NIO - Networking

Remarks#

SelectionKey defines the different selectable operations and information between its Selector and Channel. In particular, the attachment can be used to store connection-related information.

Handling OP_READ is pretty straight-forward. However, care should be taken when dealing with OP_WRITE: most of the time, data can be written to sockets so the event will keep firing. Make sure to register OP_WRITE only before you want to write data (see that answer).

Also, OP_CONNECT should be cancelled once the Channel has connected (because, well, it is connected. See this and that answers on SO). Hence the OP_CONNECT removal after finishConnect() succeeded.

Using Selector to wait for events (example with OP_CONNECT)

NIO appeared in Java 1.4 and introduced the concept of “Channels”, which are supposed to be faster than regular I/O. Network-wise, the SelectableChannel is the most interesting as it allows to monitor different states of the Channel. It works in a similar manner as the C select() system call: we get woken-up when certain types of events occur:

connection received (OP_ACCEPT)
connection realized (OP_CONNECT)
data available in read FIFO (OP_READ)
data can be pushed to write FIFO (OP_WRITE)

It allows for separation between detecting socket I/O (something can be read/written/…) and performing the I/O (read/write/…). Especially, all I/O detection can be done in a single thread for multiple sockets (clients), while performing I/O can be handled in a thread pool or anywhere else. That allows for an application to scale easily to the number of connected clients.

The following example shows the basics:

Create a Selector
Create a SocketChannel
Register the SocketChannelto the Selector
Loop with the Selector to detect events

Selector sel = Selector.open(); // Create the Selector SocketChannel sc = SocketChannel.open(); // Create a SocketChannel sc.configureBlocking(false); // … non blocking sc.setOption(StandardSocketOptions.SO_KEEPALIVE, true); // … set some options

// Register the Channel to the Selector for wake-up on CONNECT event and use some description as an attachement sc.register(sel, SelectionKey.OP_CONNECT, “Connection to google.com”); // Returns a SelectionKey: the association between the SocketChannel and the Selector System.out.println(“Initiating connection”); if (sc.connect(new InetSocketAddress(”www.google.com“, 80))) System.out.println(“Connected”); // Connected right-away: nothing else to do else { boolean exit = false; while (!exit) { if (sel.select(100) == 0) // Did something happen on some registered Channels during the last 100ms? continue; // No, wait some more
```
    // Something happened...
    Set<SelectionKey> keys = sel.selectedKeys(); // List of SelectionKeys on which some registered operation was triggered
    for (SelectionKey k : keys) {
        System.out.println("Checking "+k.attachment());
        if (k.isConnectable()) { // CONNECT event
            System.out.print("Connected through select() on "+k.channel()+" -> ");
            if (sc.finishConnect()) { // Finish connection process
                System.out.println("done!");
                k.interestOps(k.interestOps() & ~SelectionKey.OP_CONNECT); // We are already connected: remove interest in CONNECT event
                exit = true;
            } else
                System.out.println("unfinished...");
        }
        // TODO: else if (k.isReadable()) { ...
    }
    keys.clear(); // Have to clear the selected keys set once processed!
}
```
} System.out.print(“Disconnecting … ”); sc.shutdownOutput(); // Initiate graceful disconnection // TODO: emtpy receive buffer sc.close(); System.out.println(“done”);

Would give the following output:

Initiating connection
Checking Connection to google.com
Connected through 'select()' on java.nio.channels.SocketChannel[connection-pending remote=www.google.com/216.58.208.228:80] -> done!
Disconnecting ... done