My app crashes when connecting to remote tcp socket

Question

i have been trying for some hours now to get my app (running on Android 7.1.2) to connect to a server socket on my desktop PC in my home LAN.

public class MainActivity extends AppCompatActivity {

    static final String SERVER_IP   = "192.168.XXX.XXX"; // X's are replaced by actual IP
    static final int    PORT        = 30000;

    Socket clientSocket;

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

        final Button btReceive  = (Button) findViewById(R.id.btReceive);

        final TextView txtReceivedText  = (TextView) findViewById(R.id.txtReceivedText);
        final TextView txtStatus        = (TextView) findViewById(R.id.txtStatus);



        btReceive.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v) {

                txtStatus.setText("Received button pressed.");

                try {

                    Socket s = new Socket(SERVER_IP, PORT);


                    BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));
                    final String st = input.readLine();

                    txtReceivedText.setText(st);

                    s.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

        });

    }


}

When executed I get the output on the console in Android studio:

...
Waiting for process to come online
Connected to process 16980 on device samsung-gt_i9515-a72266ed
Capturing and displaying logcat messages from application. This behavior can be disabled in the "Logcat output" section of the "Debugger" settings page.
W/art: Before Android 4.1, method int android.support.v7.widget.ListViewCompat.lookForSelectablePosition(int, boolean) would have incorrectly overridden the package-private method in android.widget.ListView
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.jonas.network_connector, PID: 16980
                  android.os.NetworkOnMainThreadException
                      at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1303)
                      at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:333)
                      at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:196)
                      at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:178)
                      at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:356)
                      at java.net.Socket.connect(Socket.java:605)
                      at java.net.Socket.connect(Socket.java:554)
                      at java.net.Socket.<init>(Socket.java:431)
                      at java.net.Socket.<init>(Socket.java:210)
                      at com.example.jonas.network_connector.MainActivity$2.onClick(MainActivity.java:88)
                      at android.view.View.performClick(View.java:5637)
                      at android.view.View$PerformClick.run(View.java:22433)
                      at android.os.Handler.handleCallback(Handler.java:751)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6186)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:889)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:779)
I/Process: Sending signal. PID: 16980 SIG: 9
Application terminated.

Since i'm new to socket programming (and android as well) I don't know how to proceed to find the error.

My manifest has the permission lines:

<uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

Answer 1

You need to do Network work on worker thread.

btReceive.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v) {
                txtStatus.setText("Received button pressed.");
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            Socket s = new Socket(SERVER_IP, PORT);
                            BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));
                            final String st = input.readLine();
                            txtReceivedText.post(new Runnable(){

                                @Override
                                public void run() {
                                    txtReceivedText.setText(st);
                                }
                            });

                            s.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                    }
                }).start();
            });