Java decompress zlib

Question

I have an array of bytes, The Hex starts (it's big) with this:

78 9C 9D 5B 4B 6F A4 38 10 FE 2F 9C 19 89 F2 83 47 1F 77 46 7B DA D3 CE 6A 2F A3 08 91 B4 93 B4 96 40 0B E8 64 A2 28 FF 7D A1 B1 3B 2E C0 ED 72 4F 94 E9 04 EA 61 57 7D F5 32 E4 23 7A AB 5E 55 D9 9C 5E A2 5D 01 10 47 55 B3 EF DA C3 BE 7C A8 0F AA 19 A2 DD D0 9D

It starts with 0x789c which is the lzma headers.

I am running it through the following method:

public byte[] decompress(byte[] bytesToDecompress)
    {
        byte[] returnValues = null;

        Inflater inflater = new Inflater();

        int numberOfBytesToDecompress = bytesToDecompress.length;

        inflater.setInput
        (
            bytesToDecompress,
            0,
            numberOfBytesToDecompress
        );

        int bufferSizeInBytes = numberOfBytesToDecompress;

        int numberOfBytesDecompressedSoFar = 0;
        List<Byte> bytesDecompressedSoFar = new ArrayList<Byte>();

        try
        {
            while (inflater.needsInput() == false)
            {
                byte[] bytesDecompressedBuffer = new byte[bufferSizeInBytes];

                int numberOfBytesDecompressedThisTime = inflater.inflate
                (
                    bytesDecompressedBuffer
                );

                numberOfBytesDecompressedSoFar += numberOfBytesDecompressedThisTime;

                for (int b = 0; b < numberOfBytesDecompressedThisTime; b++)
                {
                    bytesDecompressedSoFar.add(bytesDecompressedBuffer[b]);
                }
            }

            returnValues = new byte[bytesDecompressedSoFar.size()];
            for (int b = 0; b < returnValues.length; b++) 
            {
                returnValues[b] = (byte)(bytesDecompressedSoFar.get(b));
            }

        }
        catch (DataFormatException dfe)
        {
            dfe.printStackTrace();
        }

        inflater.end();

        return returnValues;
    }

    public String decompressToString(byte[] bytesToDecompress)
    {   
        byte[] bytesDecompressed = this.decompress
        (
            bytesToDecompress
        );

        String returnValue = null;

        try
        {
            returnValue = new String
            (
                bytesDecompressed,
                0,
                bytesDecompressed.length,
                "UTF-8"
            );  
        }
        catch (UnsupportedEncodingException uee)
        {
            uee.printStackTrace();
        }

        return returnValue;
    }

Like this: System.out.println(decompressToString(jsonDecompressed));

However I am getting nothing printed out.. There is also no stacktrace...

How come this is happening?

Try to use the IDE debuging capabilites, soon or later you will need to use it, probably numberOfBytesDecompressedThisTime is 0 or inflater.needsInput() = true — Petter Friberg, Oct 10 '15 at 12:13

score 1 · Accepted Answer · edited May 23 '17 at 10:29

Your code is already correct you just have to convert the hex string to a byte array using the method from the following question:

Convert a string representation of a hex dump to a byte array using Java?

public static byte[] hexStringToByteArray(String s) {
    int len = s.length();
    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
        data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                             + Character.digit(s.charAt(i+1), 16));
    }
    return data;
}

You can then use the previous method with the hex string (without spaces)

 public static void main(String[] args) {
    String strData = "789C9D5B4B6FA43810FE2F9C1989F283471F77467BDAD3CE6A2FA30891B493B496400BE864A228FF7DA1B13B2EC0ED724F94E904EA61577DF532E4237AAB5E55D99C5EA25D01104755B3EFDAC3BE7CA80FAA19A2DDD09D";
    byte[] jsonDecompressed = hexStringToByteArray(strData);
    System.out.println(decompressToString(jsonDecompressed));
}

and you obtain the correct decompressed value:

{"wave_num":911,"android_client":tr

Java decompress zlib

1 Answers1