TCP from android to PC not working using java

Question

I am trying to send a simple message over TCP from my android phone (using java application) to my computer. I have an socket that is listening on my computer but as soon as I run this app, it crashes. I am really new to Android developing so please bear with me...

Here is my Java code:

package com.scorekeep.clienttcp;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.net.UnknownHostException;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;


public class MainActivity extends Activity {

    EditText textOut;


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

        Socket socket = null;
        try {
            socket = new Socket("10.0.0.10",5000);
            DataOutputStream DOS = new DataOutputStream(socket.getOutputStream());
            DOS.writeUTF("HELLO_WORLD");
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

what is the code of your Server? What crashes exactly ? The server or the client? — Nicolas Filotto
– Nicolas Filotto, Commented Sep 3, 2016 at 11:10

user6657161 · Accepted Answer · 2016-09-03 12:27:22Z

1

As requested an extended example :

import android.app.*;
import android.os.*;
import android.util.*;
import java.io.*;
import java.net.*;

public class MainActivity extends Activity 
{
    @Override
     protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new connection().execute();
    }
}

 class connection extends AsyncTask<String,String,String> {
public static PrintWriter out;
BufferedReader in;
public static boolean running = true;
    @Override
    protected String doInBackground(String... message) {

        try
        {
            InetAddress serverAddr = InetAddress.getByName("localhost");
            Socket socket = new Socket(serverAddr, 8008);
            // send the message to the server
            out = new PrintWriter(new BufferedWriter(
                              new OutputStreamWriter(socket.getOutputStream())), true);

            in = new BufferedReader(new InputStreamReader(
                                    socket.getInputStream()));

        while(running) {
            String msgfromserver = in.readLine();
        }
        }
        catch (Exception e)
        {}

        return null;
    }

    public static void sendmsg(String msg){
        if(out!=null){
            out.println(msg);
            out.flush();
        }
    }
}

Usage: Call connection.sendmsg("some text"); from OnClick method of button

And set connection.running = false; onbackpress. (Or before finishing activity)

edited Sep 3, 2016 at 12:27

answered Sep 3, 2016 at 11:59

user6657161

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1 Comment

Simon Erasmus Over a year ago

One problem, when adding the connection.sendmsg("some text") the editor gives error and tells met to make sendmsg static. How do I fix this? Error: Error:(22, 19) error: non-static method sendmsg(String) cannot be referenced from a static context

user6657161 · Accepted Answer · 2016-09-03 11:37:07Z

1

You cannot execute background tasks like socket connection in ui thread. You should use AsyncTask.

Example

import android.app.*;
import android.os.*;
import android.util.*;
import java.io.*;
import java.net.*;

public class MainActivity extends Activity 
{
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        new connection().execute();
    }
}

class connection extends AsyncTask<String,String,String> {

    @Override
    protected String doInBackground(String... message) {

        try
        {
            InetAddress serverAddr = InetAddress.getByName("ip here");
            Socket socket = new Socket(serverAddr, 5000); //port here
    // send the message to the server
        PrintWriter out = new PrintWriter(new BufferedWriter(
                              new OutputStreamWriter(socket.getOutputStream())), true);
        out.println("hi");
        out.flush(); //optional
        socket.close();
        }
        catch (Exception e)
        {}

        return null;
    }
}

Note Untested. May contain errors.

edited Sep 3, 2016 at 11:37

answered Sep 3, 2016 at 11:12

user6657161

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4 Comments

Simon Erasmus Over a year ago

Can you please give me an example? @user6657161

user6657161 Over a year ago

Wait i am coding an example for you.

Simon Erasmus Over a year ago

Thanks @user6657161

Simon Erasmus Over a year ago

Thanks, the example works. Could you maybe just show me how I could send a message to the server each time a button is pressed without closing and opening the server each time.

user6657161 · Accepted Answer · 2016-09-03 12:29:01Z

1

Two changes in code. public static void sendmsg and public public static PrintWriter out at begining of AsyncTask class

answered Sep 3, 2016 at 12:29

user6657161

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2 Comments

user6657161 Over a year ago

Added it as answer by mistake.

user6657161 Over a year ago

I can't delete my answer as i am on android. Alas!

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