How to force evaluation of lazy seqs in Clojure when running from an uberjar?

Question

Consider the following MWE:

(ns toto.core
(:gen-class))

(defn write-something [i]
  (spit (str "out-" (str i) ".txt") "Hi there!"))

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (dorun (pmap write-something (range 16))))

If I run -main from the REPL, it works as expected: it creates the files and returns. However, if I create an uberjar and run java -jar <toto.whatever-standalone.jar> it will create the files, but fail to exit the program. I suspect it is an issue with laziness, but can't figure why the dorun is not working as I'd expect. Any ideas on what I'm doing wrong?

It doesn't look like a laziness issue. If all files were created, then lazy sequence was realized. — Leonid Beschastny, Sep 25 '17 at 10:37
dorun walks through the successive nexts of the seq in any case so what is the fail message you get? — Ertuğrul Çetin, Sep 25 '17 at 10:38
There's no error message, it's just that the `java -jar ...` call never returns. — mvarela, Sep 25 '17 at 10:49
Leonid, you are right, the seqs were realized. Yet, the application doesn't return from main... — mvarela, Sep 25 '17 at 11:04
@mvarela could you provide additional information about your environment: os type and version, `java`, `clojure` and `lein` versions? — Leonid Beschastny, Sep 25 '17 at 11:13

score 4 · Accepted Answer · answered Sep 25 '17 at 11:52

In Clojure the pmap function uses future calls that utilize a common thread pool. The threads in the pool stay alive for a 1 minute after they have finished their tasks in order to reduce the cost of thread allocation.

If you are sure that the thread pool can be shut down (because the application quits) you need to call shutdown-agents after the (dorun ...) so the thread pool is killed.

Alternatively, you can call (System/exit <status>) to shut down the application instantly.

Thanks! the `shutdown-agents` call did the trick. – mvarela Sep 25 '17 at 13:55 — mvarela, Sep 25 '17 at 13:55

How to force evaluation of lazy seqs in Clojure when running from an uberjar?

1 Answers1