android.os.NetworkOnMainThreadException and non blocking sockets

Question

I using Java nio sockets in non blocking mode, but when i call write i still get NetworkOnMainThreadException.

java.nio.channels.SocketChannel fd = Network.createTcpSocket();
java.nio.channels.SocketChannel.open();
java.net.Socket socket = fd.socket();
socket.setTcpNoDelay(true);
socket.setKeepAlive(true);
fd.configureBlocking(false);

//.. conect the socket

int ret = fd.write(data);
if(ret == 0)
{
  // It will block.
}

I will expect this to work as the socket will not block and just return 0 in case the call need to block. But i still receive the NetworkOnMainThreadException

12-07 18:50:25.704: W/dalvikvm(14931): threadid=1: thread exiting with uncaught exception (group=0x40c501f8)
12-07 18:50:25.834: E/AndroidRuntime(14931): FATAL EXCEPTION: main
12-07 18:50:25.834: E/AndroidRuntime(14931): android.os.NetworkOnMainThreadException
12-07 18:50:25.834: E/AndroidRuntime(14931):    at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1099)
12-07 18:50:25.834: E/AndroidRuntime(14931):    at libcore.io.BlockGuardOs.sendto(BlockGuardOs.java:168)
12-07 18:50:25.834: E/AndroidRuntime(14931):    at libcore.io.IoBridge.sendto(IoBridge.java:477)
12-07 18:50:25.834: E/AndroidRuntime(14931):    at java.nio.SocketChannelImpl.writeImpl(SocketChannelImpl.java:369)
12-07 18:50:25.834: E/AndroidRuntime(14931):    at java.nio.SocketChannelImpl.write(SocketChannelImpl.java:327

Answer 1

android.os.NetworkOnMainThreadException :

The exception that is thrown when an application attempts to perform a networking operation on its main thread.

use AsyncTask for making network related task from Ui Thread

or you are using API LEVEL 9 then just set StrictMode for Current Activity in onCreate() method before making network request :

   StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
   .detectNetwork() // or .detectAll() for all detectable problems
   .penaltyDialog()  //show a dialog
   //.permitNetwork() //permit Network access 
   .build());

Answer 2

It's a normal behavior. I guess there's a check inside of Socket class, no matter is it blocking and non-blocking. If thread is UI - throw NetworkOnMainThreadException

Answer 3

Android's justification for throwing a NetworkOnMainThreadException
https://developer.android.com/reference/android/os/NetworkOnMainThreadException

is detailed in the document "Designing for Responsiveness"
https://developer.android.com/training/articles/perf-anr

But Android makes the mistake of assuming all network IO can cause UI unresponsiveness.

Java non-blocking NIO (and Java NIO.2) channel methods by definition do not block and thus cannot cause an unresponsive UI.
https://developer.android.com/reference/java/nio/channels/SelectableChannel#bm

A selectable channel is either in blocking mode or in non-blocking mode. In blocking mode, every I/O operation invoked upon the channel will block until it completes. In non-blocking mode an I/O operation will never block ...

It is a shame Android does not recognize this and thus causes developers to add an unnecessary background layer of abstraction (AsyncTask be the most clumsy) between the UI and Java non-blocking NIO network operations.