How to send/receive messages via bluetooth android studio

Question

I am trying to create an app that allows a string to be sent from one Android phone to another. The code for this is provided below. However, it isn't working as I keep getting exceptions from the try catch piece of code under the pairDevice() section. Does anyone know why I might be getting this?

import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.Intent;
import android.os.ParcelUuid;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Scanner;
import java.util.Set;

public class MainActivity extends AppCompatActivity {

    InputStream inStream;
    OutputStream outputStream;
    private static final int REQUEST_ENABLE_BT = 1;

    public void pairDevice() {
        BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
        if (bluetoothAdapter != null && !bluetoothAdapter.isEnabled()) {
            Intent enableBtIntent = new 
            Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);}

        Set<BluetoothDevice> pairedDevices = bluetoothAdapter.getBondedDevices();
        if (pairedDevices.size() > 0) {
            Object[] devices = pairedDevices.toArray();
            BluetoothDevice device = (BluetoothDevice) devices[0];
            ParcelUuid[] uuid = device.getUuids();
            try {
                BluetoothSocket socket = device.createInsecureRfcommSocketToServiceRecord(uuid[0].getUuid());
                socket.connect();
                Toast.makeText(this, "Socket connected", Toast.LENGTH_LONG).show();
                outputStream = socket.getOutputStream();
                inStream = socket.getInputStream();
            } catch (IOException e) {
                Toast.makeText(this, "Exception found", Toast.LENGTH_LONG).show();
            }

        }
    }


public void SendMessage(View v) {
    EditText outMessage = (EditText) findViewById(R.id.editText);
    try {
        if (outputStream != null)
            outputStream.write(outMessage.toString().getBytes());
            TextView displayMessage = (TextView) findViewById(R.id.textView);
            Scanner s = new Scanner(inStream).useDelimiter("\\A");
            displayMessage.setText(s.hasNext() ? s.next() : "");
    } catch (IOException e) {/*Do nothing*/}
    Toast.makeText(this,"No output stream", Toast.LENGTH_LONG).show();
}





@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    pairDevice();
}

}

Answer 1

I have made few changes to your app:-

Firstly, I shifted the code responsible for creating the Bluetooth connection to ConnectThread .

2) Added AcceptThread responsible for listening incoming connections and ConnectedThread maintaining the BTConnection, Sending the data, and receiving incoming data through input/output streams respectively. 3) Created 2 buttons to start ConnectThread and AcceptThread.

NOTE: Make sure both the devices are paired and the device that you are trying to connect to is at the top of the list(or just remove all the paired devices from both the devices and only pair the devices that you want to connect). Also, you must start the AcceptThread before ConnectThread

MAINACTIVITY.JAVA

public class MainActivity extends AppCompatActivity {

    private static final UUID MY_UUID_INSECURE =
            UUID.fromString("8ce255c0-200a-11e0-ac64-0800200c9a66");

    private static final int REQUEST_ENABLE_BT = 1;
    BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
    private BluetoothDevice mmDevice;
    private UUID deviceUUID;
    ConnectedThread mConnectedThread;
    private Handler handler;

    String TAG = "MainActivity";
    EditText send_data;
    TextView view_data;
    StringBuilder messages;






    public void pairDevice(View v) {

        Set<BluetoothDevice> pairedDevices = bluetoothAdapter.getBondedDevices();
        Log.e("MAinActivity", "" + pairedDevices.size() );
        if (pairedDevices.size() > 0) {
            Object[] devices = pairedDevices.toArray();
            BluetoothDevice device = (BluetoothDevice) devices[0];
            //ParcelUuid[] uuid = device.getUuids();
            Log.e("MAinActivity", "" + device );
            //Log.e("MAinActivity", "" + uuid)

            ConnectThread connect = new ConnectThread(device,MY_UUID_INSECURE);
            connect.start();

        }
    }

    private class ConnectThread extends Thread {
        private BluetoothSocket mmSocket;

        public ConnectThread(BluetoothDevice device, UUID uuid) {
            Log.d(TAG, "ConnectThread: started.");
            mmDevice = device;
            deviceUUID = uuid;
        }

        public void run(){
            BluetoothSocket tmp = null;
            Log.i(TAG, "RUN mConnectThread ");

            // Get a BluetoothSocket for a connection with the
            // given BluetoothDevice
            try {
                Log.d(TAG, "ConnectThread: Trying to create InsecureRfcommSocket using UUID: "
                        +MY_UUID_INSECURE );
                tmp = mmDevice.createRfcommSocketToServiceRecord(MY_UUID_INSECURE);
            } catch (IOException e) {
                Log.e(TAG, "ConnectThread: Could not create InsecureRfcommSocket " + e.getMessage());
            }

            mmSocket = tmp;

            // 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) {
                // Close the socket
                try {
                    mmSocket.close();
                    Log.d(TAG, "run: Closed Socket.");
                } catch (IOException e1) {
                    Log.e(TAG, "mConnectThread: run: Unable to close connection in socket " + e1.getMessage());
                }
                Log.d(TAG, "run: ConnectThread: Could not connect to UUID: " + MY_UUID_INSECURE );
            }

            //will talk about this in the 3rd video
            connected(mmSocket);
        }
        public void cancel() {
            try {
                Log.d(TAG, "cancel: Closing Client Socket.");
                mmSocket.close();
            } catch (IOException e) {
                Log.e(TAG, "cancel: close() of mmSocket in Connectthread failed. " + e.getMessage());
            }
        }
    }

    private void connected(BluetoothSocket mmSocket) {
        Log.d(TAG, "connected: Starting.");

        // Start the thread to manage the connection and perform transmissions
        mConnectedThread = new ConnectedThread(mmSocket);
        mConnectedThread.start();
    }

    private class ConnectedThread extends Thread {
        private final BluetoothSocket mmSocket;
        private final InputStream mmInStream;
        private final OutputStream mmOutStream;

        public ConnectedThread(BluetoothSocket socket) {
            Log.d(TAG, "ConnectedThread: Starting.");

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



            try {
                tmpIn = mmSocket.getInputStream();
                tmpOut = mmSocket.getOutputStream();
            } catch (IOException e) {
                e.printStackTrace();
            }

            mmInStream = tmpIn;
            mmOutStream = tmpOut;
        }

        public void run(){
            byte[] buffer = new byte[1024];  // buffer store for the stream

            int bytes; // bytes returned from read()

            // Keep listening to the InputStream until an exception occurs
            while (true) {
                // Read from the InputStream
                try {
                    bytes = mmInStream.read(buffer);
                    final String incomingMessage = new String(buffer, 0, bytes);
                    Log.d(TAG, "InputStream: " + incomingMessage);

                    runOnUiThread(new Runnable() {

                        @Override
                        public void run() {
                            view_data.setText(incomingMessage);
                        }
                    });


                } catch (IOException e) {
                    Log.e(TAG, "write: Error reading Input Stream. " + e.getMessage() );
                    break;
                }
            }
        }


        public void write(byte[] bytes) {
            String text = new String(bytes, Charset.defaultCharset());
            Log.d(TAG, "write: Writing to outputstream: " + text);
            try {
                mmOutStream.write(bytes);
            } catch (IOException e) {
                Log.e(TAG, "write: Error writing to output stream. " + e.getMessage() );
            }
        }

        /* Call this from the main activity to shutdown the connection */
        public void cancel() {
            try {
                mmSocket.close();
            } catch (IOException e) { }
        }
    }


    public void SendMessage(View v) {
        byte[] bytes = send_data.getText().toString().getBytes(Charset.defaultCharset());
        mConnectedThread.write(bytes);
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        send_data =(EditText) findViewById(R.id.editText);
        view_data = (TextView) findViewById(R.id.textView);

        if (bluetoothAdapter != null && !bluetoothAdapter.isEnabled()) {
            Intent enableBtIntent = new
                    Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
        }


    }

    public void Start_Server(View view) {

        AcceptThread accept = new AcceptThread();
        accept.start();

    }

    private class AcceptThread extends Thread {

        // The local server socket
        private final BluetoothServerSocket mmServerSocket;

        public AcceptThread(){
            BluetoothServerSocket tmp = null ;

            // Create a new listening server socket
            try{
                tmp = bluetoothAdapter.listenUsingInsecureRfcommWithServiceRecord("appname", MY_UUID_INSECURE);

                Log.d(TAG, "AcceptThread: Setting up Server using: " + MY_UUID_INSECURE);
            }catch (IOException e){
                Log.e(TAG, "AcceptThread: IOException: " + e.getMessage() );
            }

            mmServerSocket = tmp;
        }

        public void run(){
            Log.d(TAG, "run: AcceptThread Running.");

            BluetoothSocket socket = null;

            try{
                // This is a blocking call and will only return on a
                // successful connection or an exception
                Log.d(TAG, "run: RFCOM server socket start.....");

                socket = mmServerSocket.accept();

                Log.d(TAG, "run: RFCOM server socket accepted connection.");

            }catch (IOException e){
                Log.e(TAG, "AcceptThread: IOException: " + e.getMessage() );
            }

            //talk about this is in the 3rd
            if(socket != null){
                connected(socket);
            }

            Log.i(TAG, "END mAcceptThread ");
        }

        public void cancel() {
            Log.d(TAG, "cancel: Canceling AcceptThread.");
            try {
                mmServerSocket.close();
            } catch (IOException e) {
                Log.e(TAG, "cancel: Close of AcceptThread ServerSocket failed. " + e.getMessage() );
            }
        }

    }

ACTIVITY_MAIN.XML

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.hpi5.bluethoothshot.MainActivity"
    tools:layout_editor_absoluteY="81dp"
    tools:layout_editor_absoluteX="0dp">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        android:id="@+id/textView"
        tools:layout_constraintTop_creator="1"
        tools:layout_constraintRight_creator="1"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_marginTop="58dp"
        tools:layout_constraintLeft_creator="1"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Name"
        tools:layout_constraintTop_creator="1"
        tools:layout_constraintRight_creator="1"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_marginTop="153dp"
        tools:layout_constraintLeft_creator="1"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        android:onClick="SendMessage"
        tools:layout_constraintTop_creator="1"
        tools:layout_constraintRight_creator="1"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_marginTop="22dp"
        app:layout_constraintTop_toBottomOf="@+id/editText"
        tools:layout_constraintLeft_creator="1"
        app:layout_constraintLeft_toLeftOf="parent" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Server"
        android:onClick="Start_Server"
        android:layout_marginEnd="53dp"
        tools:layout_constraintRight_creator="1"
        tools:layout_constraintBottom_creator="1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_marginBottom="84dp" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="ConnectionREq"
        android:onClick="pairDevice"
        android:layout_marginStart="34dp"
        tools:layout_constraintTop_creator="1"
        tools:layout_constraintBottom_creator="1"
        app:layout_constraintBottom_toBottomOf="@+id/button2"
        tools:layout_constraintLeft_creator="1"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="@+id/button2"
        android:layout_marginLeft="30dp"
        app:layout_constraintVertical_bias="0.0" />

</android.support.constraint.ConstraintLayout>

Answer 2

Exception description will defenitely help, but i'm 99% sure that system prevents you from executing socket connection code inside UI thread - so you need to create a new thread, move your socket creation and socket connection code to that thread and, finally, create a callback, that will indicate listeners that connection was performed or failed.

Note that android will block most of your attempts to do network-related time-consuming operations inside UI threads, because it makes UI so sloooow ;-)

Answer 3

TWO MAJOR PROBLEMS-

1) connect() is a blocking call, you should always perform this connection procedure in a thread that is separate from the main activity (UI) thread. You are doing this on the main thread.

Note: You should always call cancelDiscovery() to ensure that the device is not performing device discovery before you call connect(). If discovery is in progress, then the connection attempt is significantly slowed, and it's more likely to fail.

2) If you are using the same code on the second device too(so that you could send or receive data) then I don't see any call to accept() . accept() listens for connection requests. Again, accept() call is a blocking call, it should not be executed in the main activity UI thread so that your application can still respond to other user interactions.

Simplified thread for the server component that accepts incoming connections:

private class AcceptThread extends Thread {
    private final BluetoothServerSocket mmServerSocket;

    public AcceptThread() { 
        // Use a temporary object that is later assigned to mmServerSocket 
        // because mmServerSocket is final. 
        BluetoothServerSocket tmp = null;
        try { 
            // MY_UUID is the app's UUID string, also used by the client code. 
            tmp = mBluetoothAdapter.listenUsingRfcommWithServiceRecord(NAME, MY_UUID);
        } catch (IOException e) {
            Log.e(TAG, "Socket's listen() method failed", e);
        } 
        mmServerSocket = tmp;
    } 

    public void run() { 
        BluetoothSocket socket = null;
        // Keep listening until exception occurs or a socket is returned. 
        while (true) { 
            try { 
                socket = mmServerSocket.accept();
            } catch (IOException e) {
                Log.e(TAG, "Socket's accept() method failed", e);
                break; 
            } 

            if (socket != null) {
                // A connection was accepted. Perform work associated with 
                // the connection in a separate thread. 
                manageMyConnectedSocket(socket);
                mmServerSocket.close();
                break; 
            } 
        } 
    } 

    // Closes the connect socket and causes the thread to finish. 
    public void cancel() { 
        try { 
            mmServerSocket.close();
        } catch (IOException e) {
            Log.e(TAG, "Could not close the connect socket", e);
        } 
    } 
} 

Android documentation - BLUETOOTH