NIO SocketChannel saying there is no data when there is (or selector is not informing me)

Question

I have a functioning client-server apparatus which can successfully connect and send messages to each other using NIO.

Right now my only confusion is how I'm supposed to continue reading when socketChannel.read() returns zero.

I have a protocol that sends the first 4 bytes as the number of incoming bytes to expect. Even with that amount, I'm running into a potential issue.

For testing, However, there are times where I might read something like:

5 // Read 5 bytes when calling socketChannel.read()
0 // Read 0 bytes when calling socketChannel.read() immediately after

When I hit the zero, I assumed that I was done reading and need to wait for more data to come to me.

When I do this however, OP_READ doesn't seem to be triggered when I perform selectNow() again later on. I checked the key and it has it's readyops() and interestops() set to 1 (which is OP_READ), but it does not want to recognize that it's time to read again.

I found that if I continue looping to read, I might get something like:

5 // socketChannel.read()
0 // socketChannel.read()
7 // socketChannel.read() (Done since I have all my bytes)
0
0
0
...

I'm confused here because this means one of:

• There is no data there, so the 0 available is legitimate, but then when the rest of the data comes in, the selector refuses to return the key with selectNow()

• The data is all there, but for some reason returns 0 on reading.

Am I supposed to re-register the channel after a selectNow() returns it as an active key? (Though I didn't have to between switching from OP_CONNECT to OP_READ... so I'm guessing not). I feel like blindly circling in a loop is dangerous and will waste processing cycles.

Am I just supposed to keep polling them? That makes me confused at when OP_READ actually fires then.

Answer 1

This was due to an error on my part, where I did not call .clear() on the bytebuffer that reads. This causes it to return 0 read even though the data has streamed in.

This example may also be of use to people who want to see how a simple client works (though with really bad exception handling). There is no guarantee this will work properly and may likely have issues since it was designed to be a quick and dirty test.

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;

public class Test {

    public static final int PORT = 22222;

    public static void main(String[] args) throws IOException {
        Thread s = new Thread(new Server());
        Thread c = new Thread(new Client());
        s.start();
        c.start();
    }
}

class Client implements Runnable {

    public Selector selector;

    public SocketChannel sc;

    public Client() throws IOException {
        selector = Selector.open();
    }

    @Override
    public void run() {
        try {
            sc = SocketChannel.open();
            sc.socket().setTcpNoDelay(true);
            sc.configureBlocking(false);
            SelectionKey k = sc.register(selector, SelectionKey.OP_CONNECT);
            boolean firstConnect = sc.connect(new InetSocketAddress("localhost", Test.PORT));
            if (firstConnect) {
                System.out.println("Connected on first connect, de-registering OP_CONNECT");
                k.interestOps(SelectionKey.OP_READ);
            }

            while (true) {
                int keys = selector.selectNow();
                if (keys > 0) {
                    for (SelectionKey key : selector.selectedKeys()) {
                        if (key.isConnectable()) {
                            boolean finishConnectResult = sc.finishConnect();
                            key.interestOps(SelectionKey.OP_READ);
                            System.out.println("Finished connection: " + finishConnectResult);
                        }

                        if (key.isReadable()) {
                            ByteBuffer bb = ByteBuffer.allocate(2);
                            int bytesRead = 0;
                            while ((bytesRead = sc.read(bb)) > 0) {
                                bb.flip();
                                System.out.println(bytesRead + " bytes read");
                                System.out.println(bb.get() + ", " + bb.get());
                                //bb.clear(); // If this is not commented, it will not be handled properly.
                            }
                            System.out.println("Last bytes read value = " + bytesRead);
                            System.exit(0);
                        }
                    }
                }

                Thread.sleep(5);
            }
        } catch (Exception e) { 
            e.printStackTrace();
            throw new RuntimeException();
        }
    }
}

class Server implements Runnable {

    public Selector selector;

    public SocketChannel sc;

    public Server() throws IOException {
        selector = Selector.open();
        ServerSocketChannel ssc = ServerSocketChannel.open();
        ssc.configureBlocking(false);
        ssc.bind(new InetSocketAddress(Test.PORT));
        ssc.register(selector, SelectionKey.OP_ACCEPT);
    }

    @Override
    public void run() {
        boolean notSentData = true;
        try {
            while (true) {
                int keys = selector.selectNow();
                if (keys > 0) {
                    for (SelectionKey key : selector.selectedKeys()) {
                        if (key.isAcceptable()) {
                            ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                            sc = ssc.accept();
                            if (sc != null) {
                                sc.configureBlocking(false);
                                sc.socket().setTcpNoDelay(true); // Required in my application
                                sc.register(selector, SelectionKey.OP_WRITE);
                                System.out.println("Server accepted connection");
                            } else {
                                System.out.println("Got null connection");
                            }
                        }
                    }
                }

                if (sc != null && notSentData) {
                    ByteBuffer bb = ByteBuffer.allocate(4);
                    bb.put(new byte[]{ 1, 2, 3, -1});
                    bb.flip();
                    int wrote = sc.write(bb);
                    System.out.println("Wrote " + wrote + " bytes");
                    notSentData = false;
                }

                Thread.sleep(5);
            }
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException();
        }
    }
}