at least in interop with java.lang.Thread, as the following code demonstrates:

def run 
  puts 'in def run'
  # super # invoking super allows the thread blocks to run 
end 

latch = java.util.concurrent.CountDownLatch.new(2)
t1 = java.lang.Thread.new { puts 'executing t1'; latch.count_down }
t2 = java.lang.Thread.new { puts 'executing t2'; latch.count_down }

t1.start
t2.start

success = latch.await(1, java.util.concurrent.TimeUnit::SECONDS) 
puts success ? 'succeeded' : 'failed'
exit (success ? 0 : 1)

I'm seeing this behavior on all JRuby 1.7 versions up to 1.7.18 as well as
1.6.8, using Oracle JDK 1.7.0_72.

The problem doesn't occur with Ruby threads, probably because no implicit Runnable is created.

Note that the test-unit gem defines run in such a way that only the first thread block never executes.

A workaround is to undef run right after requiring test-unit.