RFR: 8177107: Reduce memory footprint of java.lang.reflect.Constructor/Method

[Claes Redestad]

On Wed, 6 Apr 2022 07:58:33 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> As an alternative to #7667 I took a look at injecting an empty class array from the VM. Turns out we already do this for exception types - see https://github.com/openjdk/jdk/blob/master/src/hotspot/share/oops/method.cpp#L918 - and we can do similarly for the parameter types array. We still need to parse the signature for the return type, though.
>> 
>> I've verified by dumping and inspecting heaps that this means we are not allocating extra `Class<?>[]` on `Method` reflection.
>
> Looks fine. I would have thought to put `assert(parameter_count > 0)` near `mirrors->obj_at_put(index++, mirror);`, but I think it is fine as it is, since `obj_at_put` does its own bounds checking.

Thanks for reviewing! As @shipilev point out `obj_at_put` would assert if we tried to put anything into an empty array, so it seems redundant to add more checks here. Perhaps a comment to point that out since it was brought up here.

Do I need a second reviewer for the hotspot changes?

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PR: https://git.openjdk.java.net/jdk/pull/8089