Java JPEG 十六进制转 ASCII

Question

I have to read JPEG pictures from a database. They are stored as Hex Strings. For tests I have saved the original hex String to a file, opened it with Notepad++ and applied Convert -> HEX --> ASCII and saved the result. This is a valid JPEG and can be rendered in a browser. I have tried to reproduce this in java.

private String hexToASCII(String hex) {
    StringBuilder output = new StringBuilder();
    for (int i = 0; i < hex.length(); i+=2) {
        String str = hex.substring(i, i+2);
        output.append((char)Integer.parseInt(str, 16));
    }
    return output.toString();
}

When I save the result to disk, the resulting file is no jpg it begins with

ÿØÿà JFIF      ÿÛ C             

If have also tried to convert the original hex string with https://www.rapidtables.com/convert/number/hex-to-ascii.html this gives the same result as my code. The resulting file is no jpg. What is Notepad++ doing and how can I reproduce this in Java? Any advise would be greatly appreciated.

Answer 1

Do not convert the file contents to ASCII, UTF-8, ISO-8859-1 or other character encodings! JPEG files are binary files, and don't need encoding.

I suggest:

private void hexToBin(String hex, OutputStream stream) {
    for (int i = 0; i < hex.length(); i += 2) {
        String str = hex.substring(i, i + 2);
        stream.write(Integer.parseInt(str, 16));
    }
}

And perhaps:

private byte[] hexToBin(String hex) {
    ByteArrayOutputStream stream = new ByteArrayOutputStream(hex.length() / 2);
    hexToBin(hex, stream);
    return stream.toByteArray();
}

Answer 2

You should write the string to a file like this.

    String s = hexToASCII(hex);
    Files.writeString(Paths.get("filename.jpg"), s, StandardCharsets.ISO_8859_1);

In Java, when writing a string to a file, char is encoded into byte depending on the character set. The character set ISO_8859_1 encodes one character straight into one byte.