Has been running on another Thread, but still thrown android.os.NetworkOnMainThreadException?

Question

public class Client extends AppCompatActivity  {

public static final String TAG = Client.class.getSimpleName();
public static final int ServerPORT = 3000;
public static final String ServerIP = "10.146.166.86";
EditText message;
TextView mainView;
ClientThread clientThread;
Thread thread;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_client);
    message = (EditText)findViewById(R.id.message);
    mainView = (TextView)findViewById(R.id.mainView);
}

public void updateMessage(final String message) {
    runOnUiThread(new Runnable() {
        @Override
        public void run() {
            mainView.append(message + "\n");
        }
    });
}

public void onClick(View view) {

    if (view.getId() == R.id.connectServer) {
        mainView.setText("");
        clientThread = new ClientThread();
        thread = new Thread(clientThread);
        thread.start();
        return;
    }

    if (view.getId() == R.id.sendMessage) {
        Log.i(TAG, "Message sent from client");
        clientThread.sendMessage(message.getText().toString());
    }
}

class ClientThread implements Runnable {

    private Socket socket;
    private BufferedReader input;

    @Override
    public void run() {
        try {
            InetAddress serverAdd = InetAddress.getByName(ServerIP);
            socket = new Socket(serverAdd, ServerPORT);
            while (!Thread.currentThread().isInterrupted()) {
                this.input = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                String message = input.readLine();
                if (null == message || "Disconnect".contentEquals(message)) {
                    Thread.interrupted();
                    message = "Server Disconnected.";
                    updateMessage(getTime() + " | Server : " + message);
                    break;
                }

                updateMessage(getTime() + " | Server : " + message);
            }
        } catch (UnknownHostException e1) {
            e1.printStackTrace();
        } catch (IOException e2) {
            e2.printStackTrace();
        }
    }

    void sendMessage(String message) {
        try {
            if (null != socket) {
                PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);
                out.println(message);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

String getTime() {
    SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
    return sdf.format(new Date());
}
@Override
protected void onDestroy() {
    super.onDestroy();
    if (null != clientThread) {
        clientThread.sendMessage("Disconnect");
        clientThread = null;
    }
}
}

I wrote a simple server-client TCP socket connection, and have been trying to connect between the host and the client. Although I already ran the network operations on the other Thread (not the mainThread), when I try to send a string from the client to my server, it throws

android.os.NetworkOnMainThreadException. 

It seems even more weird to me because when I try switching the host and client devices, now I am able to send from the client to my server, but not from the server to the client. Then, I conclude that it should be due to one of my devices. One runs Android 8 and one runs Android 6, and it seems the problem belongs to the one running Android 8. Although I succesfully overcame this problem by adding:

StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy); 

It seems really weird to me because the way it happens. Does it actually depend on the device?

Edit: Full error message. It's weird because it only happens with one of my devices. When I switch the roles between devices, it does its job successfully.

02-10 16:42:17.719 827-827/com.dev.kvuong2711.clienttcp I/Client: Message sent from client
02-10 16:42:17.720 827-827/com.dev.kvuong2711.clienttcp W/System.err: android.os.NetworkOnMainThreadException
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1448)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:108)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.net.SocketOutputStream.write(SocketOutputStream.java:153)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:221)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:291)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at sun.nio.cs.StreamEncoder.implFlush(StreamEncoder.java:295)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at sun.nio.cs.StreamEncoder.flush(StreamEncoder.java:141)
02-10 16:42:17.721 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.io.OutputStreamWriter.flush(OutputStreamWriter.java:229)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.io.BufferedWriter.flush(BufferedWriter.java:254)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.io.PrintWriter.newLine(PrintWriter.java:482)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at com.dev.kvuong2711.clienttcp.Client$ClientThread.sendMessage(Client.java:105)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at com.dev.kvuong2711.clienttcp.Client.onClick(Client.java:68)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.view.View.performClick(View.java:6256)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.view.View$PerformClick.run(View.java:24779)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.os.Handler.handleCallback(Handler.java:789)
02-10 16:42:17.722 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:98)
02-10 16:42:17.723 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.os.Looper.loop(Looper.java:180)
02-10 16:42:17.723 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:6950)
02-10 16:42:17.723 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
02-10 16:42:17.723 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
02-10 16:42:17.723 827-827/com.dev.kvuong2711.clienttcp W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:835)

Answer 1

Your code in sendMessage is not running on another thread just because you put it in another class called "Thread". Only the run() method of a runnable is running on the other thread, when you call execute(). So you either have to call sendMessage in a new Thread() or put it in some kind of threadsafe variable or queue, that you check in your loop in the run() method and then send away.