Clojure - Reflection Warning with primitive array .length

Question

I'm having some trouble trying to use instance methods on a java float array in clojure. Specifically, the following code gives a reflection warning on the call to .length

(defn make-matrix
  ([^floats fs]
   (let [len-fs (.length fs)]
     (cond (>= len-fs 16) (Matrix4. fs)
           (>= len-fs 9)  (Matrix3. fs)
           :else (throw (IllegalArgumentException. (str "Array must have at least 9 elements (found" len-fs ")")))))))

By my understanding, the type-hint should eliminate the need for reflection to resolve the call to .length. What am I missing?

Answer 1

Arrays on the JVM are not quite like regular objects and their lengths are not obtained through a field lookup, but rather through a specialized opcode arraylength. Java's array.length notation is just syntactic sugar that compiles down to this opcode. Therefore, (.length fs) will simply fail at runtime, because fs will be found to have neither a length method nor a length field (assuming it is in a fact an array).

Thus, you should use the clojure.core/alength function:

(alength fs)

Without a type hint, this will result in a reflective call; with a type hint, it will be a direct call to a static method returning fs.length (in Java notation).

Answer 2

Primitive arrays are a somewhat special case in the JVM, a proof of this is that there's a bytecode instruction for getting the length of them: arraylength. So the reason why Clojure is showing a reflection warning is because the class for arrays of floats [F does not declare a field with the name length, so when it tries to get it (here) the field is not there.

You can check it yourself doing the following:

(-> (float-array [1])
  class
  (.getField "length"))