RFR: 8350550: Preload classes from AOT cache during VM bootstrap [v10]

[Vladimir Kozlov]

On Sat, 20 Sep 2025 00:57:09 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> This PR loads the classes for the boot/platform/app loaders with `AOTLinkedClassBulkLoader::preload_classes()`. This happens at the very beginning of `vmClasses::resolve_all()`, before any Java code is executed. 
>> 
>> - We essentially iterate over all the classes inside the `AOTLinkedClassTable` and adds them into the system dictionary using the new method `SystemDictionary::preload_class()`.
>> - `SystemDictionary::preload_class(..., k)` is lightweight because it's called in a single thread after all super types of `k` have been loaded. So most of the complicated work (such as place holders, circularity detection, etc) in `SystemDictionary::resolve_or_null(..., k)` can be skipped. We also don't need to call into `ClassLoader::load_class()` as the boot/platform/app loaders are well-behaved.
>> - In the assembly phase, we record the mirror, package, protection domain, code source, etc, of these classes. So there's no need to programmatically create them in the production run. See `HeapShared::copy_java_mirror()` and also changes in ClassLoader.java and SecureClassLoader.java.
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   @ashu-mehra comment - AOTLinkedClassBulkLoader::link_or_init_javabase_classes() should also call exit_on_exception()

Few comments.

src/hotspot/share/cds/aotLinkedClassBulkLoader.cpp line 60:

> 58: // Preloading requires that the Java heap objects of java.lang.Class, java.lang.Package and
> 59: // java.security.ProtectionDomain already exist for the preloaded classes. Therefore, we support preloading
> 60: // only for the classes in the static CDS archive. Classes in the dynamic archive are not supported because

"for the classes in the static CDS archive"? Do you mean "AOT cache"?  Or this feature also works for old static CDS archive?

src/hotspot/share/cds/aotLinkedClassBulkLoader.cpp line 97:

> 95:   }
> 96: 
> 97:   ClassLoaderData* loader_data = ClassLoaderData::class_loader_data(loader());

`loader_data` is not used in this method.

src/hotspot/share/cds/aotLinkedClassBulkLoader.hpp line 52:

> 50: //
> 51: // [2] load_javabase_classes() and load_non_javabase_classes():
> 52: //     This happens after some Java code is executed, to load aot-linked classes in the dynamic archive.

Previous comment say that dynamic archive is not supported.

src/hotspot/share/cds/aotLinkedClassTable.hpp line 45:

> 43: 
> 44:   Array<InstanceKlass*>* _boot1; // boot classes in java.base module
> 45:   Array<InstanceKlass*>* _boot2; // boot classes in all other (named and unnamed) modules

Do you mean JDK's modules or user's modules too?

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PR Review: https://git.openjdk.org/jdk/pull/26375#pullrequestreview-3255743679
PR Review Comment: https://git.openjdk.org/jdk/pull/26375#discussion_r2370759818
PR Review Comment: https://git.openjdk.org/jdk/pull/26375#discussion_r2370778138
PR Review Comment: https://git.openjdk.org/jdk/pull/26375#discussion_r2370797359
PR Review Comment: https://git.openjdk.org/jdk/pull/26375#discussion_r2370798743