On Fri, 13 Mar 2026 12:20:03 GMT, David Beaumont <[email protected]> wrote:
>> Implementation of preview-mode support for jimage modules file, migrated
>> from Valhalla related work (see JDK-8352750).
>>
>> This PR (the first of several) migrates work from Valhalla (lworld) to the
>> JDK mainline repository in relation to "preview mode" support. It affects
>> the creation and reading of the jimage file, both in Java
>> (BasicImageReader/ImageReader) and C++ (imageFile.xpp/jimage.xpp).
>>
>> Preview mode is a mechanism by which alternate version of JDK class files
>> and resources can be made available for class loading and reflection when
>> the '--enable-preview' flag is passed to the runtime.
>>
>> Alternate classes/resource appear in each module under the:
>>
>> /<module>/META-INF/preview/<path-to>/<resource-or-class>
>>
>> and replace the original:
>>
>> /<module>/<path-to>/<resource-or-class>
>>
>> files when preview mode is enabled.
>>
>> While initially useful for Valhalla work, this mechanism will be used for
>> other cases where preview features (in the JEP 12 sense) require alternate
>> classes/resources to be provided. None of the changes in this (or the
>> follow-up PRs) are Valhalla specific.
>>
>> In this PR:
>> * the writing of jimage files is modified to recognize and handle preview
>> mode paths
>> * flags in the jimage file are added or modified to support preview mode
>> efficiently
>> * (C++) the class loader is modified to permit reading preview versions of
>> classes
>> * (Java) the image reader and associated JRT file-system classes are
>> modified to permit reading preview files
>> * unit tests are added to ensure preview mode works as expected when enabled
>> * (temporary) any code calling into the affected API (other than tests)
>> specifies that preview mode is disabled
>>
>> Future PRs will add the plumbing to enable preview mode correctly, but with
>> the PR there should be no observable change in behaviour (especially since
>> no preview classes or resources are being supplied at this point).
>
> David Beaumont has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Feedback tweaks
src/hotspot/share/classfile/classLoader.cpp line 265:
> 263: static bool jimage_exists() {
> 264: return JImage_file != nullptr;
> 265: }
Maybe this should be jimage_isOpen as the jimage file exists before it opened.
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PR Review Comment: https://git.openjdk.org/jdk/pull/29414#discussion_r2976392681
