How to invoke StaticMethodExpr.eval?

Question

I am following this series to learn how clojure compiler works.

I tried to invoke eval method of StaticMethodExpr using below codes

(ns clojure.lang
  (:use clojure.core)
  (:import [clojure.lang Compiler Compiler$C]))

(def form (read-string "(+ 1 1)") )

(def expr (Compiler/analyze Compiler$C/EXPRESSION form))

(.eval expr)

but no luck and throw IllegalArgumentException:

Unhandled java.lang.IllegalArgumentException
   Can't call public method of non-public class: public
   java.lang.Object clojure.lang.Compiler$StaticMethodExpr.eval()

Is there anything I was missing or is this a bug in jdk ?

Michał Marczyk · Accepted Answer · 2017-02-05 06:18:10Z

1

You can still call the method using reflection:

(let [m (.getDeclaredMethod clojure.lang.Compiler$Expr "eval" (make-array Class 0))]
  (.setAccessible m true)
  (defn -eval [expr]
    (.invoke m expr (object-array 0))))

(-eval (clojure.lang.Compiler/analyze clojure.lang.Compiler$C/EXPRESSION '(+ 1 2)))
;= 3

This should be good enough if your goal is to explore.

A reflection-free Java program could call this method on the result of analysing (+ 1 2) via a reference of type clojure.lang.Compiler.HostExpr, which is public, even though neither the declaring interface of clojure.lang.Compiler.Expr nor the actual runtime type are public, so I suppose it is somewhat surprising that setAccessible is required.

answered Feb 5, 2017 at 6:18

Michał Marczyk

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