Java NIO Programming Cookbook 6 / 73
final int numSelectedKeys = selector.select();
if (numSelectedKeys > 0) {
handleSelectionKeys(selector.selectedKeys(), serverSocket);
}
}
}
• line 1: We create a Selector using the systems default SelectorProvider.
• line 2: We create a ServerSocketChannel which is a SelectableChannel.
• line 6: We configure the ServerSocketChannel for non-blocking mode.
• line 7: We then register the ServerSocketChannel with the Selector, we receive a SelectionKey from the registration although I
discard it, having no use for it. The serverSocket.validOps() call will return an operation set that is supported by the
Channel, which in this case is only the "Accept Connection" event. The returned SelectionKey contains an "interest set" which
indicates the set of IO events the Selector must monitor the Channel for.
• line 10: We call select() on the Selector which is blocking until some IO occurs on any of the SelectableChannel instances
that are registered with it. It will return the number of keys which are ready for IO activity.
The following code snippet demonstrates iterating through all the SelectionKey instances that indicate IO "ready" events from
Channel instances managed by the single Selector. We are only interested in "Accept" and Readable" events. For every new
connection accepted an "Accept" event is signaled and we can act on it. Likewise with a "read" ready event we can read
incoming data. It is important to remove the SelectionKey from the set after handling it, as the Selector does not do this and you
will continue to process that stale event.
Working with SelectionKeys
final Iterator<SelectionKey> selectionKeyIterator = selectionKeys.iterator();
while (selectionKeyIterator.hasNext()) {
final SelectionKey key = selectionKeyIterator.next();
if (key.isAcceptable()) {
acceptClientSocket(key, serverSocket);
} else if (key.isReadable()) {
readRequest(key);
} else {
System.out.println("Invalid selection key");
}
selectionKeyIterator.remove();
}
• line 13: Remember to remove the SelectionKey from the selected set as the Selector does not do this for us, if we don’t do it,
we will continue to process stale events.
The following code snippet demonstrates the use of registration of a SocketChannel with the same Selector that manages the
ServerSocketChannel. Here, however, the interest set is only for IO "read" events.
Registering a Channel with a Selector
final SocketChannel client = serverSocket.accept();
client.configureBlocking(false);
client.register(key.selector(), SelectionKey.OP_READ);
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