BluetoothServerSocket doesn't return from accept()

Question

I'm trying to build a little Bluetooth-Android-App for a project in school.

I'm quite new to Android (got my phone since 2 days). I'm experimenting since 2 weeks with android programming on my laptop. Installed a VirtualBox with Android x86 (eeepc) so I can use the BluetoothAdapter of the laptop. Emulator doesn't support Bluetooth and is quite slow. That's about the project...

The problem/question: A Bluetoothconnection has 2 devices - a connecting and a listening one. The listening device has a BluetoothServerSocket, that loops accept() method until accept() returns a BluetoothSocket. In my case the accept() method doesn't return so I get stuck and the app freezes with blackscreen asking mit to stop the app or just to wait. When I pass a timeout to accept() --> accept(10000) I get an IOException after the timeout.

listening device:

private class AcceptThread extends Thread {

    private BluetoothSocket tSocket;
    private BluetoothServerSocket bss = null;

    public void run() {
        try {
            Log.d(TAG, "erzeuge ServerSocket");
            bss = BluetoothAdapter.getDefaultAdapter().listenUsingInsecureRfcommWithServiceRecord("BluetoothChatInsecure", MainActivity.BT_UUID);
            Log.d(TAG, "ServerSocket OK");
        } catch (IOException e) {
            e.printStackTrace();
            Log.e(TAG, "Fehler Serversocket");
        }

        while (true)  {

            Log.d(TAG, "Versuche zu akzeptieren");
            try {
                Log.d(TAG, "Akzeptieren Anfang");
                tSocket = bss.accept(10000);
                //this line is never reached
                Log.d(TAG, "Akzeptieren Ende");

                if (tSocket != null){
                    //Hier wollen wir hin!
                    Log.d(TAG, "Verbindung akzeptiert");
                    ConnectedThread conThread = new ConnectedThread(tSocket);
                    conThread.run();
                    bss.close();
                    break;
                } else {
                    Log.e(TAG, "Fehler, keine Verbindung");
                }
            } catch (IOException e) {
                Log.e(TAG, "IOException währent accept-loop");
                //this exception is triggered every 10 sec, when the accept(10000) times out
                e.printStackTrace();
            }   
        }
        Log.i(TAG, "Acceptthread hat fertig");          
    }

}

connecting device:

try {
    socket = device.createInsecureRfcommSocketToServiceRecord(MainActivity.BT_UUID);

    outstr = socket.getOutputStream();
    instr = socket.getInputStream();

    ois = new ObjectInputStream(instr);
    oos = new ObjectOutputStream(outstr);

} catch (IOException e) {
    e.printStackTrace();
}

I've read a lot of threads on stackoverflow and some other forums about this topic, but I didn't got a solution for the problem. Sorry about my English, but I am not a native speaker.

Thanks for any help!

EDIT:

I forgot to write, that I test the app with 2 devices. My laptop does accept-loop, while I use my phone and try to connect.

Answer 1

This is just the normal behavior: accept() will "wait" (block) until a connection has been made from another device. Then it returns the socket representing that connection for further data transfer.

As you have seen, the timeout is signalled via an IOException. The contract of accept() is that it never returns null but always a valid socket, or fails with an exception thrown.

Therefore, thejh is right in saying that you should have a dedicated thread which waits for connections in accept().
When accept() returns a new socket, you may want to spawn another thread to handle further communication over that socket, while the accept() thread loops to wait for the next connection.

N.b.: You cannot shut down a thread blocked in IO (as in accept()) via Thread.interrupt(), but you have to close the ServerSocket from another thread to cause an IOException to 'wake up' the blocked thread.