分析这部分代码：涉及哪些高并发的问题

    private Cache<StringWrapper, AtomicReference<ConfigClient>> mtConfigClientCache = CacheBuilder.newBuilder()
            .expireAfterWrite(7, TimeUnit.DAYS)
            .maximumSize(2000)
            .build();
    private final Map<String, StringWrapper> key2LockKey = new ConcurrentHashMap<String, StringWrapper>();

    private LoadingCache<String, MtConfigClient> mtConfigClientLoadingCache = CacheBuilder.newBuilder()
            .expireAfterWrite(7, TimeUnit.DAYS)
            .maximumSize(2000)
            .build(new CacheLoader<String, MtConfigClient>() {
                @Override
                public ConfigClient load(String key) throws Exception {
                    return buildClient(key);
                }
            });

    private ConfigClient getMccClient(String appKey) {
        return this.getMccClientByLoader(appKey);
    }

    private ConfigClient getMccClient2(String appKey) {
        final StringWrapper appKeyLock = key2LockKey.computeIfAbsent(appKey, StringWrapper::new);
        // appKey should not be used again，we force appKey = null to remind others here
        appKey = null;
        AtomicReference<MtConfigClient> mtConfigClientAtomicReference = mtConfigClientCache.getIfPresent(appKeyLock);
        if (mtConfigClientAtomicReference == null) { // mtConfigClientAtomicReference.get() == null, we wont allow this happen
            synchronized (appKeyLock) {
                mtConfigClientAtomicReference = mtConfigClientCache.getIfPresent(appKeyLock);
                if (mtConfigClientAtomicReference == null) {
                    try {
                        MtConfigClient mtConfigClientCreating = new MtConfigClient();
                        mtConfigClientCreating.setAppkey(appKeyLock.s);
                        mtConfigClientCreating.setModel("v2");
                        mtConfigClientCreating.setId("UpdateRateLimitConfig_" + System.nanoTime());
                        mtConfigClientCreating.init();
                        mtConfigClientAtomicReference = new AtomicReference<>();
                        mtConfigClientAtomicReference.set(mtConfigClientCreating);
                        this.mtConfigClientCache.put(appKeyLock, mtConfigClientAtomicReference);
                    } catch (Exception ex) {
                        LOGGER.info("mcc客户端初始化失败:", ex);
                    }
                }
            }
        }
        return Optional.ofNullable(this.mtConfigClientCache.getIfPresent(appKeyLock))
                .orElseGet(() -> null)
                .get();
    }

    private MtConfigClient getMccClientByLoader(String appKey) {
        try {
            return this.mtConfigClientLoadingCache.get(appKey);
        } catch (ExecutionException e) {
            LOGGER.error("获取mcc客户端失败", e);
            return null;
        }
    }

    private MtConfigClient buildMccClient(String appKey) {
        try {
            MtConfigClient mtConfigClientCreating = new MtConfigClient();
            mtConfigClientCreating.setAppkey(appKey);
            mtConfigClientCreating.setModel("v2");
            mtConfigClientCreating.setId("UpdateRateLimitConfig_" + System.nanoTime());
            mtConfigClientCreating.init();
            return mtConfigClientCreating;
        }catch (Exception ex) {
            LOGGER.error("mcc客户端初始化失败:", ex);
            throw ex;
        }
    }


    static class StringWrapper {
        public String s;
        public StringWrapper(String s) {
            this.s = s;
        }
    }

这已经是修改后的代码，原先的代码有单例模式的典型错误，在将 client 放到cache之前，在做init操作，但是init操作耗时较长，约9s。这就导致其他线程拿到的是，未初始化完成的client。

这里的修改优化一个是： 使用 LoadingCache，这是单线程加载的，有他就够了。
如果不想用cache，第二种优化方式是：
final StringWrapper appKeyLock = key2LockKey.computeIfAbsent(appKey, StringWrapper::new);
想要针对每个key来创建单例，可以把他们转化到同一个对象，同时，使用AtomicReference<MtConfigClient> 来保证init方法一定会执行完之后，才会执行放到map里，对其他线程可见，因为AtomicReference的内部值引用是 volatile，有防止重排序的语意。