public String getRandomStr(int length)
	{
		StringBuilder code = new StringBuilder();   
	    String charOrNum;    // 输出字母还是数字   
	    int caseOption;      //取得大写字母还是小写字母  (ASCII码值) 
	    for(int i = 0; i < length; i++){   
	        charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; 
	        if("char".equalsIgnoreCase(charOrNum)){                // 字符串
	            caseOption = random.nextInt(2) % 2 == 0 ? 65 : 97; 
	            code.append((char) (caseOption + random.nextInt(26)));   
	        }else if("num".equalsIgnoreCase(charOrNum)){            // 数字
	            code.append((random.nextInt(10)));   
	        }   
	    }   
	    return code.toString().toLowerCase(); 
	}