Send text through Bluetooth from Java Server to Android Client

Question

First of all... don't redirect me to Bluetooth Chat and I have fully read it.

I have an Android Client which stablishes the connection properly with the server, and what's most I can send text to the server in my pc and show it correctly, but I can't do the opposite action, send a simple string from the server to the client and show it in my android app.

I don't want to implement a chat is just to show how BT communication works between a Java Server and Android Client.

To make it easy:

I send the text at the end of the startServer() method in the server class.

I try to read the text from the server at the beginning of onPause().

**

[Solved] Solution Below

**.

The code is as follows:

Android BT Client:

/*...libraries here...*/
  public class ConnectTest extends Activity {
  TextView out;
  private static final int REQUEST_ENABLE_BT = 1;
  private BluetoothAdapter btAdapter = null;
  private BluetoothSocket btSocket = null;
  private OutputStream outStream = null;

  // Well known SPP UUID
  private static final UUID MY_UUID =
      UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");

  // Insert your server's MAC address
  private static String address = "00:10:60:AA:B9:B2";

  /** Called when the activity is first created. */
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    out = (TextView) findViewById(R.id.out);

    out.append("\n...In onCreate()...");

    btAdapter = BluetoothAdapter.getDefaultAdapter();
    CheckBTState();
  }

  public void onStart() {
    super.onStart();
    out.append("\n...In onStart()...");
  }

  public void onResume() {
    super.onResume();

    out.append("\n...In onResume...\n...Attempting client connect...");

    // Set up a pointer to the remote node using it's address.
    BluetoothDevice device = btAdapter.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);
    } 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.
    btAdapter.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();
      } catch (IOException e2) {
        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...");
    String message = "Hello from Android.\n";
    out.append("\n\n...The message that we will send to the server is: "+message);

    try {
      outStream = btSocket.getOutputStream();
    } catch (IOException e) {
      AlertBox("Fatal Error", "In onResume() and output stream creation failed:" + e.getMessage() + ".");
    }

    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);       
    }
  }

  public void onPause() {
    super.onPause();

    //out.append("\n...Hello\n");
    InputStream inStream;
    try {
        inStream = btSocket.getInputStream();
        BufferedReader bReader=new BufferedReader(new InputStreamReader(inStream));
        String lineRead=bReader.readLine();
        out.append("\n..."+lineRead+"\n");

    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    out.append("\n...In onPause()...");



    if (outStream != null) {
      try {
        outStream.flush();
      } catch (IOException e) {
        AlertBox("Fatal Error", "In onPause() and failed to flush output stream: " + e.getMessage() + ".");
      }
    }

    try     {
      btSocket.close();
    } catch (IOException e2) {
      AlertBox("Fatal Error", "In onPause() and failed to close socket." + e2.getMessage() + ".");
    }
  }

  public void onStop() {
    super.onStop();
    out.append("\n...In onStop()...");
  }

  public void onDestroy() {
    super.onDestroy();
    out.append("\n...In onDestroy()...");
  }

  private void CheckBTState() {
    // Check for Bluetooth support and then check to make sure it is turned on

    // Emulator doesn't support Bluetooth and will return null
    if(btAdapter==null) { 
      AlertBox("Fatal Error", "Bluetooth Not supported. Aborting.");
    } else {
      if (btAdapter.isEnabled()) {
        out.append("\n...Bluetooth is enabled...");
      } else {
        //Prompt user to turn on Bluetooth
        Intent enableBtIntent = new Intent(btAdapter.ACTION_REQUEST_ENABLE);
        startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
      }
    }
  }

  public void AlertBox( String title, String message ){
    new AlertDialog.Builder(this)
    .setTitle( title )
    .setMessage( message + " Press OK to exit." )
    .setPositiveButton("OK", new OnClickListener() {
        public void onClick(DialogInterface arg0, int arg1) {
          finish();
        }
    }).show();
  }
}

and the Java BT Server:

    import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;

import javax.bluetooth.*;
import javax.microedition.io.*;

/**
* Class that implements an SPP Server which accepts single line of
* message from an SPP client and sends a single line of response to the client.
*/
public class SimpleSPPServer {

    //start server
    private void startServer() throws IOException{

        //Create a UUID for SPP
        UUID uuid = new UUID("1101", true);
        //Create the servicve url
        String connectionString = "btspp://localhost:" + uuid +";name=Sample SPP Server";

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

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

        //read string from spp client
        InputStream inStream=connection.openInputStream();
        BufferedReader bReader=new BufferedReader(new InputStreamReader(inStream));
        String lineRead=bReader.readLine();
        System.out.println(lineRead);

        //send response to spp client
        OutputStream outStream=connection.openOutputStream();
        BufferedWriter bWriter=new BufferedWriter(new OutputStreamWriter(outStream));
        bWriter.write("Response String from SPP Server\r\n");

        /*PrintWriter pWriter=new PrintWriter(new OutputStreamWriter(outStream));
        pWriter.write("Response String from SPP Server\r\n");
        pWriter.flush();
        pWriter.close();*/

        streamConnNotifier.close();

    }


    public static void main(String[] args) throws IOException {

        //display local device address and name
        LocalDevice localDevice = LocalDevice.getLocalDevice();
        System.out.println("Address: "+localDevice.getBluetoothAddress());
        System.out.println("Name: "+localDevice.getFriendlyName());

        SimpleSPPServer sampleSPPServer=new SimpleSPPServer();
        sampleSPPServer.startServer();

    }
}

Solution: It's just a small change in the server side. I don't know why but instead of using BufferedWrite to write in the socket, we need to use PrinterWriter to do so. I add that piece of code modified:

BT Server:

    ....
    //read string from spp client
    InputStream inStream=connection.openInputStream();
    BufferedReader bReader=new BufferedReader(new InputStreamReader(inStream));
    String lineRead=bReader.readLine();
    System.out.println(lineRead);

    //send response to spp client
    OutputStream outStream=connection.openOutputStream();
    PrintWriter pWriter=new PrintWriter(new OutputStreamWriter(outStream));
    pWriter.write("Response String from SPP Server\r\n");
    pWriter.flush();

    pWriter.close();

    streamConnNotifier.close();
    ...

Answer 1

It looks like your BufferedWriter cache was not being flushed, the data was simply remaining in the buffer without being sent. Calling bWriter.flush() after your bWriter.write() should fix the issue and cause the data to be flushed to the output stream. You can also consider changing your code to follow this:

PrintWriter pWriter = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outStream)));

This will cause the PrintWriter output to be buffered, instead of being written immediately to the output stream, which can be inefficient.

For your purposes though, omitting the BufferedWriter should be fine, as you will most likely want an immediate flush to the output stream (which PrintWriter does).