clj-reflector-graal-java11-fix

This library offers a fix for an issue with clojure.lang.Reflector when used together with GraalVM native-image on java11.

Usage

Include this library in your leiningen profile or deps.edn alias so it's available on the classpath for GraalVM native-image. E.g.:

(defproject foo "0.0.1-SNAPSHOT"
  :profiles {:native-image {:dependencies [[borkdude/clj-reflector-graal-java11-fix "0.0.1-graalvm-19.3.2"]]}})

Use the right GraalVM version modifier: graalvm-19.3.1 or graalvm-20.0.0. The modifier must exactly match the version of GraalVM native-image.

Do NOT distribute this library as part of libraries or applications that are run with a JVM. Use it for compiling to native binaries only.

E.g. to produce an uberjar that is fed to native-image you can do:

$ lein with-profiles +native-image do clean, uberjar

and then:

$ native-image -jar target/foo-0.0.1-SNAPSHOT-standalone.jar

The problem

JDK11 is supported since GraalVM 19.3.0. The class clojure.lang.Reflector uses a MethodHandle to maintain compatibility with java8 and java11 at the same time:

static {
	MethodHandle pred = null;
	try {
		if (! isJava8())
			pred = MethodHandles.lookup().findVirtual(Method.class, "canAccess", MethodType.methodType(boolean.class, Object.class));
	} catch (Throwable t) {
		Util.sneakyThrow(t);
	}
	CAN_ACCESS_PRED = pred;
}

GraalVM does not support MethodHandles that cannot be analyzed as a compile time constant. It will complain when it analyzes clojure.lang.Reflector on JDK11:

Exception in thread "main" com.oracle.svm.core.jdk.UnsupportedFeatureError: Invoke with MethodHandle argument could not be reduced to at most a single call or single field access. The method handle must be a compile time constant, e.g., be loaded from a `static final` field. Method that contains the method handle invocation: java.lang.invoke.Invokers$Holder.invoke_MT(Object, Object, Object, Object)
    at com.oracle.svm.core.util.VMError.unsupportedFeature(VMError.java:101)
    at clojure.lang.Reflector.canAccess(Reflector.java:49)
    ...

See the issue on the GraalVM repo.

The solution

GraalVM supports substitutions. For JDK11 or later the method canAccess is replaced as follows:

@TargetClass(className = "clojure.lang.Reflector")
final class Target_clojure_lang_Reflector {

    @Substitute
    @TargetElement(onlyWith = JDK11OrLater.class)
    private static boolean canAccess(Method m, Object target) {
        // JDK9+ use j.l.r.AccessibleObject::canAccess, which respects module rules
        try {
            return (boolean) m.canAccess(target);
        } catch (Throwable t) {
            throw Util.sneakyThrow(t);
        }
    }
}

License

Copyright © 2020 Michiel Borkent

Distributed under the EPL License. See LICENSE.

This project contains code from:

• Clojure, which is licensed under the same EPL License.