RFR: 8311302: Implement JEP 493: Linking Run-Time Images without JMODs [v41]

[Severin Gehwolf]

On Wed, 30 Oct 2024 23:23:11 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> Severin Gehwolf has updated the pull request incrementally with 20 additional commits since the last revision:
>> 
>>  - Some test fixes
>>  - Remove period in jlink.properties
>>  - Revert changes to ResourcePoolEntry
>>  - Fix comment in RuntimeImageLinkException
>>  - Remove ImageReader (like JmodsReader)
>>  - More comment fixes (JlinkTask)
>>  - Move some comments around
>>  - More comment fix-ups (JRTArchive)
>>  - Fix description of configure option
>>  - Some more wording updates
>>  - ... and 10 more: https://git.openjdk.org/jdk/compare/83a86d06...e6b3aeb1
>
> src/jdk.jlink/share/classes/jdk/tools/jlink/internal/ImageFileCreator.java line 399:
> 
>> 397:             String mResource = String.format(DIFF_PATH, module);
>> 398:             List<ResourceDiff> diff = perModDiffs.get(module);
>> 399:             if (diff == null) {
> 
> Can `diff` ever be null?   ` preparePerModuleDiffs` ensures that the return map has one entry for each module and empty list if no diff.
> 
> For the case with no diff, do you expect EMPTY_RESOURCE_BYTES or a `ResourceDiff` content with header & zero entry?
> 
> Perhaps line 377-386 in `preparePerModuleDiffs` can simply be removed and let `addDiffResourcesFiles` to handle no resource diff case as it does now.
> 
> 
>         Map<String, List<ResourceDiff>> allModsToDiff = new HashMap<>();
>         modules.stream().forEach(m -> {
>             List<ResourceDiff> d = modToDiff.get(m);
>             if (d == null) {
>                 // Not all modules will have a diff
>                 allModsToDiff.put(m, Collections.emptyList());
>             } else {
>                 allModsToDiff.put(m, d);
>             }
>         });
>         return allModsToDiff;

The diff can never be null. `ReasourceDiff.read(InputStream)` expects at least a proper header. So for a module without diff there will be small files with two integers, the magic and `0` (number of items). I've changed this code to no longer check for `null` and expanded the comment. Thanks!

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PR Review Comment: https://git.openjdk.org/jdk/pull/14787#discussion_r1828178083