Learning Java, 4th Edition

Scalable I/O with NIO

We’ll now conclude the discussion of the NIO package we began in Chapter 12 by talking about nonblocking and selectable network communications. All our server examples in this chapter thus far have used a thread-bound pattern (one thread per I/O operation). In Java, this is very natural because of the ease with which we can create threads. It’s also very efficient, within limits. Problems arise when you try to build very large-scale servers using this style of client handling. While on a large machine it’s certainly possible to have hundreds or even thousands of threads (especially if they’re mostly idle, waiting for I/O), this is a resource-hungry solution. Every thread you start in Java consumes memory for its internal stack, and the performance of managing this number of threads is highly system-dependent.

An alternate approach is to take a lesson from the old, dark days before threading was available and use nonblocking I/O operations to manage numerous communications from a single thread. Better yet, our server uses a configurable pool of threads, taking advantage of machines with many processors.

At the heart of this process is the concept of selectable I/O. It’s not good enough to simply have nonblocking I/O operations if you have no way to efficiently poll for work to be done. The NIO package provides for efficient polling using selectable channels. A selectable channel allows for the registration of a special kind of listener called a selector that can ...