How to connect and send message to multiple devices via blue tooth in Android?

try to create separate threads for each connection - I had a similar problem and creating a new thread for each connection solved it very well. By the way, I even create a new thread to establish a connection - so establishing a connection doesn't block the UI. Got this from BT sample code ...

to create a new thread to establish a connection:

    mConnectBluetoothThread = new ConnectBluetoothThread(device);
    mConnectBluetoothThread.start();

      

        
        
        
      

    

where ConnectBluetoothThread is defined as:

public ConnectBluetoothThread(BluetoothDevice device) {
        if (DEBUG)
        Log.i(this.getClass().getSimpleName(),
            this.getClass().getName()
                + " ->"
                + Thread.currentThread().getStackTrace()[2]
                    .getMethodName());

        mmDevice = device;
        BluetoothSocket tmp = null;

        // Get a BluetoothSocket for a connection with the
        // given BluetoothDevice
        try {
        tmp = device.createRfcommSocketToServiceRecord(MY_UUID);
        } catch (IOException e) {
        Log.e(this.getClass().getSimpleName(), "create() failed", e);
        }
        mmSocket = tmp;
    }

    public void run() {
        if (DEBUG)
        Log.i(this.getClass().getSimpleName(),
            this.getClass().getName()
                + " ->"
                + Thread.currentThread().getStackTrace()[2]
                    .getMethodName());
        // TODO
        setName("ConnectThread");

        // Always cancel discovery because it will slow down a connection
        mBluetoothAdapter.cancelDiscovery();

        // Make a connection to the BluetoothSocket
        try {
        // This is a blocking call and will only return on a
        // successful connection or an exception
        mmSocket.connect();
        } catch (IOException e) {

        connectionFailed();

        // Close the socket
        try {
            mmSocket.close();
        } catch (IOException e2) {
            Log.e(this.getClass().getSimpleName(),
                "unable to close() socket during connection failure",
                e2);
        }

        return;
        }

        // Reset the ConnectThread because we're done
        synchronized (InterBT.this) {
        mConnectBluetoothThread = null;
        }

        // Start the connected thread
        connected(mmSocket, mmDevice);
    }

    public void cancel() {
        try {
        mmSocket.close();
        } catch (IOException e) {
        Log.e(this.getClass().getSimpleName(),
            "close() of connect socket failed", e);
        }
    }
    }

    public synchronized void connected(BluetoothSocket socket,
        BluetoothDevice device) {
    if (DEBUG)
        Log.d(this.getClass().getSimpleName(), "connected");

    // Cancel the thread that completed the connection
    if (mConnectBluetoothThread != null) {
        mConnectBluetoothThread.cancel();
        mConnectBluetoothThread = null;
    }

    // Cancel any thread currently running a connection
    if (mConnectedBluetoothThread != null) {
        mConnectedBluetoothThread.cancel();
        mConnectedBluetoothThread = null;
    }

    // Cancel the accept thread because we only want to connect to one
    // device
    // if (mAcceptThread != null) {mAcceptThread.cancel(); mAcceptThread =
    // null;}

    // Start the thread to manage the connection and perform transmissions
    mConnectedBluetoothThread = new ConnectionThreadBT(socket);
    mConnectedBluetoothThread.start();

    setState(STATE_CONNECTED);

    }

      

        
        
        
      

    

and also create a new class ConnectionThreadBT

that handles the read / write connection:

public class ConnectionThreadBT extends ConnectionThreadBase {
    private static final boolean DEBUG = true;

    private final BluetoothSocket mmSocket;
    private final InputStream mmInStream;
    private final OutputStream mmOutStream;
    byte[] responseBuffer = new byte[4096 * 4]; 
    int responseBufferLen = 0;


    public ConnectionThreadBT(BluetoothSocket socket) {
    if (DEBUG)
        Log.i(this.getClass().getSimpleName(),
            this.getClass().getName()
                + " ->"
                + Thread.currentThread().getStackTrace()[2]
                    .getMethodName());

    mmSocket = socket;
    InputStream tmpIn = null;
    OutputStream tmpOut = null;

    // Get the BluetoothSocket input and output streams
    try {
        tmpIn = socket.getInputStream();
        tmpOut = socket.getOutputStream();
    } catch (IOException e) {
        Log.e(this.getClass().getSimpleName(), "temp sockets not created",
            e);
    }

    mmInStream = tmpIn;
    mmOutStream = tmpOut;
   }

    public void run() {
    if (DEBUG)
        Log.i(this.getClass().getSimpleName(),
            this.getClass().getName()
                + " ->"
                + Thread.currentThread().getStackTrace()[2]
                    .getMethodName());
    //we have successfully connected to BT

    //now inform UI 

    Home_Screen.sendMessageToHomeScreen(
        Home_Screen.MESSAGE_INTERBT_CONNECTION_TESTED,
        Home_Screen.CONNECTION_SUCCESS, true);
  }

      

        
        
        
      

    

and then just write this method which is also defined in ConnectionThreadBT

public void sendMsg(MyBuffer buffer){
        try {
        mmOutStream.write(buffer);
        mmOutStream.flush();
        successfullyWritten = true;
        } catch (IOException e) {
        Log.e(this.getClass().getSimpleName(),
            "Exception during write", e);
        successfullyWritten = false;
        }

      

        
        
        
      

    

to read, either do the same, or start a monitoring loop in the startup method that continues reading as long as the connected Thread is alive and reports any read information through a handler like updating the UI screen.