java java.lang.NullPointerException 空指針異常

Doctor doctor = doctorMapper.selectList(

new LambdaQueryWrapper()

.eq(Doctor::getCode,doctorCode)

).get(0);

寫了個這，一直出問題，空指針異常。後面發現是

DoctorMapper doctorMapper;究其原因還是因為對象沒有初始化。沒有加上注解@Resource，加上注解就好了