Sending a constant rate data stream from Droid via TCP/IP

Question

in my quest to learn Java / Android development, I'm running into lots of roadblocks. Mainly because I don't really know much about threading and communication between threads/processes. I'm trying to stream the IMU data from an android device to a python application on a computer. Whenever the sensor values change, a sensor listener saves the current values into a variable for the network handler to access.

The network handler in turn is supposed to run on a timer, sending the values and a current timestamp at a more or less fixed rate of 33Hz (perhaps a bit fast? well, I'd be willing to accept as slow as 10Hz, but no slower than that). Anyway, when I tested this, I could see on the computer interface that the data isn't nearly coming in at a steady pace of 30 per second, but rather comes in surges, sometimes not coming at all for a second, and overall accumulating quite the delay (ie. the later the values are, the more delayed they come in). I understand there might be some variability in the network and some lags, but I would at least like the overall pace to at least be correct, ie that it doesn't get worse and worse the longer I'm sending.

Considering the devices are both on a common wifi network, and I'm capable of streaming 1080p video without any lags over wifi, I'm fairly confident that the protocol should be able to handle a 64 Byte string every 30ms without troubles. To eliminate the sensor reader as an problem source, I made a minimum working example that simply sends a string every 30ms, without any sensor reading. I basically got this code from various stackoverflow posts, and modified it until it more or less did what I wanted. The problem is that the network interface runs in an AsynchronousTask, for which I am uncertain how to access it once it has been started. My theory is that it's wasting resources to open a new socket for every new data packet, but I'm unsure how to open the socket once in the background and then pass the values to it on a timer and tell it to send.

Here's the basic activity I made to test this:

package com.jamesdoesntlikejava.motionlearning15;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Toast;
import java.util.Timer;
import java.util.TimerTask;


public class SendValuesActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_send_values);
        // creating timer task, timer
        final Timer timer = new Timer();
        TimerTask taskNew = new TimerTask() {
            @Override
            public void run() {
                int counter = 0;
                int numsteps = 333;
                String params[] = new String[2];
                if (counter < numsteps) {
                    params[0] = "192.168.1.33";
                    long currentTime = System.currentTimeMillis();
                    params[1] = Long.toString(currentTime)+"blablabla";
                    new ServerCommunicationTask().execute(params);
                    counter++;
                } else  {
                    timer.cancel();
                    timer.purge();
                }
            }
        };
        // scheduling the task at fixed rate delay
        Toast.makeText(this, "Sending Values in 1s...", Toast.LENGTH_SHORT).show();
        timer.scheduleAtFixedRate(taskNew,1000,30);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_send_values, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

And this is the class for doing the networking:

package com.jamesdoesntlikejava.motionlearning15;

import android.os.AsyncTask;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.net.Socket;
import java.net.UnknownHostException;

public class ServerCommunicationTask extends AsyncTask<String, Void, String> {

    public final static int TCP_SERVER_PORT = 13337;
    // params are 0: the target IP and 1: the message to send.
    @Override
    protected String doInBackground(String[] params) {

        String TCP_SERVER_IP = params[0];
        try {
            Socket s = new Socket(TCP_SERVER_IP, TCP_SERVER_PORT);
            BufferedWriter out = new BufferedWriter(new OutputStreamWriter(s.getOutputStream()));
            //send output msg
            String outMsg = params[1];
            out.write(outMsg);
            out.flush();
            //close connection
            s.close();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return "success";
    }

    @Override
    protected void onPostExecute(String response) {
    }
}

Running on a Moto G LTE (updated 1st gen) with android 5.1. Any hints are appreciated, thanks!

Answer 1

Instead of AsyncTask and always opening new connection, you can use Thread.

import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.ArrayList;

public class ServerCommunicationThread extends Thread {

    public final static int TCP_SERVER_PORT = 13337;

    private ArrayList<String> mMessages = new ArrayList<>();
    private String mServer;

    private boolean mRun = true;

    public ServerCommunicationThread(String server) {
        this.mServer = server;
    }

    @Override
    public void run() {

        while (mRun) {
            Socket s = null;
            try {
                s = new Socket(mServer, TCP_SERVER_PORT);
                BufferedWriter out = new BufferedWriter(new OutputStreamWriter(s.getOutputStream()));

                while (mRun) {
                    String message;

                    // Wait for message
                    synchronized (mMessages) {
                        while (mMessages.isEmpty()) {
                            try {
                                mMessages.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        // Get message and remove from the list
                        message = mMessages.get(0);
                        mMessages.remove(0);
                    }

                    //send output msg
                    String outMsg = message;
                    out.write(outMsg);
                    out.flush();
                }

            } catch (UnknownHostException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                //close connection
                if (s != null) {
                    try {
                        s.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

    public void send(String message) {
        synchronized (mMessages) {
            mMessages.add(message);
            mMessages.notify();
        }
    }

    public void close() {
        mRun = false;
    }
}

You can keep the thread running with connection opened and send message when needed.

ServerCommunicationThread thread = new ServerCommunicationThread("192.168.1.33");
thread.start();
...
thread.send("blablabla");
...
thread.send("blablabla");
...
thread.close();

Please note that this code is not tested.