[FIXED] UnsupportedOperationException when using enum in Java 11, works fine in Java 8

Issue

I am porting an existing application Java 7 to Java 11 (Drools 7.42.0). The application works as expected if I compile as Java 1.8 and deploy on Java 11 container (Weblogic 14). But if I compile the same code in Java 11, I get the following error. The only change between 1.8 and 11 is the java compiler level in Maven pom. I have ASM7 as a dependency in Maven.

Caused by: java.lang.UnsupportedOperationException: This feature requires ASM7
        at org.mvel2.asm.ClassVisitor.visitNestMember(ClassVisitor.java:236)
        at org.mvel2.asm.ClassReader.accept(ClassReader.java:651)
        at org.mvel2.asm.ClassReader.accept(ClassReader.java:391)
        at org.drools.core.util.asm.ClassFieldInspector.processClassWithByteCode(ClassFieldInspector.java:107)
        at org.drools.core.util.asm.ClassFieldInspector.processClassWithByteCode(ClassFieldInspector.java:114)
        at org.drools.core.util.asm.ClassFieldInspector.<init>(ClassFieldInspector.java:86)
        at org.drools.core.util.asm.ClassFieldInspector.<init>(ClassFieldInspector.java:74)
        at org.drools.core.base.ClassFieldAccessorFactory.getClassFieldInspector(ClassFieldAccessorFactory.java:157)
        at org.drools.core.base.ClassFieldAccessorFactory.getFieldType(ClassFieldAccessorFactory.java:141)
        at org.drools.core.base.ClassFieldAccessorStore.getFieldType(ClassFieldAccessorStore.java:295)
        at org.drools.compiler.rule.builder.PatternBuilder.getValueType(PatternBuilder.java:1079)
        at org.drools.compiler.rule.builder.PatternBuilder.normalizeExpression(PatternBuilder.java:1047)
        at org.drools.compiler.rule.builder.PatternBuilder.buildExpression(PatternBuilder.java:965)
        at org.drools.compiler.rule.builder.PatternBuilder.buildCcdDescr(PatternBuilder.java:942)
        at org.drools.compiler.rule.builder.PatternBuilder.build(PatternBuilder.java:767)
        at org.drools.compiler.rule.builder.PatternBuilder.processConstraintsAndBinds(PatternBuilder.java:620)
        at org.drools.compiler.rule.builder.PatternBuilder.build(PatternBuilder.java:187)
        at org.drools.compiler.rule.builder.PatternBuilder.build(PatternBuilder.java:154)
        at org.drools.compiler.rule.builder.PatternBuilder.build(PatternBuilder.java:136)
        at org.drools.compiler.rule.builder.GroupElementBuilder.build(GroupElementBuilder.java:66)
        at org.drools.compiler.rule.builder.RuleBuilder.build(RuleBuilder.java:107)
        at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.internalAddRule(KnowledgeBuilderImpl.java:1183)
        at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.addRule(KnowledgeBuilderImpl.java:1178)
        at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.lambda$null$3(KnowledgeBuilderImpl.java:1134)
        at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
        at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:177)
        at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
        at java.base/java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:290)
        at java.base/java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:746)

I noticed while debugging that this was caused by an "enum" in our code. Drools works fine if we remove the enum and use constants. The error seems to suggest that this could happen to any nested classes. Is there a better way to handle this other than changing our code to remove nested classes and enums?

I see that in the class org.drools.core.util.asm.ClassFieldInspector, the API is set to ASM5, but MVEL is expecting ASM7. But not sure why it would work in Java 8 and not in Java 11. Any suggestions?

ClassFieldVisitor(final Class< ? > cls,
                  final boolean includeFinalMethods,
                  final ClassFieldInspector inspector) {
    super(Opcodes.ASM5);
    this.clazz = cls;
    this.includeFinalMethods = includeFinalMethods;
    this.inspector = inspector;
}

Solution

Disclaimer: I'm one of the Drools developers

That's interesting as we might not emit valid ASM code. Please file a bug on https://issues.redhat.com so that we can fix it

Thank you very much

Answered By - Luca Molteni
Answer Checked By - Marilyn (JavaFixing Volunteer)