java按字节数截取字符串

 /**
     * 
     * @param str  要截取的字符串 如输入：测试testing嘻嘻
     * @param length 截取的字符串位数   :12
     * 结果：测试testing...
     * @return
     */
    public String subStringByBytes(String str, int length) {
        String result = "";
        int i = 0;
        int j = 0;

        StringBuffer buff = new StringBuffer(str);
        String stmp;
        int len = buff.length();
        for (i = 0; i < len; i++) {
            if (j < length) {
            stmp = buff.substring(i, i + 1);
            try {
                stmp = new String(stmp.getBytes("utf-8"));
            } catch (Exception e) {
                e.printStackTrace();
            }
            if (stmp.getBytes().length > 1) {
                j += 2;
            } else {
                j += 1;
            }
            result += stmp;
            }else{
                break;
            }
        }
        if (j > length) {
            result = result.substring(0, result.length() - 1);
        }
        result += "...";
        return result;
        }