Server.java

import io.undertow.Undertow;
import io.undertow.server.HttpHandler;
import io.undertow.server.HttpServerExchange;
import io.undertow.util.HttpString;
import org.xnio.Options;

import java.nio.ByteBuffer;
import java.util.concurrent.atomic.AtomicInteger;

public class Server
{
    private static final AtomicInteger _numOverlappingRequests = new AtomicInteger(0);

    private static final Object _maxSeenOverlappingRequestsMutex = new Object();

    private static int _maxSeenOverlappingRequests = 0;

    private static final int IO_THREAD_COUNT = 8; // we hope to see this number printed on the screen!

    public static void main(String[] args)
    {
        int listenPort = 8080;
        Undertow server = Undertow.builder()
                .addHttpListener(listenPort, "0.0.0.0") // 0.0.0.0 means listen on any ip addr
                .setSocketOption(Options.BACKLOG, 1000)
                .setIoThreads(IO_THREAD_COUNT)
                .setHandler(new HttpHandler()
                {
                    @Override
                    public void handleRequest(HttpServerExchange httpServerExchange) throws Exception
                    {
                        int overlappingRequestsCount = _numOverlappingRequests.incrementAndGet();

                        synchronized (_maxSeenOverlappingRequestsMutex)
                        {
                            if (overlappingRequestsCount > _maxSeenOverlappingRequests)
                            {
                                _maxSeenOverlappingRequests = overlappingRequestsCount;
                                System.out.println(_maxSeenOverlappingRequests);
                            }
                        }

                        Thread.sleep(1000);

                        httpServerExchange.getResponseHeaders().put(new HttpString("Content-Type"), "text/plain");
                        httpServerExchange.setStatusCode(200);
                        httpServerExchange.getResponseSender().send(ByteBuffer.wrap("Hello".getBytes()));
                        httpServerExchange.endExchange();

                        _numOverlappingRequests.decrementAndGet();
                    }
                })
                .build();
        server.start();

        System.out.println("\n\nServer started on port " + listenPort);
    }
}