Java Language NIO - Networking Using Selector to wait for events (example with OP_CONNECT)
Example
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 theSelector - Loop with the
Selectorto 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
