Question

I’m trying to make a client in Android. This clients runs a thread that creates the client socket and launches another thread that is always listening to the socket to receive Strings. Everything is OK when I send a String from the client to a Java server running in a PC, but when I send a String from the server to the Android client the app finishes. Why do I get this error?

Here is the code of the main Activity of the client:

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

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

    public class MainActivity extends Activity {
        TextView chatHistorial;
        EditText msg;
        Socket client;
        DataInputStream in;
        DataOutputStream out;
        Boolean cerrar;

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

            Thread t = new Thread(new Runnable(){
                @Override
                public void run() {
                    try{
                        client = new Socket("192.168.1.33", 4444);
                        in = new DataInputStream(client.getInputStream());
                        out = new DataOutputStream(client.getOutputStream());
                        cerrar = false;

                        chatHistorial = (TextView) findViewById(R.id.chatHistorial);
                        msg = (EditText) findViewById(R.id.msg);

                        ThreadLectura tl = new ThreadLectura(in, cerrar, chatHistorial);
                        tl.start();
                    }
                    catch(Exception e){
                        // ...
                    }
                }
            });

            t.start();

            findViewById(R.id.enviar).setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    String cadena = msg.getText().toString();
                    try {
                        out.writeUTF(cadena);
                    } catch (IOException e) {
                        // ...
                    }
                    if(cadena.equals("exit"))
                        cerrar = true;
                }
            });
        }

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

    }

    class ThreadLectura extends Thread{
        DataInputStream in;
        String cadena;
        TextView chatHistorial;
        Boolean cerrar;

        public ThreadLectura(DataInputStream in, Boolean cerrar, TextView tv){
            this.in = in;
            this.cerrar = cerrar;
            chatHistorial = tv;
        }

        @Override
        public void run(){
            try{
                while(!cerrar){
                    cadena = in.readUTF();
                    chatHistorial.append("Has recibido: " + cadena);
                }
            }
            catch(IOException ioe){
                System.out.println("Error de entrada/salida: "+ioe.getMessage());
            }
        }
    }

Answer 1

It’s hard to say without seeing your logcat output, but I’m betting it’s because you are attempting to modify the UI from within your background thread. This line within ThreadLectura:

chatHistorial.append("Has recibido: " + cadena);

is probably the issue, as chatHistorial is a TextView. You need to only modify the UI from within the main UI thread.