What static type is needed to fix this "Illegal reflective access"?

Question

Attempting to get the length of a Unicode string, I found this code on RosettaCode [1] called grapheme-length :

(def grapheme-length
  #(->> (doto (BreakIterator/getCharacterInstance)
          (.setText %))
        (partial (memfn next))
        repeatedly
        (take-while (partial not= BreakIterator/DONE))
        count))

I invoked it from the REPL to test it like this:

(star-traders.util/grapheme-length "\uD83D\uDE04⌘\uD83D\uDE00") ; Expecting 3

While it did return the expected value, I also got an unexpected warning in red text asking me to "consider reporting this to the maintainers of clojure.lang.InjectedInvoker"...

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by clojure.lang.InjectedInvoker/0x0000000800cc8440 (file:/Users/myusername/.m2/repository/org/clojure/clojure/1.10.1/clojure-1.10.1.jar) to method sun.text.RuleBasedBreakIterator.setText(java.text.CharacterIterator)
WARNING: Please consider reporting this to the maintainers of clojure.lang.InjectedInvoker/0x0000000800cc8440
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

Apparently this is a known issue with the JVM due to the addition of the Java 9 module system and is mentioned in the Clojure FAQ. [2]

While the problem does go away if --illegal-access=deny is added to the JVM Args, it appears that the recommended solution is to fix the code with type hinting.

I've tried using --illegal-access=warn to locate the spot and fix it, like this:

          (.setText ^java.text.BreakIterator %)) ; Problem happens here (I think)

However, the problem still occurs, and this has me curious what the right type I should be using is.

How do I fix the code to get rid of the compiler warning, rather than suppress warnings?

[1] https://rosettacode.org/wiki/String_length#Grapheme_Length_2
[2] https://clojure.org/guides/faq#illegal_access

Answer 1

There are actually reflection warnings coming from a couple of lines in this: the .setText call and the .next call. I believe you need the following to resolve both of them:

user=> (def grapheme-length
  #(->> (doto (BreakIterator/getCharacterInstance)
          (.setText ^String %)) ; hint the argument here
        (partial (fn [^BreakIterator o] (.next o))) ; hint the object here
        repeatedly
        (take-while (partial not= BreakIterator/DONE))
        count))
#'user/grapheme-length
user=> (grapheme-length "\uD83D\uDE04⌘\uD83D\uDE00")
3
user=> 

Comments

Just in general, memfn will always introduce reflection, just by the nature of its impl.

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This did the trick, Sean. Thank you.  I'd be curious how you figured this out, but that's a conversation for another day. I suspect I wasn't groking what the error messages were telling me.  (Thanks, also, Alex -- this was my first encounter with memfn.)

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I just copied my REPL session into a Gist and annotated it: https://gist.github.com/seancorfield/01e42dbf3de68bab6ef3d7d6245af192