refl

An example project and script to clean up reflection configurations produced by the GraalVM native-image-agent for Clojure projects.

Problem description

The reflection configs produced by the GraalVM native-image-agent contain many false positives for Clojure programs. This can be alleviated using a caller-based filter, like described here. But excluding all calls from clojure.lang.RT may be too coarse for some programs. This repo offers a finer-grained solution.

How it works

This project invokes GRAALVM_HOME/bin/java on an AOT-ed Clojure program that does runtime reflection, twice. In both runs the native-image-agent is used. The first time it is invoked a trace-file.json will be produced. The second time a reflect-config.json will be produced. Unfortunately the reflect-config.json isn't very usable for GraalVM native-image yet, since it contains a lot of false positives. Using the script script/gen-reflect-config.clj the information from both JSON files is combined to create a cleaned up version of the reflect config, called reflect-config-cleaned.json. This config is then used for native compilation.

How to use

This project is intended as a reference example on how you could clean up your generated reflection config. Feel free to copy the code and change it to your needs. See the tasks section to see what you can do in this project.

Requirements

Download GraalVM and set GRAALVM_HOME. You will also need clojure for Clojure compilation. Scripts are executed with bb (babashka).

The example

This is the example Clojure program that performs reflection at runtime:

(ns refl.main
  (:require [clojure.java.io :as io])
  (:gen-class))

(defn refl-str [s]
  s)

(defn file [f]
  (io/file f))

(defn -main [& _]
  (let [res (refl-str "foo")]
    (println (.length res)) ;; reflect on string
    (prn (type (into-array [res res]))) ;; make array of unknown type
    (println (.getPath (file "."))))) ;; reflect on file

This is the generated reflection config, after executing bb gen-reflect-config:

[ {
  "name" : "java.io.File",
  "allPublicMethods" : true
}, {
  "name" : "java.lang.Object[]"
}, {
  "name" : "java.lang.String",
  "allPublicMethods" : true
}, {
  "name" : "java.lang.String[]"
}, {
  "name" : "java.lang.reflect.AccessibleObject",
  "methods" : [ {
    "name" : "canAccess",
    "parameterTypes" : [ "java.lang.Object" ]
  } ]
} ]

Note that the raw reflect-config.json is 527 lines long and contains many false positives, mainly due to calls to Class/forName in clojure.lang.RT and some other places. Unfortunately ignoring all calls from clojure.lang.RT is too coarse, since it also does reflection to create arrays.

The java.lang.reflect.AccessibleObject is needed because the clojure.lang.Reflector reflectively looks up the canAccess method on Method here. How meta.

Tasks

See bb tasks:

The following tasks are available:

compile-clj
classpath
gen-reflect-config
compile-native

License

Unlicense.