返回指定字节长度的字符串

/** 
     * 功能描述：返回指定字节长度的字符串 
     *  
     * @param str 
     *            String 字符串 
     * @param length 
     *            int 指定长度 
     * @return String 返回的字符串 
     */
    public static String toLength(String str, int length) { 
        if (str == null) { 
            return null; 
        } 
        if (length <= 0) { 
            return ""; 
        } 
        try { 
            if (str.getBytes("GBK").length <= length) { 
                return str; 
            } 
        } catch (Exception e) { 
        } 
        StringBuffer buff = new StringBuffer(); 
  
        int index = 0; 
        char c; 
        length -= 3; 
        while (length > 0) { 
            c = str.charAt(index); 
            if (c < 128) { 
                length--; 
            } else { 
                length--; 
                length--; 
            } 
            buff.append(c); 
            index++; 
        } 
        buff.append("..."); 
        return buff.toString(); 
    }