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POHow to detect a Selector.wakeup call
 

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  1. POHow to detect a Selector.wakeup call
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    <p>If I would write:</p> <pre> int selectedChannels = selector.select(); Set selectedKeys = selector.selectedKeys(); if ( selectedChannels != selectedKeys.size() ) { // Selector.select() returned because of a call to Selector.wakeup() // so do synchronization. } // Continue with handling selected channels. </pre> <p>would it correctly detect the wakeup-call?</p> <p><strong>Backgroundinformation:</strong></p> <p>I'm writing a server which most of the time just receives packets and stores them in a file. Very rarely the application has the need to send itself a special packet. For this it initiates a connection (from a different thread) to the server socket:</p> <pre> SocketChannel channel = SocketChannel.open(); channel.configureBlocking( false ); channel.connect( new InetSocketAddress( InetAddress.getLocalHost(), PORT )); selector.wakeup(); SelectionKey key = channel.register( selector, SelectionKey.OP_CONNECT ); </pre> <p>The problem is that SelectableChannel.register() might block if the main thread is already in Selector.select(). To prevent this from happening I'm calling Selector.wakeup() which let's the main thread return prematurely from select(). To make sure the other thread has the chance to complete the register-call, I would have to synchronize the main thread, but I would have to do it after <strong>every</strong> return from select(). If I could detect whether it returned from select() because of a wakeup() call, then I could optimize it for just this case.</p> <p>So, in theory the top code snippet should work, but I was wondering whether it would only do so, because it relies on some unspecified behavior?</p> <p>Thanks for any hints.</p>
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    1. COWhat's your motivation for wanting to avoid locks? Is it that you're worried about execution time, or do you call select() in many places and want to avoid code duplication?
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    2. COI'm worried about execution time, although this might be a moot point since select() and register() already synchronize on the registered keys.
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    3. COAgreed, worrying about this here is a classic example of premature optimization. Unless you're really talking about single microsecond tolerances you're not going to notice any slowdown. I'll throw up an answer using locks.
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