@RestController
public class CacheController {

    @Autowired
    CacheService cacheService;

    @GetMapping("/item/{id}")
    public Item getItem(@PathVariable("id") Long id) {
        // 从缓存中获取 Item
        Item item = cacheService.getItemFromCache(id);

        // 没有在缓存中找到 Item，从数据库中获取
        if (item == null) {
            item = itemRepository.findById(id).orElse(null);
            if (item != null) {
                cacheService.putItemInCache(item);
            }
        }

        return item;
    }
}

@Service
public class CacheService {

    private ConcurrentHashMap<Long, Item> cache = new ConcurrentHashMap<>();

    public Item getItemFromCache(Long id) {
        return cache.get(id);
    }

    public void putItemInCache(Item item) {
        cache.put(item.getId(), item);
    }
}