Question

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?

Answer 1

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.