code: public static void setMainCache(String cacheName) {
  		if (StrKit.isBlank(cacheName))
  			throw new IllegalArgumentException("cacheName can not be blank");
  		cacheName = cacheName.trim();
  		Cache cache = cacheMap.get(cacheName);
  		if (cache == null)
  			throw new IllegalArgumentException("the cache not exists: " + cacheName);
  		
  		Redis.mainCache = cache;
  	}
docs: 提供一个设置设置主缓存 mainCache 的机会，否则第一个被初始化的 Cache 将成为 mainCache