RFR: 8356975: Provide alternative way to generate preview API docs

[Chen Liang]

On Fri, 11 Jul 2025 13:49:21 GMT, Hannes Wallnöfer <hannesw at openjdk.org> wrote:

> Please review a change to allow marking API elements as preview features via a new hidden `--preview-feature-tag` option. In conjunction with the already existing hidden `--preview-note-tag` option this allows documenting elements as preview features without relying on JDK-internal types. 
> 
> Part of this change is a refactoring of how `SummaryListWriter` allows its subclasses to extend the content of summary pages. The old `addExtra*` hook methods were too unflexible as they always add extra content before other page content. I replaced them with protected `getIndexLinks` and `addSummaries` methods that allow overriding methods to add extra content before or after the default content returned by the super method.
> 
> The change also includes the fix for [JDK-8355933](https://bugs.openjdk.org/browse/JDK-8355933) to change the presentation and wording for elements with preview notes as requested by the Valhalla team. 
> 
> The output generated by the new test with preview features marked by tags [can be viewed here](https://cr.openjdk.org/~hannesw/8356975/out.00/api/package-summary.html).

src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/HtmlDocletWriter.java line 2506:

> 2504:                     previewDiv.setId(htmlIds.forPreviewSection(forWhat));
> 2505:                     previewDiv.add(HtmlTree.DIV(HtmlStyles.previewComment,
> 2506:                             commentTagsToContent(forWhat, tags.getFirst().getContent(), false)));

Should we add a check to assert there is exactly one occurrence of such a preview note tag?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26267#discussion_r2202949167