java生成短信验证码 字母加数字的验证码

/**
 * 获取随机验证码
 * @param length 验证码的长度
 * @return
 */
public static String getRandomString(int length) {
    String randomStr = "abcdefghijklmnopqrstuvwxyz0123456789";   
    Random random = new Random();   
    StringBuffer strBuf = new StringBuffer();   
    for (int i = 0; i < length; i++) {   
        int number = random.nextInt(randomStr.length());   
        strBuf.append(randomStr.charAt(number));   
    }   
    return strBuf.toString();   
 }/**
 * 获取随机验证码
 * @param length 验证码的长度
 * @retu