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What options for async io (socket-based) are there in java other then java.nio? Also does java.nio use threads in the backround (as I think .NET's async-socket-library does, maybe it's been changed) or is it "true" async io using a proper select call?

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    Where the underlying stream supports it, .NET uses IO completion ports and a ThreadPool thread for executing callbacks. Commented Feb 26, 2009 at 20:31
  • Jon, when might the underlying stream NOT support it? Commented May 4, 2010 at 21:10
  • Any specific reason about why not to use java.nio? Actually, I am trying to implement some asynchronous i/o in my project at work and I haven't used either of these before. Hence wanted to know. Thanks. Commented Jun 23, 2011 at 17:31
  • I usually use async-io.org/games.html for java and you can get examples of code for game or chat. Commented Jan 11, 2017 at 19:54

6 Answers 6

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Java's NIO package (as of Java6), provides support for non-blocking I/O only, via Selectors. Java7 is hopefully going to ship with NIO.2, which includes asynchronous I/O support. Today, your best bet is to make use of a framework. ARMistice mentioned Mina. Here are some others.

  1. Grizzly. This is the I/O core for Sun's GlassFish server. Grizzly provides a facility for doing asynchronous reads/writes (via a queue model). It supports TCP and UDP alike. I've used Grizzly in a couple of projects. There are things I like and dislike about the framework, but to detail this is really another topic. I will say that it's quite easy to get something up and running and Grizzly does a lot of the heavy lifting for you.
  2. Netty. This project comes from one of the original authors of the Mina project. I haven't used this one so I don't know about its support for asynchronous I/O. You should take a look.

Now, with regard to your question about threads, NIO Selectors do not use threads for non-blocking I/O. In JDK6 they use select() under Windows and the epoll facility on newer Linux kernels. For asynchronous I/O, threading details depend on the framework.

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7 Comments

Java's NIO also supports blocking IO. ;)
Netty is asynchronous and event-driven like MINA. Check out the testimonials and performance reports written by real users in the home page. :)
What's the difference between non-blocking IO & async IO?
@oconnor0 Non-blocking IO the kernel calls don't block under buffer-full(on writes) and buffer-empty(on reads), these states result in soft-error returns from the respective APIs.However non-blocking IO still copies data between user-space and kernel-space causing an unwanted extra copying of data which has a significant overhead when a lot of data is involved.The kernel then(may)copy the data yet again into packet size pieces with networking protocol overhead around it.Some network stacks/hardware drivers may support scatter gather to optimize the in kernel stages,but worst case is 3 copies.
@oconnor0 Async IO accepts to remove the extra copying action during the data transition between user-space and kernel-space.This allows the kernel to directly access the data from user-space.To achieve this the application prepares memory and posts an IO request to the kernel.Control returns back to the application (the API is much like non-blocking).At some future point in time the kernel may access the user-space memory while performing the IO.Once done the kernel fires a signal like callback into user-space via the completion handler to notify the application of the IO result of request.
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JAVA 7 arrived so new answer is NIO.2 with Future class. Example :

On server side:

final AsynchronousServerSocketChannel serverSocket=
  AsynchronousServerSocketChannel.open().bind(new InetSocketAddress("127.0.0.1", 2587)); // Listening on port 2587 for client connection
Future<AsynchronousSocketChannel> future= serverSocket.accept();
final AsynchronousSocketChannel clientSocket= future.get(); // now it's blocking, useful: future.isDone() and .isCancelled()

//Do whatever you want ..
InputStream stream = Channels.newInputStream(clientSocket) (...)

On client side:

AsynchronousSocketChannel clientChannel = AsynchronousSocketChannel.open();
Future connected = localSocket.connect(ourServerSocketAddress);
// later: if(future.isDone())
connected.get();

//Send something
OutputStream os = Channels.newOutputStream(clientChannel );
os.write (...)

Update: If you can use actor model then AKKA TCP IO would be even better.

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2 Comments

Future is not true async. It's thread/semaphore based: docs.oracle.com/javase/7/docs/api/java/util/concurrent/…
What is "true async"? - Just because it is implemented using threads does not mean it is not asynchronous...
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Another suggestion in regards to libs would be Naga (http://naga.googlecode.com). It is a bit less like a framework and more like a library. It tries to look more like the ordinary java sockets, if that is your cup of tea. It's minimalistic compared to Grizzly, Mina and Netty.

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Naga actually seems like a very nice wrapper around the asynchronous stuff.
If you just want to do asynchronous Socket I/O without a framework getting in your way, Naga is what you want.
I seem to recall people having used it for android.
I can confirm that naga works with Android. I haven't finished a production-quality app with it yet, but so far all my testing has been OK.
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java.nio is just a package - a collection of "dumb" classes - by itself it does not employ any use of threads. When used properly, such as in the Reactor design pattern, you can achieve proper, fully-scalable, asynchronous I/O.

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Your answer seems legit to me, but can you explain more? A little bit more explanations please.
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If you are interested in using it for Network Stuff. A really good choice is:

http://mina.apache.org/

Have a look there its easy to use and very powerfull.

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To the original question, the implementation only consumes a thread per I/O operation in one case, AsynchronousFileChannel on Unix/Linux systems.

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