Sample request: detect cycles among static initializers

[vlsi]

Sample issues: google/guava#1977 , apache/calcite#2255

It would be nice if there was a documented sample that configures ArchUnit to detect cycles among class initializers.

[codecholeric]

Unfortunately I think at the moment the cycle algorithm of ArchUnit is not public API for use cases outside of the slices API 🤔
So if you only want to check static initializers it would probably be quite some implementation work. You could do it somewhat like this

ArchRuleDefinition.codeUnits()
    .that().haveName(JavaStaticInitializer.STATIC_INITIALIZER_NAME)
    .should(new ArchCondition<JavaCodeUnit>("be free of cycles") {
        @Override
        public void check(JavaCodeUnit codeUnit, ConditionEvents events) {
            Set<String> targetsReachableThroughStaticInitializers = new HashSet<>();
            follow(codeUnit, targetsReachableThroughStaticInitializers);
            if (targetsReachableThroughStaticInitializers.contains(codeUnit.getOwner().getName())) {
                events.add(SimpleConditionEvent.violated(codeUnit,
                        codeUnit.getDescription() + " introduces a cycle within " + targetsReachableThroughStaticInitializers));
            }
        }

        private void follow(JavaCodeUnit codeUnit, Set<String> targetsReachableThroughStaticInitializers) {
            for (JavaAccess<?> access : codeUnit.getAccessesFromSelf()) {
                if (!access.getTargetOwner().equals(access.getOriginOwner())) {
                    boolean encounteredNew = targetsReachableThroughStaticInitializers.add(access.getTargetOwner().getName());
                    if (encounteredNew) {
                        follow(access, targetsReachableThroughStaticInitializers);
                    }
                }
            }
        }

        private void follow(JavaAccess<?> access, Set<String> targetsReachableThroughStaticInitializers) {
            Optional<JavaStaticInitializer> initializer = access.getTargetOwner().getStaticInitializer();
            if (initializer.isPresent()) {
                follow(initializer.get(), targetsReachableThroughStaticInitializers);
            }
        }
    })

It's just a quick hack and not very sophisticated, also doesn't report exactly, how the cycle looks like, but only that within a set of classes a cycle between static initializers occurred. And reports the same cycle once for each static initializer 😉
But maybe you can use it as an inspiration how you could go forward with this. Other than that I think we should maybe open up the cycle algorithm to be used with custom implementations, because in the end it already is pretty generic and could be used for this 🤔