[Cojen]

From what I can tell, async system calls are used whenever possible. A blocking call on a socket doesn't make a blocking system call, thus permitting the carrier OS thread to go do something else. As for file I/O, things are a bit messy. Older linux kernels don't support "true" async file I/O, and the Java NIO async file channels do use thread pools to emulate async behavior in that case.