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.

Important:

• Use the right GraalVM version modifier: e.g. graalvm-20.2.0. The modifier must exactly match the version of GraalVM native-image you are using.
• 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.

Leiningen

Relevant config for project.cljc:

(defproject foo "0.0.1-SNAPSHOT"
  :profiles {:native-image {:dependencies [[borkdude/clj-reflector-graal-java11-fix "0.0.1-graalvm-20.2.0"]]}})

To produce an uberjar that is fed to native-image you can:

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

and then:

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

Tools Deps

Relevant config for deps.edn:

{:aliases
 {:native-image {:extra-deps {borkdude/clj-reflector-graal-java11-fix
                              {:mvn/version "0.0.1-graalvm-20.2.0"
                               :exclusions [org.graalvm.nativeimage/svm]}}}}}

Notice the exclusion? This project depends on native dep org.graalvm.nativeimage/svm. While technically correct, tools deps does not support native deps and fails for this native dep. Since the native dep is provided by the Graal installation anyway, it can be safely excluded for tools deps.

Your compile script would get the classspath for native-image via:

clojure -A:native-image -Spath

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.