入的接口只有一个实现的话，那么spring框架可以自动将interface于实现组装起来。如果没有配置component scan，那么你必须在application-config.xml（或等同的配置文件）定义这个bean。
2.需要@Qualifier和@Resource注解吗?
Once you have more than one implementation, then you need to qualify each of them and during auto-wiring, you would need to use the @Qualifier annotation to inject the right implementation, along with @Autowired annotation. If you are using @Resource (J2EE semantics), then you should specify the bean name using the name attribute of this annotation.
一旦一个接口有多个实现，那么就需要每个特殊化识别并且在自动装载过程中使用@Qualifier和@Autowired一起使用来标明。如果是使用@Resource注解，那么你应该使用resource中属性名称来标注@Autowired.
3.为什么@Autowired使用在interface上而不是实现类上？
Firstly, it is always a good practice to code to interfaces in general. Secondly, in case of spring, you can inject any implementation at runtime. A typical use case is to inject mock implementation during testing stage.
首先，一般使用接口是很常用并且有益的变成技术。其次，在spring中，你可以在运行过程中注入各种实现。一个很经典的情况就是在测试阶段，注入模拟的实现类。