IO#select always returns nil with JRuby, when Files or standard streams are used: p select(nil, [STDOUT], nil, 40) # == > nil f = File.open("/tmp/blah.txt", "r+") p select(nil, [f], nil, 40) #==> nil f.close Looking at the code, in registerSelect(), there is a check: channel instanceof SelectableChannel And the method does nothing for non-selectable channels. Both, file channels and stdio channels are non-selectable, so they don't work.