RFR: 8304425: ClassHierarchyResolver from Reflection

[Adam Sotona]

On Mon, 20 Mar 2023 11:07:23 GMT, Adam Sotona <asotona at openjdk.org> wrote:

>> Add API to explore Class Hierarchy with a `ClassLoader` or a `Lookup` with proper privileges, with tests.
>> 
>> This addition is useful in case classes at runtime are not loaded from the system class loader, such as Proxy. This is also useful to APIs that generate bytecode with a `Lookup` object, such as a custom single-abstract-method class implementations from a method handle.
>> 
>> See https://mail.openjdk.org/pipermail/classfile-api-dev/2023-March/000249.html as well.
>> 
>> Current questions, which I wish to discuss with @asotona:
>> 1. Should the resolver fail fast on `IllegalAccessException` from the lookup? This usually indicates the hierarchy resolver is set up improperly, and proceeding may simply yield verification errors in class loading that are hard to track. For bytecode-generating APIs, throwing access errors for the Lookup eagerly is also more preferable than later silent generation failure.
>> 2. Whether the default resolver should be reading from jrt alone, reflection alone, or jrt then reflection. I personally believe reflection alone is more reliable, for classes may redefined with instrumentation or jfr, which may not be reflected in the system resources.
>> 3. In addition, I don't think chaining system class loader reflection after system resource retrieval is really meaningful: is there any case where reflection always works while the system resource retrieval always fails?
>
> Marked as reviewed by asotona (Committer).

> @asotona So should I simply throw an `IllegalAccessError` when the Lookup encounters a `IllegalAccessException`, or should I return null? I favor throwing an `IllegalAccessError` as the lookup represents bytecode accessibility, and shall it fail to access, the generated bytecode will fail to access the specified superclass as well.

Yes, I also prefer throwing `IllegalAccessError` to directly indicate what is wrong instead of later indirect exceptions. I think `IllegalAccessError` is not a case for fallback resolver, so it should not be masked.

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PR: https://git.openjdk.org/jdk/pull/13082