RFR: 8342283: CDS cannot handle a large number of classes

[Calvin Cheung]

On Fri, 25 Apr 2025 14:49:37 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> CDS cannot handle large number of classes, because `ClassLoader` data structures get too large for CDS archival. The new test captures such an occasion. We do `clear()`-s, but that is not enough to trim the backing storages for the affected collections.
> 
> In contrast with previous attempts to fix this (https://github.com/openjdk/jdk/pull/21797 and my VM-side field overwrite), we can just reinitialize the fields using `Unsafe`, which bypasses normal `final` field write restriction. `ClassLoader` already does a similar thing nearby! This allows us to stay fully in Java, and makes the patch fairly straightforward.
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, new test reliably fails without the fix, passes with it
>  - [x] Linux x86_64 server fastdebug, `runtime/cds`

LGTM.

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Marked as reviewed by ccheung (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24877#pullrequestreview-2795241761