Sending text over wifi using sockets causes app to crash

Question

I'm trying to send a text from my phone to my pc over wifi but my app keeps crashing and I have no clue why. Here is the code which crashes:

try {
 Socket socket = new Socket(RecieverIP,1755);
 DataOutputStream DOS = new DataOutputStream(socket.getOutputStream());
 DOS.writeUTF(String.valueOf(progressvalue));
 socket.close();
 } catch (IOException e) 
{
  e.printStackTrace();
 }

and on my pc's end:

 import java.io.BufferedInputStream;
    import java.io.DataInputStream;
    import java.io.IOException;
    import java.net.InetAddress;
    import java.net.ServerSocket;
    import java.net.Socket;
    import java.net.UnknownHostException;
    import java.util.logging.Level;
    import java.util.logging.Logger;

    /**
     *
     * @author fares Part of a project to control the led brightness on a pi, the job of this program is to receive a text over wifi and then launch a python program and pass the brightness as a parameter
     */
    public class WebSocketReciever {

        /**
         * @param args the command line arguments
         */
        public static void main(String[] args) throws IOException {

            System.out.println("Program started");
            String msg_received;
            System.out.println("Creating socket");
            ServerSocket socket = new ServerSocket(1755);
            System.out.println("Socket created");
            Socket clientSocket = socket.accept();       //This is blocking. It will wait.
            System.out.println("Client socket created");

            while(true){
            System.out.println("Reading data");
            DataInputStream DIS = new DataInputStream(clientSocket.getInputStream());
            msg_received = DIS.readUTF();
            System.out.println(msg_received);

            clientSocket.close();
            socket.close();}

        }
    }

If I run the pc code it always only reaches the output: "Socket created"

The whole code for the app is:

package com.example.fares.ledslider;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.SeekBar;
import android.widget.TextView;
import android.widget.Toast;

import java.io.DataOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.Socket;

import static com.example.fares.ledslider.R.id.seekBar;

public class MainActivity extends AppCompatActivity {

    private static TextView textView;
    private static SeekBar seek_Bar;
    private static String RecieverIP;
    private static EditText IPText;

    public void seekbarmethod(){
        seek_Bar = (SeekBar) findViewById(seekBar);
        textView = (TextView) findViewById(R.id.textview);
        seek_Bar.setMax(100); //Max value of the seekbar
        seek_Bar.setProgress(50);//Initial seekbar value
        textView.setText("Brightness = " +seek_Bar.getProgress() + " %"); //Notify user of percentage brightness
        seek_Bar.setOnSeekBarChangeListener(
                new SeekBar.OnSeekBarChangeListener(){
                    int count =0;
                    int progressvalue;
                    @Override
                    public void onProgressChanged(SeekBar seekBar, int i, boolean b) {
                        progressvalue = i;
                        textView.setText("Brightness = " + progressvalue + " %");

                          //  Toast.makeText(MainActivity.this,"Change in progress" + count,Toast.LENGTH_SHORT).show();
                            //count +=1;
                        //Send data from app to pc

                        try {
                           Socket socket = new Socket(RecieverIP,1755);
                            DataOutputStream DOS = new DataOutputStream(socket.getOutputStream());

                            DOS.writeUTF(String.valueOf(progressvalue));
                            socket.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }


                    }



                    @Override
                    public void onStartTrackingTouch(SeekBar seekBar) {
                        Toast.makeText(MainActivity.this,"Change initiated",Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public void onStopTrackingTouch(SeekBar seekBar) {
                        Toast.makeText(MainActivity.this,"Change ended",Toast.LENGTH_SHORT).show();
                        count = 0 ;

                    }
                }
        );

    }

    public void IPBtnMethod(View v){
    IPText = (EditText) findViewById(R.id.IPBox);
    RecieverIP = IPText.getText().toString();
    Toast.makeText(MainActivity.this,"IP = " + RecieverIP,Toast.LENGTH_SHORT).show();
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        seekbarmethod();

    }
}

Why does my app crash and how can I fix it? Thanks in advance

In Android you are not doing networking on the main thread. See this: https://stackoverflow.com/questions/6343166/how-do-i-fix-android-os-networkonmainthreadexception — Ervin Szilagyi, Aug 26 '17 at 15:43
@ErvinSzilagyi alright, thanks, do you know why this is though? — Fares Al Ghazy, Aug 26 '17 at 15:46
It was a design decision by the Android team. Probably to not freeze your application by locking your main thread. — Ervin Szilagyi, Aug 26 '17 at 15:50
thanks again, if you would like to answer instead of comment so i can vote you as correct then that'd be good — Fares Al Ghazy, Aug 26 '17 at 16:03

score 0 · Accepted Answer · answered Aug 26 '17 at 16:07

You are probably getting NetworkOnMainThreadException exception. This exception is thrown when you are doing networking on the main thread. The Android operating system forbids doing this: https://developer.android.com/reference/android/os/NetworkOnMainThreadException.html

Sending text over wifi using sockets causes app to crash

1 Answers1