默认的注入方式为 byType ，也就是根据类型匹配，当有多个实现时，则通过 byName 注入，也可以通过配合@Qualifier注解来显式指定name值，指明要使用哪个具体的实现类
@Service
public class UserServiceImpl implements UserService {
@Autowired
private UserMapper userMapper;
@Autowired
private RoleMapper roleMapper;
}
二：Java JSR-250中提供的注解 @Resource
默认通过 byName 注入，如果没有匹配则通过 byType 注入
@Service
public class UserServiceImpl implements UserService {
@Resource
private UserMapper userMapper;
@Resource
private RoleMapper roleMapper;
}
三：构造器注入
@Service
public class UserServiceImpl implements UserService {
private final UserMapper userMapper;
private final RoleMapper roleMapper;
public UserServiceImpl(UserMapper userMapper, RoleMapper roleMapper) {
this.userMapper = userMapper;
this.roleMapper = roleMapper;
}
}
四：Lombok提供的 @RequiredArgsConstructor 方式（会生成一个包含常量，和标识了NotNull的变量的构造方法）
也可写作 @RequiredArgsConstructor(onConstructor =@_(@Autowired))
@Service
@RequiredArgsConstructor
public class UserServiceImpl implements UserService {
private final UserMapper userMapper;
private final RoleMapper roleMapper;
}
