问为什么我的Spring @Autowired字段为空？

@Controlle

public class MileageFeeController {    

    @RequestMapping("/mileage/{miles}")

    @ResponseBody

    public float mileageFee(@PathVariable int miles) {

        MileageFeeCalculator calc = new MileageFeeCalculator();

        return calc.mileageCharge(miles);

    }

}

@Service

public class MileageFeeCalculator {

    @Autowired

    private MileageRateService rateService; // <--- should be autowired, is null

    public float mileageCharge(final int miles) {

        return (miles * rateService.ratePerMile()); // <--- throws NPE

    }

}

@Service

public class MileageRateService {

    public float ratePerMile() {

        return 0.565f;

    }

}