How to make a UDP connection in android

Question

I'm trying to make a connection between my galaxy tab and my laptop . So I'm trying to run server activity on my laptop and client activity on my tab, but it doesn't work. Here is the server and client code. Where is the mistake?

SERVER:

public class MainActivity extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    TextView txt = (TextView)findViewById(R.id.textView1);

    int port = 12345;
    byte [] message = new byte [1500];
    DatagramPacket p = new DatagramPacket (message,message.length);
    try {
        InetAddress serveraddr = InetAddress.getByName("192.168.1.116");
        DatagramSocket s = new DatagramSocket (port,serveraddr);
        while (true){
            s.receive(p);
            String text = new String (message,0,p.getLength());
            txt.setText(text);
        }
    } catch (SocketException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

CLIENT:

public class MainActivity extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final EditText edt = (EditText)findViewById(R.id.editText1);
    Button btn = (Button)findViewById(R.id.button1);
    btn.setOnClickListener(new View.OnClickListener() {

        public void onClick(View arg0) {
            String msg = edt.getText().toString();
            int port = 12345;
            try {
                DatagramSocket s = new DatagramSocket();
                InetAddress local  = InetAddress.getByName("192.168.1.116");
                int msg_lenght = msg.length();
                byte []message = msg.getBytes();
                DatagramPacket p = new DatagramPacket(message,msg_lenght,local,port);
                s.send(p);
            } catch (SocketException e) {                   
                e.printStackTrace();
            } catch (UnknownHostException e) {                  
                e.printStackTrace();
            } catch (IOException e) {               
                e.printStackTrace();
            }               
        }
    });
}

Here is the log :

09-17 23:49:55.190: D/dalvikvm(5892): Late-enabling CheckJNI 09-17 23:49:55.690: D/CLIPBOARD(5892): Hide Clipboard dialog at Starting input: finished by someone else... ! 09-17 23:49:59.590: D/AndroidRuntime(5892): Shutting down VM 09-17 23:49:59.590: W/dalvikvm(5892): threadid=1: thread exiting with uncaught exception (group=0x40c4f1f8) 09-17 23:49:59.590: E/AndroidRuntime(5892): FATAL EXCEPTION: main 09-17 23:49:59.590: E/AndroidRuntime(5892): android.os.NetworkOnMainThreadException 09-17 23:49:59.590: E/AndroidRuntime(5892): at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1099) 09-17 23:49:59.590: E/AndroidRuntime(5892): at libcore.io.BlockGuardOs.sendto(BlockGuardOs.java:175) 09-17 23:49:59.590: E/AndroidRuntime(5892): at libcore.io.IoBridge.sendto(IoBridge.java:463) 09-17 23:49:59.590: E/AndroidRuntime(5892): at java.net.PlainDatagramSocketImpl.send(PlainDatagramSocketImpl.java:182) 09-17 23:49:59.590: E/AndroidRuntime(5892): at java.net.DatagramSocket.send(DatagramSocket.java:307) 09-17 23:49:59.590: E/AndroidRuntime(5892): at com.example.udpclient.MainActivity$1.onClick(MainActivity.java:36) 09-17 23:49:59.590: E/Androi dRuntime(5892): at android.view.View.performClick(View.java:3620) 09-17 23:49:59.590: E/AndroidRuntime(5892): at android.view.View$PerformClick.run(View.java:14322) 09-17 23:49:59.590: E/AndroidRuntime(5892): at android.os.Handler.handleCallback(Handler.java:605) 09-17 23:49:59.590: E/AndroidRuntime(5892): at android.os.Handler.dispatchMessage(Handler.java:92) 09-17 23:49:59.590: E/AndroidRuntime(5892): at android.os.Looper.loop(Looper.java:137) 09-17 23:49:59.590: E/AndroidRuntime(5892): at android.app.ActivityThread.main(ActivityThread.java:4507) 09-17 23:49:59.590: E/AndroidRuntime(5892): at java.lang.reflect.Method.invokeNative(Native Method) 09-17 23:49:59.590: E/AndroidRuntime(5892): at java.lang.reflect.Method.invoke(Method.java:511) 09-17 23:49:59.590: E/AndroidRuntime(5892): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:978) 09-17 23:49:59.590: E/AndroidRuntime(5892): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745) 09-17 23:49:59 .590: E/AndroidRuntime(5892): at dalvik.system.NativeStart.main(Native Method) 09-17 23:50:34.320: I/Process(5892): Sending signal. PID: 5892 SIG: 9

Answer 1

09-17 23:49:59.590: E/AndroidRuntime(5892): android.os.NetworkOnMainThreadException
09-17 23:49:59.590: E/AndroidRuntime(5892):     at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1099)
09-17 23:49:59.590: E/AndroidRuntime(5892):     at libcore.io.BlockGuardOs.sendto(BlockGuardOs.java:175)

You shouldn't do network or time intensiv operations in ui thread

See also: Android: NoClassDefFoundError for some app users or android developers information

Checkout: activity.runOnUi

Answer 2

You have an infinite loop in onCreate of the Server. You shouldn't! Create a thread for polling the socket.

Answer 3

Have 2 problems with your code

1. Work with Network on Main thread (UI Thread)

09-17 23:49:59.590: E/AndroidRuntime(5892): FATAL EXCEPTION: main 09-17 23:49:59.590: E/AndroidRuntime(5892): android.os.NetworkOnMainThreadException

2. Loop while(true) on the Main thread:

    while(true) { s.receive(p); String text = new String (message,0,p.getLength()); txt.setText(text); } 

Answer 4

• you cannot send udp packets on ui thread, so a new seperate thread must be created.

Just a quick solution...

create a udpOutputData string:
String udpOutputData;

create a new thread in your code:

    //-----UDP send thread
Thread udpSendThread = new Thread(new Runnable() {

        @Override
        public void run() {


                while (true) {  

                    try {
                        Thread.sleep(100);
                    } 

                    catch (InterruptedException e1) {
                        // TODO Auto-generated catch block
                        e1.printStackTrace();
                    }

                        if (sendUdp == true) {

                            try {

                                // get server name
                                InetAddress serverAddr = InetAddress.getByName(outputIP);
                                Log.d("UDP", "C: Connecting...");

                                // create new UDP socket
                                DatagramSocket socket = new DatagramSocket();

                                // prepare data to be sent
                                byte[] buf = udpOutputData.getBytes();

                                // create a UDP packet with data and its destination ip & port
                                DatagramPacket packet = new DatagramPacket(buf, buf.length, serverAddr, broadcastPort);
                                Log.d("UDP", "C: Sending: '" + new String(buf) + "'");

                                // send the UDP packet
                                socket.send(packet);

                                socket.close();

                                Log.d("UDP", "C: Sent.");
                                Log.d("UDP", "C: Done.");


                            } 

                            catch (Exception e) {
                                Log.e("UDP", "C: Error", e);

                            }

                                try {
                                    Thread.sleep(100);
                                }

                                catch (InterruptedException e) {
                                    // TODO Auto-generated catch block
                                    e.printStackTrace();
                                }


                                sendUdp = false;
                        }

                }
        }

});

create a method to call everytime you want to send some udp data:

    public void sendUdp(String udpMsg) {

       udpOutputData = udpMsg;
       sendUdp = true;
    }

call the method and pass a string for the output data everytime you want to send a udp packet:

String s = "hello from app";
sendUdp(s);