如何使用 Spring Boot 将 @Cacheable 与 redis 一起使用

Question

I am trying to implement hashmap using redis but I would like to control the key myself, so I have implemented following service class.

@Slf4j
@Service
public class RedisService {
    Map<Long, Student> studentMap = new HashMap<>();
    @Cacheable(cacheNames = "studentCache")
    public Map<Long, Student> getStudentCache(){
        return studentMap;
    }
}

my pojo class is

@Data
public class Student implements Serializable {
    private static final long serialVersionUID = -2722504679276149008L;
    public enum Gender {
        MALE, FEMALE
    }
    private Long id;
    private String name;
    private Gender gender;
    private int grade;

}

and the data loader

@Slf4j
@Component
public class DataLoader implements CommandLineRunner {

    @Autowired
    RedisService redisService;

    @Override
    public void run(String... args) {
      log.info("================ loading data now ========================");
        Student student = getStudent();
        redisService.getStudentCache().put(student.getId(), student);
        System.out.println("student is "+redisService.getStudentCache().get(student.getId()));
        log.info("================= program ends ==============");
    }

    private Student getStudent() {
        Student student = new Student();
        student.setId(ThreadLocalRandom.current().nextLong());
        student.setName("first last");
        student.setGender(Student.Gender.MALE);
        student.setGrade(85);
        return student;
    }
}

Main class

@EnableCaching
@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

}

I have successfull connection with redis but it does not seems to be putting anything in the cache. When I run the program I got following message as outcome.

2020-09-10 15:26:37.605  INFO 28540 --- [           main] c.w.s.components.DataLoader              : ================ loading data now ========================
student is null
2020-09-10 15:26:38.295  INFO 28540 --- [           main] c.w.s.components.DataLoader              : ================= program ends ==============

So I am not sure why student is returning NULL any help would be appreciated.

Answer 1

Cache is being initialized when you access cached data for the first time (to be specific - data for the specific caching key - in this case constant). In your case it's happenning at the end of first call to redisService.getStudentCache() . At that time studentMap is an empty map, hence empty map is being cached. The fact that moment later you add there some entry (via .put(student.getId(), student); ) doesn't matter because caching system is using its own copy of cached map, hence null on getting value from this map.