In MRI, running the following code:

$$
# => 11989
10.times { f = File.open("/tmp/foo", "w") }

Creates 10 open file descriptors; even though f is not referenced, the file descriptor remains. JRuby also behaves this way.

lsof -p 11989 | grep "/tmp/foo" | wc -l
#10

Now if I do:

GC.start

lsof will no longer show any open file descriptors. wc -l is 1. JRuby does not do this. I can't seem to get JRuby to collect open file descriptors on a Garbage Collection. Any thoughts? I discovered this bug while sanity-checking my fix for this issue:

padrino/padrino-framework#877

Initially, I "fixed" the issue by no longer caching the file descriptor. This should have resolved the issue, but the file descriptors stuck around, even after a garbage collection. GC.start seems to do nothing. One thing I did notice, is if I forced a garbage collection from VisualVM then all but one of the file descriptors would be collected (just like MRI).

So, I'm not sure.