code: public String put(String group, String key, String value) {
  		group = StrUtil.nullToEmpty(group).trim();
  		writeLock.lock();
  		try {
  			LinkedHashMap<String, String> valueMap = this.get(group);
  			if (null == valueMap) {
  				valueMap = new LinkedHashMap<>();
  				this.put(group, valueMap);
  			}
  			this.size = -1;
  			return valueMap.put(key, value);
  		} finally {
  			writeLock.unlock();
  		}
  	}
docs: 将键值对加入到对应分组中
  
  @param group 分组
  @param key 键
  @param value 值
  @return 此key之前存在的值，如果没有返回null