Java NIO从远程机器上读写

Question

I'd like to use the NIO to send/receive data to/from a distant machine. I can send or receive data at any time, when i need to send data i just send it without any queries from the distant machine, and the distant machine send me data at regular interval. I don't understand the NIO mechanism. What generates and read or write event on the Selector SelectionKey ? Is it possible to use only one ServerSocketChannel on my side, to read data from the distant machine et to write data to it ? That is what i understand but i don't see how the writing event can be triggered... Thank you for your explanation.

I already did some coding and i can read data coming in from the distant machine, but cannot write. I use Selector and i don't know how can i write data. The logged message "handle write" is never written, but in wireshark i can see my packet.

    public class ServerSelector {

    private static final Logger logger = Logger.getLogger(ServerSelector.class.getName());
    private static final int TIMEOUT = 3000; // Wait timeout (milliseconds)
    private static final int MAXTRIES = 3;
    private final Selector selector;

    public ServerSelector(Controller controller, int... servPorts) throws IOException {
        if (servPorts.length <= 0) {
            throw new IllegalArgumentException("Parameter(s) : <Port>...");
        }
        Handler consolehHandler = new ConsoleHandler();
        consolehHandler.setLevel(Level.INFO);
        logger.addHandler(consolehHandler);

        // Create a selector to multiplex listening sockets and connections
        selector = Selector.open();

        // Create listening socket channel for each port and register selector
        for (int servPort : servPorts) {
            ServerSocketChannel listnChannel = ServerSocketChannel.open();
            listnChannel.socket().bind(new InetSocketAddress(servPort));

            listnChannel.configureBlocking(false); // must be nonblocking to register
            // Register selector with channel.  The returned key is ignored
            listnChannel.register(selector, SelectionKey.OP_ACCEPT);
        }

        // Create a handler that will implement the protocol
        IOProtocol protocol = new IOProtocol();

        int tries = 0;
        // Run forever, processing available I/O operations
        while (tries < MAXTRIES) {
            // Wait for some channel to be ready (or timeout)
            if (selector.select(TIMEOUT) == 0) { // returns # of ready chans
                System.out.println(".");
                tries += 1;
                continue;
            }

            // Get iterator on set of keys with I/O to process
            Iterator<SelectionKey> keyIter = selector.selectedKeys().iterator();
            while (keyIter.hasNext()) {
                SelectionKey key = keyIter.next(); // Key is a bit mask
                // Server socket channel has pending connection requests?
                if (key.isAcceptable()) {
                    logger.log(Level.INFO, "handle accept");
                    protocol.handleAccept(key, controller);
                }

                // Client socket channel has pending data?
                if (key.isReadable()) {
                    logger.log(Level.INFO, "handle read");
                    protocol.handleRead(key);
                }

                // Client socket channel is available for writing and
                // key is valid (i.e., channel not closed) ?
                if (key.isValid() && key.isWritable()) {
                    logger.log(Level.INFO, "handle write");
                    protocol.handleWrite(key);
                }
                keyIter.remove(); // remove from set of selected keys
                tries = 0;
            }
        }
    }
}

The protocol

    public class IOProtocol implements Protocol {

    private static final Logger logger = Logger.getLogger(IOProtocol.class.getName());

    IOProtocol() {
        Handler consolehHandler = new ConsoleHandler();
        consolehHandler.setLevel(Level.INFO);
        logger.addHandler(consolehHandler);
    }

    /**
     *
     * @param key
     * @throws IOException
     */
    @Override
    public void handleAccept(SelectionKey key, Controller controller) throws IOException {
        SocketChannel clntChan = ((ServerSocketChannel) key.channel()).accept();
        clntChan.configureBlocking(false); // Must be nonblocking to register
        controller.setCommChannel(clntChan);
        // Register the selector with new channel for read and attach byte buffer
        SelectionKey socketKey = clntChan.register(key.selector(), SelectionKey.OP_READ | SelectionKey.OP_WRITE, controller);
    }

    /**
     * Client socket channel has pending data
     *
     * @param key
     * @throws IOException
     */
    @Override
    public void handleRead(SelectionKey key) throws IOException {
        Controller ctrller = (Controller)key.attachment();
        try {
            ctrller.readData();
        } catch (CommandUnknownException ex) {
            logger.log(Level.SEVERE, null, ex);
        }
        key.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE);
    }

    /**
     * Channel is available for writing, and key is valid (i.e., client channel
     * not closed).
     *
     * @param key
     * @throws IOException
     */
    @Override
    public void handleWrite(SelectionKey key) throws IOException {

        Controller ctrl = (Controller)key.attachment();
        ctrl.writePendingData();
        if (!buf.hasRemaining()) { // Buffer completely written ?
            // Nothing left, so no longer interested in writes
            key.interestOps(SelectionKey.OP_READ);
        }
    buf.compact();
    }
}

The controller

    /**
     * Fill buffer with data.
     * @param msg The data to be sent
     * @throws IOException 
     */
    private void writeData(AbstractMsg msg) throws IOException {
//        
        writeBuffer = ByteBuffer.allocate(msg.getSize() + 4);
        writeBuffer.putInt(msg.getSize());
        msg.writeHeader(writeBuffer);
        msg.writeData(writeBuffer);
        logger.log(Level.INFO, "Write data - message size : {0}", new Object[]{msg.getSize()});
        logger.log(Level.INFO, "Write data - message : {0}", new Object[]{msg});
    }

    /**
     * Write to the SocketChannel
     * @throws IOException 
     */
    public void writePendingData() throws IOException {
        commChannel.write(writeBuffer);
    }

Answer 1

ServerSocketChannel is used to make a connection, but not send data. You need one ServerSocketChannel and one SocketChannel per each connection.

Examples of reading and writing using SocketChannel :

ByteBuffer buf = ByteBuffer.allocate(48);
int bytesRead = socketChannel.read(buf);

Your program will sleep at second line until data will come. You need to put this code in infinite loop and run it in background Thread . When data came you can process it from this thread, then wait for another data to come.

ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();
buf.put("Hello!".getBytes());

buf.flip();

while(buf.hasRemaining()) {
    channel.write(buf);
}

There is no blocking methods, so if you sending small byte buffer you can call this from your main Thread .

Source

ADD: Don't set OP_WRITE key on new connection. Only OP_READ . When you want to write some data you need to notify selector that you want to send something and send it in events loop. Good solution is to make a Queue of outcoming messages. Then follow this steps:

• adding data to Queue
• setting OP_WRITE to channel's key
• in while (keyIter.hasNext()) loop you'll have writable key , write all data from queue and remove OP_WRITE key.

It's hard for me to understand your code, but I think you'll find out what's the problem. Also if you want to have only one connection there is no need to use Selector . And this is weird that you binding few ServerSocketChannels .

Answer 2

I would suggest you use blocking NIO (which is the default behaviour for SocketChannel) You don't need to use a Selector but you can use one thread for reading and another for writing.

---

Based on your example.

private final ByteBuffer writeBuffer = ByteBuffer.allocateDirect(1024*1024);

private void writeData(AbstractMsg msg) {
    writeBuffer.clear();
    writeBuffer.putInt(0); // set later
    msg.writeHeader(writeBuffer);
    msg.writeData(writeBuffer);
    writeBuffer.putInt(0, writeBuffer.position());

    writeBuffer.flip();
    while(writeBuffer.hasRemaining())
        commChannel.write(writeBuffer);
}

Answer 3

What generates and read or write event on the Selector SelectionKey?

OP_READ: presence of data or an EOS in the socket receive buffer.

OP_WRITE: room in the socket send buffer.