Support multipage reports when the goal is invoked directly

[slachiewicz]

Fixes #217.

reportToSite() passed null as the SinkFactory to generate(Sink, SinkFactory, Locale), with a TODO saying multipage reports would fail with an NPE. They do:

mvn plugin-report:3.15.1:report
...
Cannot invoke "org.apache.maven.doxia.sink.SinkFactory.createSink(java.io.File, String)"
because the return value of "PluginReport.getSinkFactory()" is null
    at PluginReport.generateMojosDocumentation (PluginReport.java:280)
    at AbstractMavenReport.reportToSite (AbstractMavenReport.java:266)

Only the site path was affected. reportToMarkup(), taken when output.format is set, already builds a sink factory, which is why use-as-direct-mojo-markup exercises the multi-page mojo happily while use-as-direct-mojo could not.

The change

Hand generate() a MultiPageSinkFactory: each createSink(File, String) builds a DocumentRenderingContext derived from the main one, wraps it in a SiteRendererSink that remembers where it belongs, and records it. After the main document is merged into the site, every collected sub-sink is merged the same way.

MultiPageSinkFactory and MultiPageSubSink are verbatim copies of the private nested classes in Maven Site Plugin's ReportDocumentRenderer, so a report behaves the same whether its goal is invoked directly or through the site, and the two copies stay diffable. apache/maven-doxia-sitetools#671 tracks moving them into doxia-site-renderer as public API and deleting both copies; the extension-less outputName StringIndexOutOfBoundsException present in both is fixed there too.

Behaviour change worth a release note: getSinkFactory() is now non-null on a direct site-mode invocation. A report that used getSinkFactory() == null to detect direct invocation now takes its multipage branch, which is what #217 asks for.

Test

The integration test for exactly this was already written and disabled with a pointer to the issue, in src/it/use-as-direct-mojo. This PR enables invoker.goals.4 = custom-reporting:multi-page again and asserts the rendered content.

The MultiPageReport fixture now renders a distinct title and body on its second page. Before, both pages were byte-identical, so an assertion on multi-second.html could not tell a sub-sink merged with the wrong context from a correct one. use-as-direct-mojo, use-as-site-report and use-as-direct-mojo-markup all assert that the second page carries the second page's content and not the first page's.

Reverting only AbstractMavenReport and rerunning use-as-direct-mojo reproduces the reported failure:

Cannot invoke "org.apache.maven.doxia.sink.SinkFactory.createSink(java.io.File, String)"
because the return value of "MultiPageReport.getSinkFactory()" is null
Passed: 1, Failed: 1

Verified: mvn verify under JDK 17 → checkstyle, RAT, 3 unit tests and 6 ITs passed; the direct-mojo run logs both pages.

--- custom-reporting:1.0-SNAPSHOT:multi-page (default-cli) @ use-as-direct-mojo ---
Rendering report to target/reports/multi-page.html
          using org.apache.maven.skins:maven-fluido-skin:jar:2.0.0-M9 site skin
Rendering report to target/reports/multi-second.html

I did not rebuild maven-plugin-report-plugin against this branch to re-verify the original report from the issue; the IT covers the same code path with the same API.

[Copilot]

Pull request overview

This PR fixes a NullPointerException when invoking reporting goals directly for multipage reports by providing a SinkFactory during site rendering, aligning behavior with Maven Site Plugin so multipage reports work consistently in both direct-goal and site modes.

Changes:

• Provide a MultiPageSinkFactory to generate(...) in reportToSite() and render any collected subpage sinks after the main page is merged into the site.
• Add internal MultiPageSubSink/MultiPageSinkFactory helpers to track subpage output locations and rendering contexts.
• Re-enable and strengthen the existing IT (use-as-direct-mojo) to verify both multipage outputs exist and contain expected content.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/main/java/org/apache/maven/reporting/AbstractMavenReport.java	Passes a sink factory for multipage sub-sinks during direct-goal site rendering and merges generated subpages into the site output.
src/it/use-as-direct-mojo/verify.groovy	Re-enables assertions for multipage output and validates rendered content.
src/it/use-as-direct-mojo/invoker.properties	Re-enables the multipage goal in the direct-mojo IT.

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[michael-o]

Does it make sense to maven the MultiReportSink public instead of duplicating the code?

[slachiewicz]

So what proposal you have?

[michael-o]

So what proposal you have?

Two steps:

• Merge this to solve the problem for now (with a TODO)
• Create a ticket to make the factory public to be usable from site and direct invocation.

[slachiewicz]

TODO added in 4489b1b, follow-up filed as apache/maven-doxia-sitetools#671 — move MultiPageSinkFactory/MultiPageSubSink into doxia-site-renderer as public API and drop both copies. That is also where the extension-less outputName StringIndexOutOfBoundsException gets fixed once instead of twice.

[michael-o]

I am fine with that. @hboutemy WDYT?