[lworld] RFR: 8368467: [lworld] Add new flag generation for jimage to support preview mode

[Roger Riggs]

On Tue, 4 Nov 2025 17:08:45 GMT, David Beaumont <duke at openjdk.org> wrote:

> Adds support for writing preview related flags into jimage files.
> 
> Preview mode is complex. It's not nearly as simple as "does something in /modules/xxx/... have an entry in /modules/xxx/META-INF/preview/...".
> 
> Specific issues include:
> 
> Supporting preview-only resources without forcing a double lookup on everything.
> Changing the set of entries in /packages/xxx directories to account for preview only packages in some modules.
> Minimising the work done during image reader initialization to only need to process the small number of preview resources (rather than scanning the whole file to look for them).
> The new flags added by this code address these issues, but calculating them correctly with only minor adjustments to the existing code was not feasible, it just became a lot larger and very complex.
> 
> To address this, a new type (ModuleReference) is introduced to track and then merge information about packages seen in each module. This allows a much simpler inner loop for processing resource paths when building the node tree, combined with a subsequent merging stage to produce the final package information for each module.
> 
> Not that since ModuleReference is needed during jimage reading, that class is already present in the previous PR on which this is based, but it starts to be used to calculate the module flags in this PR.
> 
> This PR can also adds the ImageReader unit tests for preview mode, which rely on being able to generate jimage files with preview mode flags in.

src/java.base/share/classes/jdk/internal/jimage/PreviewMode.java line 89:

> 87:         }
> 88:     }
> 89:     ;

Extra "'"

src/java.base/share/classes/jdk/internal/jimage/PreviewMode.java line 95:

> 93:     // will be always 'true' and this code, and all related dead-code can be removed.
> 94:     private static final boolean DISABLE_PREVIEW_PATCHING_DEFAULT = false;
> 95:     private static final boolean ENABLE_PREVIEW_MODE = Boolean.parseBoolean(

the naming is confusing: stick to the same sense as the system property; (Or change the system property)

src/jdk.jlink/share/classes/jdk/tools/jlink/internal/ImageResourcesTree.java line 204:

> 202:                 try {
> 203:                     processPath(fullPath, modulesRoot, packageToModules);
> 204:                 } catch (InvalidTreeException err) {

Exception varialble "err" is ambiguous with System.err used below.
"ex" is more conventional for exceptions.

test/jdk/tools/jlink/whitebox/ImageResourcesTreeTestDriver.java line 24:

> 22:  */
> 23: 
> 24: /*

Why is this separate from the test itself?

test/micro/org/openjdk/bench/jdk/internal/jrtfs/ImageReaderBenchmark.java line 318:

> 316:     }
> 317: 
> 318:     /// Note: This list is inherently a little fragile and may end up being more

Yes, this kind of test/benchmark will bit rot very fast.
The existing startup measurements are more stable and reflect actual startup times, not a simulation thereof.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2492033684
PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2492038687
PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2492084995
PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2492060924
PR Review Comment: https://git.openjdk.org/valhalla/pull/1718#discussion_r2492057544