how to decode x-www-form-urlencoded form data in java which has an unescaped % as data

Question

I have a situation where I receive x-www-form-urlencoded form data and I need to decode it.

how to decode x-www-form-urlencoded form data in java which has an unescaped % as data. e.g. "APL%205%%20off" which actually shall be decoded as "APL 5% off" However, I tried a few code snippets in java as well as a few online decoders and all throw an exception or say invalid.

I need the understanding on how this can be achieved? If anyone can share the knowledge would be greatly appreciated.

Answer 1

'%' in urlEncode encoding is '%25', so decoding 'APL%205%%20off' will cause issues. Normally, it should be encoded as 'APL%205%25%20off' to decode and get the expected result "APL 5% off". If you must decode this string, you can try the following code:

public static String decodeFormData(String formData) {
        try {
            return URLDecoder.decode(formData, StandardCharsets.UTF_8.toString());
        } catch (Exception e) {
            // Manually handle decoding for malformed input
            StringBuilder decoded = new StringBuilder();
            for (int i = 0; i < formData.length(); i++) {
                char c = formData.charAt(i);
                if (c == '%' && i + 2 < formData.length()) {
                    String hex = formData.substring(i + 1, i + 3);
                    try {
                        int charCode = Integer.parseInt(hex, 16);
                        decoded.append((char) charCode);
                        i += 2;
                    } catch (NumberFormatException ignored) {
                        decoded.append(c);
                    }
                } else {
                    decoded.append(c);
                }
            }
            return decoded.toString();
        }
    }

    public static void main(String[] args) {
        String formData = "APL%205%%20off";
        String decoded = decodeFormData(formData);
        System.out.println(decoded); // Output: APL 5% off
    }

or try this

private static String decodeUrl(String url) {
        String decodeUrl = "";
        try {
            String transformUrl = url.replaceAll("%(?![0-9a-fA-F]{2})", "%25");
            decodeUrl = URLDecoder.decode(transformUrl, "UTF-8");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            return decodeUrl;
        }
    }