给定字符集,创建某一位数的随机字符串

public static String getRandomString(int length) {
		String base = "abcdefghijklmnopqrstuvwxyz0123456789";
		SecureRandom random = new SecureRandom();
		StringBuffer sb = new StringBuffer();
		for (int i = 0; i < length; i++) {
			int number = random.nextInt(base.length());
			sb.append(base.charAt(number));
		}
		return sb.toString();
	}

字符串拼接的效率问题:
        append的效率最高.
        str = str + "a";
        上面的实现机制是:str = new StringBuilder(str).append("JTZen9").toString();