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How can I test if a SocketChannel.read() for a non-blocking channel has finished?

I'm using a function to read bytes from non-blocking SocketChannel (socket from accept()) and from blocking SocketChannel (client side). I'm implementing a server using selector to handle multiple clients, and I'm using loopback address to use my laptop only. I wrote this
while((r = socketChannel.read(ackBuf)) != -1) {
System.out.println(name3d+" r: "+r);
}
and I expected that when the end of the content in the channel was reached, read() would returned -1 but is not what succedes.
read(), in non-blocking configuration, return 0 also if nothing is ready to read at the moment but it will be soon (if I understand well) so if I change the code to
while((r = socketChannel.read(ackBuf)) > 0) {
System.out.println(name3d+" r: "+r);
}
I will not read nothing also if something will be ready a moment later.
How can I distinguish if I got 0 because is not ready or because it is ended?
In the following snippet I can test for a second time the read after a sleep but I'm sure is not the reliable way to do what I want.
int times = 0;
while((r = socketChannel.read(ackBuf)) != -1 && times<2) {
if (r == 0)
try {
Thread.sleep(500);
times++;
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(name3d+" r: "+r);
}

" if I got 0 because is not ready or because it is ended?" Do you mean the message or the totality of the communication?
For the message, you should use a communication protocol (like json or http) for the communication, I think you should get a SocketException... You would if you using blocking and the person on the other end closed the connection... (I've written to a lot of people on SO about how SocketException is your friend)
--- edit ---
Looking over the documention for Channel, it looks like you should get an IOException of some kind (SocketException is a subclass of IOException) if/when the channcel is closed

The Non-blocking SocketChannel is used a bit different.
You first wait for the selection key to tell you that there is data, and
then you read that data from the channel.
See this code draft:
Selector selector = Selector.open();
SocketChannel sc = SocketChannel.open();
sc.configureBlocking(false);
sc.connect(addr);
sc.register(selector, SelectionKey.OP_READ);
while (true) {
// select() can block!
if (selector.select() == 0) {
continue;
}
Iterator iterator = selector.selectedKeys().iterator();
while (iterator.hasNext()) {
SelectionKey key = (SelectionKey) iterator.next();
iterator.remove();
if (key.isReadable()) {
SocketChannel sc = (SocketChannel) key.channel();
ByteBuffer bb = ByteBuffer.allocate(1024);
sc.read(bb);
System.out.println("Message received!");
}
}

In JAVA nio selector, when should I register the 'write operation'?

I am learning Java nio selector. In my understanding, I thought the steps using selector is to firstly I register the my interest operations and then I can check the ready set and finally I can do the operations corresponding my interest operations. I don't know why in this code the writing process can happen in the block of if (key.isReadable()){...} but not if (key.isWritable){...} and why writing operation is not registered?
Iterator keys = sel.selectedKeys().iterator();
while (keys.hasNext()) {
SelectionKey key = (SelectionKey)keys.next();
if (!key.isValid())
continue;
if (key.isAcceptable()) {
// increase the counter
connection++;
// remove accept request
keys.remove();
// ACCEPT: get the server channel
ServerSocketChannel ssc =
(ServerSocketChannel) key.channel();
// init a socket for a client
SocketChannel nsc = ssc.accept();
nsc.configureBlocking(false);
// register the socket for READ
nsc.register(sel, SelectionKey.OP_READ);
}
}
while (count < COUNT_MAX + NUM_CHILD - 1) {
sel.select();
// Get all pending events and iterate
Iterator keys = sel.selectedKeys().iterator();
while ( keys.hasNext() ) {
SelectionKey key = (SelectionKey)keys.next();
keys.remove();
if (!key.isValid())
continue;
if (key.isReadable()) {
// READ: get the channel
SocketChannel nsc = (SocketChannel) key.channel();
// clear buffer for reading
readBuffer.clear();
int nBytes = nsc.read(readBuffer);
if (nBytes == -1) {// Check if the client closed the socket
key.channel().close();
key.cancel();
continue;
}
// Read a message
DataInputStream ist = new DataInputStream (
new ByteArrayInputStream(readBuffer.array()));
String msg = ist.readUTF();
System.out.print(msg + "\n");
// Clear the write buffer
writeBuffer.clear();
// Write the counter value on the buffer
count++;
if (count < COUNT_MAX)
writeBuffer.putInt(count);
else
writeBuffer.putInt(-1);
// flip the buffer and write on the channel
writeBuffer.flip();
// Reply to a client
nsc.write(writeBuffer);
}
} // while (key)

You don't need to register interest in OP_WRITE because usually the channel is ready for writing. However a WritableChannel, if in non blocking mode, might not succeed in writing all content of the given ByteBuffer. See here in its java docs:
Some types of channels,
depending upon their state, may write only some of the bytes or
possibly none at all. A socket channel in non-blocking mode, for
example, cannot write any more bytes than are free in the socket's
output buffer.
In this case you need to register the interest for OP_WRITE on the selector to be notified when the channel is once again ready for writing, so you can finish writing your ByteBuffer.
See here a related SO question.

How java-nio is Non Blocking confused

I am new to NIO i understand the concept of Asynchronous Socket but i am confused on Non Blocking part.
I am using java NIO Selector . My Code for Server is
public class EcoNonBlockingIOServer_7 {
public static int PORT_NUMBER = 5555;
public static void main(String[] argv) throws Exception {
new EcoNonBlockingIOServer_7().go(argv);
}
public void go(String[] argv) throws Exception {
int port = PORT_NUMBER;
if (argv.length > 0) { // Override default listen port
port = Integer.parseInt(argv[0]);
}
System.out.println("Listening on port " + port);
// Allocate an unbound server socket channel
ServerSocketChannel serverChannel = ServerSocketChannel.open();
// Get the associated ServerSocket to bind it with
ServerSocket serverSocket = serverChannel.socket();
// Create a new Selector for use below
Selector selector = Selector.open();
// Set the port the server channel will listen to
serverSocket.bind(new InetSocketAddress(port));
// Set nonblocking mode for the listening socket
serverChannel.configureBlocking(false);
// Register the ServerSocketChannel with the Selector
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
while (true) {
// This may block for a long time. Upon returning, the
// selected set contains keys of the ready channels.
int n = selector.select();
if (n == 0) {
continue; // nothing to do
}
// Get an iterator over the set of selected keys
Iterator it = selector.selectedKeys().iterator();
// Look at each key in the selected set
while (it.hasNext()) {
SelectionKey key = (SelectionKey) it.next();
// Is a new connection coming in?
if (key.isAcceptable()) {
ServerSocketChannel server = (ServerSocketChannel) key.channel();
SocketChannel channel = server.accept();
registerChannel(selector, channel, SelectionKey.OP_READ);
sayHello(channel);
}
// Is there data to read on this channel?
if (key.isReadable()) {
readDataFromSocket(key);
}
// Remove key from selected set; it's been handled
it.remove();
}
}
}
Now My Queries are:
If we register a channel with selector on any operation it always get blocked on selector.select() then how it is non blocking.
If we admit it uses OP_ACCEPT as key and maps channel accordingly but again In key is acceptable i am modifying this channel selector to OP_READ since it already has been accepted. Again It blocks on selector.select() for read event
*Please correct my understanding if i am wrong *

If we register a channel with selector on any operation it always get blocked on selector.select() then how it is non blocking.
select() is blocking. Every operation on a non-blocking channel itself is non-blocking, i.e. read() and write().
If we admit it uses OP_ACCEPT as key and maps channel accordingly but again In key is acceptable i am modifying this channel selector to OP_READ since it already has been accepted.
Very confused. The channel whose interest-ops == OP_ACCEPT is the listening socket. The channel you accepted from the listening socket is a connected socket, and it is this socket that you put into non-blocking mode, register with OP_ACCEPT, etc.
Again It blocks on selector.select() for read event
Correct, but it doesn't block in read() or write() or accept() or finishConnect(). Using a selector is actually called multiplexed I/O: you wait for multiple channels and multiple events at the same time in a single operation.

NIO - Can state of readiness of channel be lost?

I am new to a non-blocking IO in Java. I have a question - will a readiness of a non-blocking channel be lost by the selector, if a new packet from a server will arrive after we completed reading from the channel, but before we removed a selection key for this channel from selector? Example code here:
Selector selector;
// ......
while (true) {
selector.select();
Set<SelectionKey> set = selector.selectedKeys();
Iterator<SelectionKey> iterator = set.iterator();
while (iterator.hasNext()) {
SelectionKey key = iterator.next();
SocketChannel channel = (SocketChannel) key.channel();
ByteBuffer byteBuffer = ByteBuffer.allocate(GOOD_ENOUGH_CAPACITY);
while (channel.read(byteBuffer) > 0) ;
// HERE ! What happen if server started to write new message here?
// Will this channel be selected on next selector.select() ?
iterator.remove();
}
}

Yes, key will be selected. You must use method
key.cancel();
to remove key from selector

Datagram Channel Sockets not writing Java

Simply said, I'm able to receive data from the connected socket, until about 10 seconds after it starts. The solution to fix this is, to send data to the 'client'(an ARDrone) to keep the data stream going, otherwise it will stop sending data to the phone. However, my current code for some reason only writes data to the client (the ARDrone) on first connect and never after. I need it to keep sending data to the ARDrone even after the socket is communicating.
I've tried moving around the channel.register() calls, but nothing I've done seems to send data to the ARDrone, as I need.
Creating the connection:
channel = DatagramChannel.open();
channel.configureBlocking(false);
channel.socket().bind(new InetSocketAddress(video_port));
channel.connect(new InetSocketAddress(drone_addr, video_port));
selector = Selector.open();
channel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_WRITE);
Here is the current send/receive data function.
public void run() {
try {
ByteBuffer inbuf = ByteBuffer.allocate(BUFSIZE);
done = false;
while (!done) {
selector.select();
if (done) {
disconnect();
break;
}
Set readyKeys = selector.selectedKeys();
Iterator iterator = readyKeys.iterator();
while (iterator.hasNext()) {
SelectionKey key = (SelectionKey) iterator.next();
iterator.remove();
if (key.isWritable()) {
byte[] trigger_bytes = { 0x01, 0x00, 0x00, 0x00 };
ByteBuffer trigger_buf = ByteBuffer.allocate(trigger_bytes.length);
trigger_buf.put(trigger_bytes);
trigger_buf.flip();
channel.write(trigger_buf);
channel.register(selector, SelectionKey.OP_READ);
} else if (key.isReadable()) {
inbuf.clear();
int len = channel.read(inbuf);
if (len > 0) {
inbuf.flip();
final BufferedVideoImage vi = new BufferedVideoImage();;
vi.addImageStream(inbuf);
drone.videoFrameReceived(0, 0, vi.getWidth(), vi.getHeight(), vi.getJavaPixelData(), 0, vi.getWidth());
}
}
}
} catch (Exception e) {
drone.changeToErrorState(e);
}
}

I believe you corrupt your event interest on the first write with this line:
channel.register(selector, SelectionKey.OP_READ);
Per documentation, this resets it to OP_READ only.
Edit 0:
Per your comment - yes, I think you should remove that line entirely, and do not treat read and write cases as alternatives. In general a socket could be readable and writable at the same time. So what's happening now is that the write case prevents the read case since UDP socket is always writable (versus TCP that buffers outgoing data).
So my suggestion is to not include OP_WRITE into that event set at all, but handle writing in some other fashion, say on a timer, or after each read, or whatever makes sense to your app.
Hope this helps.