按双字节长度来取String

    /*
     * String.valueOf(arr[i]).matches("[\u4e00-\u9fa5]") 只是汉字
     * String.valueOf(arr[i]).matches("[^x00-xff]") 双字节（包括汉字）
     */
    public static String getStrBylength(String str , int len)
    {
        if(null == str)
            return "";
        int sl = str.getBytes().length;
        if(sl>len*2)
        {
            StringBuffer sb = new StringBuffer();
            char[] arr = str.toCharArray();
            for (int i = 0,j=0; i<arr.length && j<len*2; i++) {
                if (String.valueOf(arr[i]).matches("[^x00-xff]")) {
                    j+=2;
                } else {
                    j++;
                }
                if(j==len*2-1 && String.valueOf(arr[i]).matches("[^x00-xff]"))
                {
                    
                }
                else
                {
                    sb.append(arr[i]);
                }            
            }
            return sb.toString()+"...";
        }
        else
        {
            return str;
        }
    }