Hi all,
I have recently been playing with futures and have stumbled on the
following situation.
(defn test-f1 [initial-v]
(let [n (atom initial-v)]
[n
(future (while (not (Thread/interrupted))
(Thread/sleep 5000)
(swap! n inc)
(println @n)))]))
(def t1 (test-f1 11))
This works as expected. I see the println outputs, can @(t1 0) and
(future-cancel (t1 1)) returns true.
Then I have:
(defn test-f2 [initial-v]
(let [n (atom initial-v)]
[n
(future (while (not (Thread/interrupted))
(try
(Thread/sleep 5000)
(swap! n inc)
(println @n)
(catch Exception e (println "whoops")))))]))
(def t2 (test -f2 11))
The same except now my while has a (try ... (catch Exception e ...).
However, now (future-cancel (t2 1)) returns false and the future cannot be
cancelled - I also see the "whoops" at the REPL. Seeing that "whoops" made
me wonder if the interrupt that would shutdown the thread is raising an
exception that is being caught by my (try .... (catch ...)) block, thus
never interrupting the thread.
I made a change to throw the exception again:
(defn test-f3 [initial-v]
(let [n (atom initial-v)]
[n
(future (while (not (Thread/interrupted))
(try
(Thread/sleep 5000)
(swap! n inc)
(println @n)
(catch Exception e (do (println "whoops") (throw
e))))))]))
and the behavior now matches that of test-f1.
I think I'm just not clear on some fundamental aspect of how future-cancel,
try/catch, and the underlying java are actually working here.
It seems *very* weird to me that after future-cancel cancels the future,
that the (try . . . (catch . . .)) would ever be invoked in the body of the
while loop. Yet, clearly the interrupt is being handled in the (try . . .
(catch . . . )). It also seems *very* weird that even in this case, that
the next time we get back to (while (not (Thread/interrupted)) that
(not(Thread/interrupted)) would be true . . . I guess the exception sets
the interrupt status for the thread. When exception gets caught in my (try
. . . (catch . . .)), the interrupt status is never set?
Thanks for any clarification.
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