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<>场景：使用 @Autowired 的时候，到底是写接口还是实现类？
<>答案：写的接口
<>当实现类唯一时，表面上注入接口，其实注入实现类
<>当实现类多个时，通过@Service(“menuService1”)指定实现类
<>解析：
这个其实是创建了实现类的对象但引用了接口类型，
即 “InjectionDao injectionDao = new InjectionDaoImpl ()”, 是 Java 多态性（向上转型）的一种应用。
在实现类处加 @Repository 注解，意思就是 new InjectionDaoImpl (),
而在 InjectionServiceImpl 中定义属性 InjectionDAO injectionDAO 就是将 new 出来的这个
InjectionDaoImpl 对象向上转型为 InjectionDao 类型。
<>注意：
若一个接口被多个实现类，实现的时候，@Autowired，Spring 会按 byType
的方式寻找接口的实现类，将其注入。存在多个实现类，应该指定名字，可以通过 byName 注入的方式。可以使用 @Resource 或 @Qualifier
注解。
//实现类1 @Service("menuService1") public class MenuServiceImpl implements
IMenuService //实现类2 @Service("menuService2") public class MenuServiceImpl
implements IMenuService //注入接口，相当于new一个实现类，指定名称menuService1实现类 @Autowired @
Qualifier("menuService1") private IMenuService menuService;
//注入接口，相当于new一个实现类，指定名称menuService2实现类 @Autowired @Qualifier("menuService2")
private IMenuService menuService;
