public class UserCache {

    public static final String KEY_REMOVED_FROM_CACHE = "key={} removed from cache";
    @Getter
    private LoadingCache<String, Optional<UserDetailDto>> cache1;
    private LoadingCache<String, Optional<UserDetailDto>> cache2;
    @Getter
    private LoadingCache<String, Optional<UserDetailDto>> cache3;
    private final UserRepository userRepository;
    private final CacheConfig cacheConfig;
    private final PrometheusMeterRegistry prometheusMeterRegistry;

    @PostConstruct
    void init() {
        if(cacheConfig.isEnabled()) {
            ExecutorService threadPool = Executors.newFixedThreadPool(4);
            cache1 = Caffeine.newBuilder()
                    .maximumSize(100_000) // 100_000 test-user data will be cached.
                    .removalListener(TestCache::onRemoval)
                    .refreshAfterWrite(2, TimeUnit.HOURS)
                    .executor(threadPool)
                    .recordStats()
                   .build(userRepository::loadCache1);

            cache2 = Caffeine.newBuilder()
                    .maximumSize(5_000) // 5000 test-user data will be cached.
                    .removalListener(TestCache::onRemoval)
                    .refreshAfterWrite(5, TimeUnit.HOURS) // refresh cache after 5 hours write
                    .executor(threadPool)
                    .build(userRepository::loadCache2);


            cache3 = Caffeine.newBuilder()
                    .maximumSize(100_000) // 100_000 test-user data will be cached.
                    .removalListener(TestCache::onRemoval)
                    .refreshAfterWrite(1, TimeUnit.HOURS)
                    .executor(threadPool)
                    .build(userRepository::loadCache3);

            CacheMetricsCollector collector = new CacheMetricsCollector();
            collector.addCache("cache1", cache1);
            collector.addCache("cache2", cache2);
            collector.addCache("cache3", cache3);

            prometheusMeterRegistry.getPrometheusRegistry().register(collector);
        }

    private static void onRemoval(@Nullable Object key, @Nullable Object value, RemovalCause cause) {
        log.atDebug().log(KEY_REMOVED_FROM_CACHE, key);
    }
}

public void registerCacheMetrics(String cacheName, PrometheusMeterRegistry prometheusMeterRegistry, LoadingCache<?, ?> loadingCache) {

        // Register custom gauges for cache metrics
        Gauge.builder(cacheName + ".hit.count", loadingCache, cache -> cache.stats().hitCount())
                .description("Number of cache hits")
                .register(prometheusMeterRegistry);

        Gauge.builder(cacheName + ".miss.count", loadingCache, cache -> cache.stats().missCount())
                .description("Number of cache misses")
                .register(prometheusMeterRegistry);

        Gauge.builder(cacheName + ".requests.count", loadingCache, cache -> cache.stats().requestCount())
                .description("Number of times cache requested")
                .register(prometheusMeterRegistry);

        Gauge.builder(cacheName + ".eviction.count", loadingCache, cache -> cache.stats().evictionCount())
                .description("Number of cache evictions")
                .register(prometheusMeterRegistry);

        Gauge.builder(cacheName + ".load.success.count", loadingCache, cache -> cache.stats().loadSuccessCount())
                .description("Number of successful cache loads")
                .register(prometheusMeterRegistry);

        Gauge.builder(cacheName + ".load.failure.count", loadingCache, cache -> cache.stats().loadFailureCount())
                .description("Number of cache load failures")
                .register(prometheusMeterRegistry);

        Gauge.builder(cacheName + ".load.total", loadingCache, cache -> cache.stats().totalLoadTime())
                .description("Time taken to load cache")
                .register(prometheusMeterRegistry);
    }