Selector.select() does not fill selectedKeys() if wakeup() is called before select()

Question

I have a somewhat complex loop processing several different NIO channels and other objects. Some of these objects trigger their processing (i.e. simulate a "select"-event) by calling Selector.wakeup().

When I accidentally put my thread into a busy-loop by always calling wakeup() before every call to select(), I noticed that my other NIO channels were no longer getting serviced at all. As soon as the condition triggering the busy-loop went away, all other channels were immediately serviced again as usual.

After a bit of messing around with this I figured out that the reason for my other channels no longer being serviced was that select() did not fill the list of selectedKeys() if it returned immediately due to a pending wakeup().

Is this expected behavior? I couldn't find anything in the docs regarding this detail.
If so, is there a way to prevent this more elegantly than by following every select() with an additional selectNow()?

score 0 · Answer 1 · answered Feb 01 '17 at 22:18

0

Of course it's expected. It retuned immediately because of the wakeup, so it didn't block looking for ready channels, populate the selected keys, etc.

answered Feb 01 '17 at 22:18

user207421

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Is this documented somewhere? I was assuming that wakeup(); select(); would be equivalent to selectNow(); – Markus A. Feb 01 '17 at 22:20
Am I correct in assuming that wakeup(); selectNow(); then also doesn't populate the selected keys? – Markus A. Feb 01 '17 at 22:21

Selector.select() does not fill selectedKeys() if wakeup() is called before select()

1 Answers1