code: public boolean exists() {
  		try {
  			if(!isBinary)return getJedisCommands(groupName).exists(key);
  			if(isCluster(groupName)){
  				return getBinaryJedisClusterCommands(groupName).exists(keyBytes);
  			}
  			return getBinaryJedisCommands(groupName).exists(keyBytes);
  		} finally {
  			getJedisProvider(groupName).release();
  		}
  		
  	}
docs: 检查给定 key 是否存在。
  
  @param keyBytes
  @return