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    <p>There are lots of factors involved in this one. I will try to summarize my findings as best as possible (aware of the fact that there is contention regarding the usefulness of reactor and proactor IO handling implementations).</p> <blockquote> <p>Is proactor based IO handling possible in Java in such a way that it is advantageous to use for specific scenarios.</p> </blockquote> <p>Java 1.4 introduced non-blocking IO which is NOT the same as asynchronous IO. Java SE 7 introduces asynchronous IO with JSR203 making "true" proactor style IO handling implementations possible.</p> <p>See <a href="http://openjdk.java.net/projects/nio/javadoc/java/nio/channels/AsynchronousSocketChannel.html">AsyncrhonousSocketChannel</a>, <a href="http://openjdk.java.net/projects/nio/javadoc/java/nio/channels/AsynchronousServerSocketChannel.html">AsynchronousServerSocketChannel</a></p> <blockquote> <p>and, if Java NIO does support proactor based IO handling (either in Java 6 or Java 7) is OS managed asynchronous IO support (i.e. completion callbacks from the OS) being utilized?</p> </blockquote> <p>Reading through the JSR 203 specs, completion handlers using new asynchronous channels are definitely supported and it is reported that native OS features are being utilized but I have not ascertained to what extent yet. I may follow up on this after an analysis of the Java 7 source (unless someone beats me to it).</p> <blockquote> <p>Furthermore, if the implementation is purely in-VM are the performance benefits so little that using proactive event handling offers nothing more than a different (possibly simpler) way of constructing concurrent network handling software.</p> </blockquote> <p>I have not been able to find any performance comparisons regarding new Asynchronous IO features in Java 7. I'm sure they will become available in the near future.</p> <p>As always, when presented with more than one way to tackle a problem the questions of which approach is better is almost always answered with "depends". Proactive event handling (using asynchronous completion handlers) is included with Java 7 and cannot simply exist without purpose. For certain applications, it will make sense to use such IO handling. Historically a common example given where proactor has good applicability is in a HTTP server where many short requests are issued frequently. For a deeper explanation <a href="http://www.cse.wustl.edu/~schmidt/PDF/proactor.pdf">give this a read</a> (provided only to highlight the advantages of proactor so try to overlook the fact that example code is C++).</p> <p>IMO it seems obvious that in many circumstances reactor/proactor complicate what would otherwise be a very simple design using a more traditional approach and in other more complex systems they offer a high degree of simplification and flexibility.</p> <p>. . .</p> <p>On a side note I highly recommend reading through <a href="http://www.mailinator.com/tymaPaulMultithreaded.pdf">the following presentation about NIO</a> which offers performance comparison between NIO and the "traditional" approach. Though I would also advise caution regarding the results presented as the NIO implementation in the benchmark was based on the pre Java 1.4 NBIO NIO library and not the NIO implementation shipped in 1.4. </p>
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    1. POJava Non-Blocking and Asynchronous IO with NIO & NIO.2 (JSR203) - Reactor/Proactor Implementations
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    1. USRoman Kagan
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    1. POJava Non-Blocking and Asynchronous IO with NIO & NIO.2 (JSR203) - Reactor/Proactor Implementations
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    1. COI came across this while trying to find something in java comparable to boost::asio. My take away is that nothing exists now, but something similar may be possible with Java7.
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