从Android到使用蓝牙的PC的文本消息有时为null

Question

Im trying to send Text from a client android to a server java on PC

here is the android side code

protected static final UUID MY_UUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

private String BluetoothMacAddress = "A8:A7:95:65:26:BA";
protected void onCreate() {
    initBluetoothDevice();
    CheckBluetoothState();
    device = mBluetoothAdapter.getRemoteDevice(getBluetoothMacAddress());
    new Send("Hello from Client").execute();
    mBluetoothAdapter.cancelDiscovery();
}

class Send extends AsyncTask<Void, Void, Void> {

    BluetoothSocket btSocket;
    //private Socket client;
    private String TextMessage;
    //private PrintWriter toServer;
    protected OutputStream outStream = null;
    public Send(String string) {
        TextMessage = string;
    }

    @Override
    protected Void doInBackground(Void... params) {
        try {
            Log.d(TAG, "doInBackground: "+TextMessage);
            //client = new Socket(IpAdress, Port); // connect to the server Ip given by my mobile hotspot
            btSocket = device.createRfcommSocketToServiceRecord(MY_UUID);
            btSocket.connect();

            outStream = btSocket.getOutputStream();
            byte[] msgBuffer = TextMessage.getBytes();
            outStream.write(msgBuffer);
            outStream.flush();
            btSocket.close(); // closing the connection

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

here is the java code

public class SimpleSPPServer {
StreamConnection connection;
StreamConnectionNotifier streamConnNotifier;
InputStream inStream;
BufferedReader bReader;

private void startServer() throws IOException{

    //Create a UUID for SPP
    UUID uuid = new UUID("0001101", true);
    //Create the servicve url
    String connectionString = "btspp://localhost:" + uuid +";name=Sample SPP Server";

    //open server url
    streamConnNotifier = (StreamConnectionNotifier)Connector.open( connectionString );

    //Wait for client connection
    System.out.println("\nServer Started. Waiting for clients to connect...");
    connection = streamConnNotifier.acceptAndOpen();

    RemoteDevice dev = RemoteDevice.getRemoteDevice(connection);
    System.out.println("Remote device address: "+dev.getBluetoothAddress());
    System.out.println("Remote device name: "+dev.getFriendlyName(true));

    while(true){
        try{
            //streamConnNotifier = (StreamConnectionNotifier)Connector.open( connectionString );
            connection = streamConnNotifier.acceptAndOpen();

            inStream = connection.openInputStream();
            bReader=new BufferedReader(new InputStreamReader(inStream));
            String lineRead = bReader.readLine();
            System.out.println(lineRead);
        inStream.close();
    }catch(Exception e){
        e.printStackTrace();
        //  break;
    }
    }
}

}

the code works but the problem is that the received message is sometimes null, can anyone tell me where is the problem and how to fix it

Answer 1

Looking up the documentation for BufferedReader#readLine() it specifies the following as return value:

A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

So when it returns a null that means the underlying data stream did not have any further data. With any communication between two computers like bluetooth it might just be a case of communication failure and should be expected to happen every so often.