code: public static String generateZeroString(int length) {
  		StringBuffer sb = new StringBuffer();
  		for (int i = 0; i < length; i++) {
  			sb.append('0');
  		}
  		return sb.toString();
  	}
docs: 生成一个定长的纯0字符串
  
  @param length 字符串长度
  @return 纯0字符串