The ClassTransform returned by ClassTransform.transformingMethods is an instance of jdk.internal.classfile.impl.TransformImpl.ClassMethodTransform. This class (well, record) overrides andThen so that when the next transform is also a ClassMethodTransform, it will return a new ClassMethodTransform with the MethodTransforms chained and a predicate equivalent to first.and(second).

The documentation of ClassTransform.andThen says “elements presented to the builder of this transform will become the input to the next transform”. Because ClassMethodTransform will present non-matching elements to the builder, I believe the overridden implementation does not comply with the definition of ClassTransform.andThen.

Example:

ClassTransform.transformingMethods(mm -> mm.methodName().equalsString("foo"), ...)
    .andThen(ClassTransform.transformingMethods(mm -> mm.methodName().equalsString("bar"), ...))

With the override, neither transform will ever be applied, because mm.methodName().equalsString("foo") && mm.methodName().equalsString("bar") is an impossible condition.

So I suppose I have two questions here:

1. If this is unintended behaviour, how should I report this issue to the OpenJDK developers. https://github.com/openjdk/jdk does not accept issues, and https://bugs.openjdk.org seems to require login to open new issues, but I believe that is intended for members of the project.
2. If this is intended behaviour, how are users of the API expected to chain such transforms “independently” (as they would be if this override did not exist)? I could create a wrapper class to foil the instanceof check, but that seems like a poor solution.