Spring Boot中服务接口类的用途

Spring Boot中服务接口类的用途

java
public interface UserService {
    User getUserById(int id);
    void saveUser(User user);
    List getAllUsers();
}
@Service
public class UserServiceImpl implements UserService {
    private List userList = new ArrayList<>();
    @Override
    public User getUserById(int id) {
        for (User user : userList) {
            if (user.getId() == id) {
                return user;
            }
        }
        return null;
    }
    @Override
    public void saveUser(User user) {
        userList.add(user);
    }
    @Override
    public List getAllUsers() {
        return userList;
    }
}
@RestController
@RequestMapping("/users")
public class UserController {
    @Autowired
    private UserService userService;
    @GetMapping("/{id}")
    public User getUserById(@PathVariable int id) {
        return userService.getUserById(id);
    }
    @PostMapping("/")
    public void saveUser(@RequestBody User user) {
        userService.saveUser(user);
    }
    @GetMapping("/")
    public List getAllUsers() {
        return userService.getAllUsers();
    }
}