Deserialize QByteArray to Java data types

Question

I am currently working on a Client-Server connection.

The client is written in Java not using QT running on an Android phone, the server is written in C++ using the Qt framework.

The client recieves QByteArrays , QStrings and QLists, but I have no clue how to deserialize and interpret the incomming data.

The C++ source code that creates the packet that I have to read on the java client side looks like this:

QByteArray body;
QString string1, string2, string3;
QList<float> list;
qint8 recognitionCount;

QDataStream bodyStream(&body, QIODevice::WriteOnly);

bodyStream << recognitionCount;

bodyStream << string1.toUtf8()
<< string2.toUtf8()
<< string3.toUtf8()
<< list;

The connection between client and server is up and running fine. I understand how to read integers that the server sends me for example. I also know how to read bytes, but what am I supposed to do with these bytes? How can I format them to a String for example?

Can anyone help me out?

I really appreciate any help!

Answer 1

You can convert from Qt Strings to Java like this (according to this definition):

final static int MAX_STRING_LENGTH = 10240; // arbitrary number
private final static ByteBuffer stringBytes = ByteBuffer.allocate(MAX_STRING_LENGTH);

static String readStringFromQTStream(final ObjectInputStream in) throws IOException {
  if (in.available() < (Integer.SIZE / 8)) { // check that there are at least 4 bytes for the length
    throw new IOException("Illegal data received: expected integer but only got " + in.available() + " bytes");
  }
  final int stringLength = in.readInt();
  if (stringLength == 0xFFFFFFFF) { // Qt for null string
    return null;
  }
  if ((stringLength < 0) || (stringLength > stringBytes.capacity())) { // check for malformed data
    throw new IOException("Illegal data received: string with supposed length of " + stringLength + " bytes");
  }

  stringBytes.clear(); // global var, not thread-safe!
  in.readFully(stringBytes.array(), 0, stringLength);
  stringBytes.flip();
  return StandardCharsets.UTF_8.decode(stringBytes).toString();
}

Please note that the ByteBuffer is reused, which is a good performance increase if you read data frequently, but this is of course not thread-safe without further code.

Answer 2

The format of QDataStream is documented at e.g. http://qt-project.org/doc/qt-4.8/datastreamformat.html. This should tell you what you need to know to deserialise it to Java types.

Answer 3

On your server you have to know what the client 'ask' and answer correctly. On the client-side the same, you have to understand what the server 'answer'. So you have to create your own 'protocol' of communication between your server and client.

What is exchanged through the wire is only a sequence of bytes and not a QByteArray nor a QList.

The API of QTcpSocket gives you everything you need to know the client connected (if the application is supposed to support more than one client is your business to make it possible) and how to write to the socket.

On client side will be almost the same, but I have never developed in Android so can't suggest anything.

This my answer explains very briefly how to build an application client-server in Qt. Maybe can be useful.

Answer 4

In case anyone comes across this thread:

I recommend using Protocol Buffers for data transfer on the wire instead.