On Tue, 1 Jul 2025 14:37:50 GMT, Alan Bateman <al...@openjdk.org> wrote:
>> Refactoring `ImageReader` to make it easy to add preview mode functionality
>> for Valhalla.
>>
>> This PR is a large change to `ImageReader` (effectively a rewrite) but
>> reduces the surface area of the API significantly, reduces code complexity
>> and increases performance/memory efficiency. The need for this sort of
>> change comes from the need to support "preview mode" in Valhalla for classes
>> loaded from core modules.
>>
>> ### Preview Mode
>>
>> In the proposed preview mode support for Valhalla, a resource with the name
>> `/modules/<module>/<path>` may come from one of two locations in the
>> underlying jimage file; `/<module>/<path>` or
>> `/<module>/META-INF/preview/<path>`. Furthermore, directories (represented
>> as directory nodes via the API) will have a potentially different number of
>> child nodes depending on whether preview mode is in effect, and some
>> directories may only exist at all in preview mode.
>>
>> Furthermore, the directories and symbolic link nodes in `/packages/...` will
>> also be affected by the existence of new package directories. To retain
>> consistency and avoid "surprises" later, all of this needs to be taken into
>> account.
>>
>> Below is a summary of the main changes for mainline JDK to better support
>> preview mode later:
>>
>> ### 1: Improved encapsulation for preview mode
>>
>> The current `ImageReader` code exposes the data from the jimage file in
>> several ways; via the `Node` abstraction, but also with methods which return
>> an `ImageLocation` instance directly. In preview mode it would not be
>> acceptable to return the `ImageLocation`, since that would leak the
>> existence of resources in the `META-INF/preview` namespace and lets users
>> see resource nodes with names that don't match the underlying
>> `ImageLocation` from which their contents come.
>>
>> As such, the PR removes all methods which can leak `ImageLocation` or other
>> "underlying" semantics about the jimage file. Luckily most of these are
>> either used minimally and easily migrated to using nodes, or they were not
>> being used at all. This PR also removes any methods which were already
>> unused across the OpenJDK codebase (if I discover any issues with
>> over-trimming of the API during full CI testing, it will be easy to address).
>>
>> ### 2. Simplification of directory child node handling
>>
>> The current `ImageReader` code attempts to create parent directories "on
>> demand" for any child nodes it creates. This results in parent directories
>> having a non-empty but incomplete set of child nodes present. This is re...
>
> src/java.base/share/classes/jdk/internal/module/SystemModuleFinders.java line
> 456:
>
>> 454: Optional<ImageReader.Node> node =
>> Optional.ofNullable(reader.findNode(nodeName));
>> 455: if (node.isPresent() && !node.get().isResource()) {
>> 456: throw new IllegalStateException("Not a resource node: "
>> + node.get());
>
> The IllegalStateException is problematic here, maybe you mean to add an
> assert?
This is called externally so cannot trust the given name really references a
resource.
I can't therefore make it an assertion, but I can just return empty if it
didn't point at a resource (even if it did point at a directory etc.). Or I can
make it an Error.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/26054#discussion_r2178417095
