RFR: 8377070: Migrate preview mode support for jimage to mainline

[David Beaumont]

On Mon, 26 Jan 2026 12:42:06 GMT, David Beaumont <duke at openjdk.org> 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