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Bluetooth connected but not sending data

I'm working on a project connecting my Android phone to an Arduino bluetooth transceiver. It works fine until the phone goes into suspend (sleep?) mode; when that happens, the bluetooth connection I establish seems to still exist, but no data is transferred. I'm using Android Sdk 30.
Here's what I do: I have a UI list with BT devices for the user to choose; after one is selected, this is called to connect:
// in my BluetoothService class
private BluetoothDevice bluetoothDevice;
private BluetoothSocket bluetoothSocket;
public void connect(BluetoothDevice device) {
try {
bluetoothDevice = device;
String uuid = getString(R.string.bluetooth_uuid);
bluetoothSocket = bluetoothDevice.createInsecureRfcommSocketToServiceRecord(UUID.fromString(uuid));
bluetoothSocket.connect();
if (bluetoothSocket.isConnected()) {
Log.i(TAG, "connected to " + device + " with socket " + bluetoothSocket);
} else {
Log.w(TAG, "still not connected to " + device + " with socket " + bluetoothSocket);
}
} catch (IOException e) {
Log.e(TAG, "failed to connect to device", e);
}
}
So again, this works fine; the connection is established and data is sent/received.
Now, I send data every 30 seconds, and on the receiver side light an led until 60 seconds have passed with no data received.
private ScheduledFuture<?> sendScheduler; // canceled in onDestroy()
public void onCreate() {
sendScheduler = Executors.newSingleThreadScheduledExecutor().scheduleAtFixedRate(() -> {
sendData(lastLevelSent); // i can post sendData, but it works in normal state
}, 30, 30, TimeUnit.SECONDS);
}
Now when the phone goes into sleep (ie I press the power button once), the led turns off after 60 seconds, ie no data is being received. However, the connection itself seems to still be established (I can tell by how the receiver led blinks).
When I wake the device back up, it does not resume sending data. The disconnect button in my app doesn't work (bluetoothSocket.close()), and trying to connect to the device again doesn't either. I have to kill the app, then I can disconnect.
The check I'm using whether the connection is still valid is bluetoothSocket != null && bluetoothSocket.isConnected().
I did add my app to the "don't put into standby" in the Android settings.
So how can I either convince the app to continue sending data while in sleep mode, or at least reliably check the connection status?

Java serversocket not detecting lost connection

I have a socket client (on android phone) and server (on PC) both on a wifi network and the server successfully reads data from the client.
However, when I turn off the wifi on the phone the server read just hangs, whereas I was hoping some error would be thrown.
I do have setSoTimeout set on the server, but the read is not timing out.
On the PC netstat still shows an established connection
netstat -na | grep 6668
TCP 192.168.43.202:6668 192.168.43.26:43076 ESTABLISHED
Is there a way to tell if the client host has disappeared, or getting the read to time out?
Here is the server read
if (ss.isConnected()) {
try {
readData();
} catch (java.net.SocketTimeoutException ex) {
logger.warning(ex.toString());
} catch (InterruptedIOException ex) {
logger.warning(ex.toString());
} catch (IOException ex) {
logger.log(Level.WARNING, "Data communication lost will close streams - IOEx - socket status {0}", ss.socketStatus());
closeStreams();
} catch (Exception ex) {
logger.log(Level.WARNING, "Data communication lost will close streams - Ex - socket status {0}", ss.socketStatus());
closeStreams();
}
}
Where readData is,
public void readData() throws IOException {
for (int i = 0; i < data.length; i++) {
data[i] = ss.readDouble();
}
}
ss.readDouble() is,
public double readDouble() throws IOException {
return in.readDouble();
}
And the server connection,
public void connect() throws IOException {
if (serverSocket == null || serverSocket.isClosed()) {
init();
}
logger.log(Level.INFO, "Wait on " + serverSocket.getLocalPort());
server = serverSocket.accept();
serverSocket.close();
logger.log(Level.INFO, "Connected to {0}", server.getRemoteSocketAddress());
out = new DataOutputStream(server.getOutputStream());
in = new DataInputStream(server.getInputStream());
}

Make a timeout, so let's say no data has been sent for 10 minutes, close it in 60 seconds!
Setting a timeout for socket operations
The answer for this question may help you.

This is nature of TCP connection, not java sockets per se. If the remote peer disconects with broken connection, how should your server know that the peer simply has no data to send?
Writting on closed socket will cause exception, read will simply block if client doesnt end tcp connection properly, for the reason above.
If you go through socket API, you will find option to set timeout ( before proceeding with blocking operation).
You could also consider TCP KEEP Alive, which is also exposed by the Socket API.
// Edit: additional information as per the OP comment
When your client connects to server, you create a client socket to communicate with the peer. Your server socket is the one at which you are listening for new client connections. It is the client socket at which you specify keep alive or read timeout because this is the socket from which you read/write.
// your server is actually reference to ClientSocket
server = serverSocket.accept();
// keep alive duh
server.setKeepAlive(true);
serverSocket.close();

Is it possible to make insecure connection between android bluetooth and laptop bluetooth device?

I am trying to connect android device with laptop or desktop which contains Bluetooth via Bluetooth socket connection.
I have created one android application (Client) which tries to connect laptop Bluetooth device where java application (Server) is running.
My concern is that, Is it possible to connect both the device insecurely (without pin authentication) using Bluetooth socket connection?
If possible, Please suggest me solution.
If not, Is there any way to programmatically auto pair both the devices?
Thanks in advance !!!

By referring java api for bluetooth, I got the solution for Insecure connection between two Android and laptop Bluetooth devices.
I have used SPP client server mechanism.
My server is in java.
In java add certain parameters to URL.
Make authentication= false; authorize=false;encrypt=false;
open this URL for connection acceptance.
//Create a UUID for SPP
UUID uuid=new UUID("0f2b61c18be240e6ab90e735818da0a7", false);
System.out.println("\n"+uuid.toString());
//Create the servicve url
String url="btspp://localhost:"+uuid.toString()+";"+"name=remoteNotifier;authenticate=false;authorize=false;encrypt=false";
//open server url
StreamConnectionNotifier streamConnNotifier = (StreamConnectionNotifier)Connector.open(url);
//Create a UUID for SPP
UUID uuid=new UUID("0f2b61c18be240e6ab90e735818da0a7", false);
System.out.println("\n"+uuid.toString());
//Create the servicve url
String url="btspp://localhost:"+uuid.toString()+";"+"name=remoteNotifier;authenticate=false;authorize=false;encrypt=false";
//open server url
StreamConnectionNotifier streamConnNotifier = (StreamConnectionNotifier)Connector.open(url);
Now on client side:
Android API 10 above contains insecure connection method. "createInsecureRfcommSocketToServiceRecord(UUID)"
So use this method for connection. It will not pop up for pairing request adn try to connect with remote Bluetooth device where Java server is already running.
Code:
// Set up a pointer to the remote node using it's address.
BluetoothDevice device = mBluetoothAdapter.getRemoteDevice(address);
// Two things are needed to make a connection:
// A MAC address, which we got above.
// A Service ID or UUID. In this case we are using the
// UUID for SPP.
try {
// btSocket = device.createRfcommSocketToServiceRecord(MY_UUID);
btSocket = device.createInsecureRfcommSocketToServiceRecord(MY_UUID);
} catch (IOException e) {
AlertBox("Fatal Error", "In onResume() and socket create failed: " + e.getMessage() + ".");
}
// Discovery is resource intensive. Make sure it isn't going on
// when you attempt to connect and pass your message.
mBluetoothAdapter.cancelDiscovery();
// Establish the connection. This will block until it connects.
try {
btSocket.connect();
out.append("\n...Connection established and data link opened...");
} catch (IOException e) {
try {
btSocket.close();
e.printStackTrace();
} catch (IOException e2) {
e2.printStackTrace();
AlertBox("Fatal Error", "In onResume() and unable to close socket during connection failure" + e2.getMessage() + ".");
}
}
// Create a data stream so we can talk to server.
out.append("\n...Sending message to server...");
try {
outStream = btSocket.getOutputStream();
} catch (IOException e) {
AlertBox("Fatal Error", "In onResume() and output stream creation failed:" + e.getMessage() + ".");
}
// Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.android_logo);
// byte[] msgBuffer = getBytesFromBitmap(bitmap);
String message = "Hello from Android.\n";
byte[] msgBuffer = message.getBytes();
try {
outStream.write(msgBuffer);
} catch (IOException e) {
String msg = "In onResume() and an exception occurred during write: " + e.getMessage();
if (address.equals("00:00:00:00:00:00")) {
msg = msg + ".\n\nUpdate your server address from 00:00:00:00:00:00 to the correct address on line 37 in the java code";
msg = msg + ".\n\nCheck that the SPP UUID: " + MY_UUID.toString() + " exists on server.\n\n";
}
// AlertBox("Fatal Error", msg);
}
I have provided only required code.
For connection UUID should be same for both the devices.
Provide server Bluetooth MAC address at "address" field at client side.
We are able to communicate with remote Bluetooth device insecurely (Without pairing).
But this code is device dependent...
certain device are able to communicate very efficiently.
Like Lenovo laptop, external bluetooth device for PC for Java server
AND
Android devices DELL venue 7, Sony, LG mobiles for client.
Tested and working properly.
But in Dell laptop, Micromaxx, xolo mobile it doesn't work.
I don't know why it is not working, If anyone knows please give the solution.

For Bluetooth 2.1 and above devices, security is mandatory.
If you are just trying to avoid the passkey entry/display, you can set the security requirements on the laptop and android device to "MITM protection not required".
This way the devices will pair automatically, but the link would be susceptible to man in the middle attacks.

BluetoothServerSocket doesn't return from accept()

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.

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.

I've been facing this problem for a couple of days. Finally, I realized why:
I was creating the Thread that accepts incoming connections in the server twice. Thus, the ServerSocket was being created to times, although only the second time the accept() method was called.
This leads to server not accepting any connection!!

It seems that you didn't call socket.connect() from client side in the shown codes.

Today I continued work on project. I got IOException after failing connect() from connecting device.
Now I managed the devices to have a socket, after pairing them before running the app.
EDIT: accept() returns a socket now, but it isn't connected when asking with isConnected().
Socket of the connecting device is connected.

How to get and/or set the link key when pairng bluetoth devices using Android(Java)?

I'm really new on this of Android development, so I have many questions. Right now I'm trying to connect two devices via Bluetooth. So far I was able to do this, I have connected two tablets, but now I'm asked to save the LinkKey and use some bytes of it to create a PIN. So, how can I get that LinkKey? I've read that it's saved on both devices for future connections, but I don't know how to get it.
Also, I've read that this devices can delete those LinkKey, what happens then? If they try to connect with each other again, will they use the same LinkKey or a new one is created?
And the last thing. I have to connect a tablet to a device that has neither a display nor a keypad, a normal bluetooth connection is enough? I'm doing this to stablish the connection:
final UUID MY_UUID = UUID.randomUUID();
//tmp = device.createRfcommSocketToServiceRecord(MY_UUID);
Method m = null;
try {
m = device.getClass().getMethod("createRfcommSocket", new Class[] {int.class});
} catch (NoSuchMethodException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
tmp = (BluetoothSocket) m.invoke(device, 1);
mmSocket = tmp;
Thread connectionThread = new Thread(new Runnable(){
public void run() {
// Cancel discovery because it will slow down the connection
mBluetoothAdapter.cancelDiscovery();
try {
// Connect the device through the socket. This will block
// until it succeeds or throws an exception
mmSocket.connect();
} catch (IOException connectException) {
// Unable to connect; close the socket and get out
try {
mmSocket.close();
} catch (IOException closeException) { }
return;
}
// Do work to manage the connection (in a separate thread)
manageConnectedSocket(mmSocket);
}
Thanks.

Link key creation is done on the stack/LMP(Link Manager) level, not on the app level. Bluetooth devices check if the link key exist or not, if exist, it will not go for pairing and bluetooth connection will proceed, if no link key is found on the bluetooth stack, LMP(Link Manager) will initiate a new pairing process , once pairing completed, bluetooth connection will proceed. If link key is deleted, a new link key will be generated for a connection.
Try checking in adb shell (/data/misc/bluetoothd or data/misc/bluetooth) for link key , it will be available in the paired device info.