On Mon, 26 Jan 2026 12:42:06 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).
src/hotspot/share/classfile/classLoader.cpp line 246:
> 244: jint error;
> 245: JImage_file = (*JImageOpen)(modules_path, &error);
> 246: if (Arguments::has_jimage() && JImage_file == nullptr) {
Error check added to mainline but refactored here for simplicity (these
functions are intended to encapsulate this sort of thing).
src/hotspot/share/classfile/classLoader.cpp line 1420:
> 1418: jio_snprintf(modules_path, JVM_MAXPATHLEN, "%s%slib%smodules",
> Arguments::get_java_home(), fileSep, fileSep);
> 1419: JImage_file =(*JImageOpen)(modules_path, &error);
> 1420: if (JImage_file == nullptr) {
This check was moved into jimage_open().
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/29414#discussion_r2758662629
PR Review Comment: https://git.openjdk.org/jdk/pull/29414#discussion_r2758667570
