Send data from phone to laptop via Bluetooth

Question

I already successfully paired my Android (6.0.1) phone (Sony Xperia Z3) with my laptop (running Ubuntu 14.04). I can send files back and forth. The Bluetooth connection menu shows that the connection switch switches to ON if files are sent.

I established a persistent connection using:

sudo rfcomm connect rfcomm0 [MAC ADDRESS] [CHANNEL]

I want to send data from my phone to my laptop via Bluetooth. If I run this code the switch goes on as well, but immediately closes the connection down (switch goes back to OFF).

Logcat shows the following warning after calling init() :

W/BluetoothAdapter: getBluetoothService() called with no BluetoothManagerCallback

and upon calling the write() method this exception:

E/error: error init:java.io.IOException: read failed, socket might closed or timeout, read ret: -1

When connecting using rfcomm some channels fail and refuse the connection. My guess is that I am using the wrong channel.

• How can I know which channel to use when calling rfcomm ?
• How can I specify this channel in my Android app?
• How can I know which UUID to use?
• In the example code the first UUID is used: why?

Answer 1

Somehow I could not get it working with the method createRfcommSocketToServiceRecord .

What I did then was removing:

ParcelUuid[] uuids = device.getUuids();
BluetoothSocket socket = device.createRfcommSocketToServiceRecord(uuids[0].getUuid());

and substitute those lines with:

int channel = 1; // substitute with channel you are listening on
Method m = device.getClass().getMethod("createRfcommSocket",new Class[] { int.class });
BluetoothSocket socket = (BluetoothSocket) m.invoke(device, channel);

Then I issued sudo rfcomm listen rfcomm0 which then shows the channel it is listening on the Linux terminal and I could finally connect!

To answer my own questions:

• How can I know which channel to use when calling rfcomm?
  The Linux terminal shows the channel when issuing sudo rfcomm listen rfcomm0

• How can I specify this channel in my Android app?
  The method I am accessing using reflection has this parameter now ( createRfcommSocket )

• How can I know which UUID to use?
  In this solution none.

• In the example code the first UUID is used: why?
  Does not matter for the found solution.