Common NIO mistakes

Just for grins, I've been writing my own NIO server. Ostensibly, it's to provide a server that does not need to create lots of String objects (whose constant creation and destruction can hammer performance). Secondarily, I'm learning more about the minutiea of NIO.

These are the silliest mistakes I've made so far:


Buffer Reading and Writing

Most Java developers never have to worry about this as it's all hidden by your container of choice. But if you roll your own, remember the order for reading is read-flip-get. That is:

ReadableByteChannel.read(ByteBuffer)
Buffer.flip()
ByteBuffer.get(...)

For writing, it's put-flip-write, that is:

ByteBuffer.put(...)
Buffer.flip()
WritableByteChannel.write(ByteBuffer)


ServerSocketChannel Backlogs

Don't forget to set this in the bind method or you may be left wondering why your clients start throwing exceptions under a modest load. If too many connections swamp the serber, client's will see exceptions that say something like "connection reset by peer".


Call register() and select() in the Same Thread

It's not obvious, but you must call AbstractSelectableChannel.register and Selector.select(...) in the same thread [1].

[1] Stack Overflow