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[fixed] Problems to pass byte arrays via JNDI for Bluetooth on Android

  • Hi,
    I am trying to use an RFCOMM socket in a Qt Application for Android mobiles. Since the Bluetooth stuff is (not yet) implemented in Qt and Android provides only a Java interface, I have to use JNI. Please correct me if that's incorrect.

    First, I wrote some quick and dirty lines of Java code to evaluate the Java API, and it works fine.

    BluetoothSocket socket=device.createRfcommSocketToServiceRecord(UUID.fromString("00001101-0000-1000-8000-00805F9B34FB"));
    InputStream istream=socket.getInputStream();
    OutputStream ostream=socket.getOutputStream();
    int avail=istream.available();
    Log.w(this.getClass().getSimpleName(),"Received "+avail+" bytes");

           BufferedReader br = new BufferedReader(new InputStreamReader(istream));
           String s=br.readLine();
           Log.w(this.getClass().getSimpleName(),"Received: "+s);


    Now I want to do the same in my Qt (c++) program. I can open the RFCOMM connection, and I can get the nput stream and output stream objects. But the remote device does not receive the command "oPA3,0" and the program does not receive the expected answer (3 bytes = "Ok\n").

    qWarning("Opening RFCOMM connection");
    QAndroidJniObject string = QAndroidJniObject::fromString("00001101-0000-1000-8000-00805F9B34FB");
    if (!string.isValid()) qFatal("Cannot create String");
    QAndroidJniObject uuid=QAndroidJniObject::callStaticObjectMethod("java/util/UUID","fromString","(Ljava/lang/String;)Ljava/util/UUID;",string.object<jstring>());
    if (!uuid.isValid()) qFatal("Cannot create UUID");
    QAndroidJniObject socket=testDevice->callObjectMethod("createRfcommSocketToServiceRecord","(Ljava/util/UUID;)Landroid/bluetooth/BluetoothSocket;",uuid.object<jobject>());
    if (!socket.isValid()) qFatal("Cannot create RFCOMM socket");
    jboolean connected=socket.callMethod<jboolean>("isConnected");
    if (!connected) qFatal("Cannot open RFCOMM connection");
    qWarning("Connection established");
    QAndroidJniObject istream=socket.callObjectMethod("getInputStream","()Ljava/io/InputStream;");
    if (!istream.isValid()) qFatal("Cannot open input stream");
    QAndroidJniObject ostream=socket.callObjectMethod("getOutputStream","()Ljava/io/OutputStream;");
    if (!ostream.isValid()) qFatal("Cannot open output stream");

        qWarning("Sending test data");
        QAndroidJniEnvironment env;
        char data[100];
        jbyteArray buffer=env->NewByteArray(100);
        jint read=istream.callMethod<jint>("available");
        qWarning("Received %i bytes",read);


    I think my probloem is that I have no idea how to pass the command string to Java and how to pass the response back to C++. Does anybody here know how to do that? Or maybe there is somewhere and example that I could look at?

  • I also tried sending single bytes instead of byte arrays, but again the bluetooth device did not receive a single character.

    At least sending byte arrays should work, because it's described here:

    Any ideas?

  • I found the problen cause: I needed to add some few type castings to ostream.callMethod and add the return type "V" to the method signature.

    So I was able to finish my class which I now use to communicate with serial bluetooth devices (RFCOMM protocol, SPP profile).

    If you are interested in the full source, checkout

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