URLDecoder异常Illegal hex characters in escape (%)

URLDecoder.decode(param,"utf-8");

public static String replacer(StringBuffer outBuffer) {
      String data = outBuffer.toString();
      try {
         data = data.replaceAll("%(?![0-9a-fA-F]{2})", "%25");
         data = data.replaceAll("\\+", "%2B");
         data = URLDecoder.decode(data, "utf-8");
      } catch (Exception e) {
         e.printStackTrace();
      }
      return data;
   }

public static String decode(String s, String enc)
        throws UnsupportedEncodingException{

        boolean needToChange = false;
        int numChars = s.length();
        StringBuffer sb = new StringBuffer(numChars > 500 ? numChars / 2 : numChars);
        int i = 0;

        if (enc.length() == 0) {
            throw new UnsupportedEncodingException ("URLDecoder: empty string enc parameter");
        }

        char c;
        byte[] bytes = null;
        while (i < numChars) {
            c = s.charAt(i);
            switch (c) {
            case '+':
                sb.append(' ');
                i++;
                needToChange = true;
                break;
            case '%':
                /*
                 * Starting with this instance of %, process all
                 * consecutive substrings of the form %xy. Each
                 * substring %xy will yield a byte. Convert all
                 * consecutive  bytes obtained this way to whatever
                 * character(s) they represent in the provided
                 * encoding.
                 */

                try {

                    // (numChars-i)/3 is an upper bound for the number
                    // of remaining bytes
                    if (bytes == null)
                        bytes = new byte[(numChars-i)/3];
                    int pos = 0;

                    while ( ((i+2) < numChars) &&
                            (c=='%')) {
                        int v = Integer.parseInt(s.substring(i+1,i+3),16);
                        if (v < 0)
                            throw new IllegalArgumentException("URLDecoder: Illegal hex characters in escape (%) pattern - negative value");
                        bytes[pos++] = (byte) v;
                        i+= 3;
                        if (i < numChars)
                            c = s.charAt(i);
                    }

                    // A trailing, incomplete byte encoding such as
                    // "%x" will cause an exception to be thrown

                    if ((i < numChars) && (c=='%'))
                        throw new IllegalArgumentException(
                         "URLDecoder: Incomplete trailing escape (%) pattern");

                    sb.append(new String(bytes, 0, pos, enc));
                } catch (NumberFormatException e) {
                    throw new IllegalArgumentException(
                    "URLDecoder: Illegal hex characters in escape (%) pattern - "
                    + e.getMessage());
                }
                needToChange = true;
                break;
            default:
                sb.append(c);
                i++;
                break;
            }
        }

        return (needToChange? sb.toString() : s);
    }