Add JDT classpath attribute support for multi-release JAR compilation

[Copilot]

Implements support for Eclipse JDT's release classpath attribute (introduced in Eclipse 4.38) to enable multi-release JAR compilation with flexible source folder naming and without requiring supplemental manifests.

Changes

Core Implementation

• SourceFolderClasspathEntry: Added getMultiReleaseVersion() method returning OptionalInt to extract Java version from release classpath attribute
• AbstractOsgiCompilerMojo: Enhanced doCompile() to check for Multi-Release: true manifest header first, then detect classpath entries with release attributes, compile them with appropriate --release flags, and output to META-INF/versions/N/ directories. Falls back to directory-based approach for backward compatibility.
• Manifest Requirement: The Multi-Release: true header must be explicitly set in the manifest as a prerequisite to enable the feature

Example

.classpath file marking source folders for specific Java versions:

<classpath>
    <classpathentry kind="src" path="src"/>
    <classpathentry kind="src" path="src9">
        <attributes>
            <attribute name="release" value="9"/>
        </attributes>
    </classpathentry>
    <classpathentry kind="src" path="src11">
        <attributes>
            <attribute name="release" value="11"/>
        </attributes>
    </classpathentry>
</classpath>

Note: Source folders can be named flexibly (e.g., src_java9, java11-src). The version is determined by the release attribute value, not the folder name.

Required manifest configuration:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-SymbolicName: my.bundle
Multi-Release: true
Require-Capability: osgi.ee;filter:="(&(osgi.ee=JavaSE)(version=1.8))"

Resulting JAR structure:

jar root
├── com/example/MyClass.class (Java 8)
└── META-INF/
    ├── MANIFEST.MF (Multi-Release: true - required)
    └── versions/
        ├── 9/com/example/MyClass.class
        └── 11/com/example/MyClass.class

Key Features

• Manifest-First: Multi-Release: true header is required as the primary trigger
• Flexible Naming: Source folders can be named anything, version determined by release attribute
• No Supplemental Manifests: Unlike the legacy approach, no supplemental manifests in META-INF/versions/N/OSGI-INF/ are required
• Backward Compatible: Falls back to directory-based approach (srcN naming) if no classpath attributes found

Demo & Documentation

• demo/multi-release-jar-classpath/: Reference implementation using classpath attribute approach
• BuildingMultiReleaseBundles.md: Comprehensive guide covering JEP 238, OSGi R8 spec, both approaches (classpath vs manifest-first), and best practices
• DemoTest: Added integration test for classpath-based demo

References

• JEP 238: Multi-Release JAR Files
• OSGi Core R8 Multi-Release JAR
• Eclipse 4.38 JDT Multi-Release Support
• Eclipse PDE PR #2138

Original prompt

Enhance Multi-Release jar support

JDT since this release supports an additional attribute in the classpath see https://github.com/eclipse-pde/eclipse.pde/pull/2138/files#diff-94c43945b1beca74bb258b7502839b468019013d2f00a33cab2fc751f11c5df7R12
and teh release notes here https://eclipse.dev/eclipse/markdown/?f=news/4.38/jdt.md#multi-release-jar-compilation-support

Tycho already contains multi-release jar support since a while see https://github.com/eclipse-tycho/tycho/tree/main/demo/multi-release-jar but it requires a certain structur to be recognized.

This should now be enhance to also bein triggered by the flag in the classpath, and Tycho contains already classes for reading claspath here https://github.com/eclipse-tycho/tycho/tree/main/tycho-metadata-model/src/main/java/org/eclipse/tycho/model/classpath

So we need:

• Add support for building MR jars also when the JDT attribute is found on the classpath without requiring the special MANIFEST
• Add a new demo and reference it in the DemoTest so we proof its working and have an example
• Add a new BuildingMultiReleaseBundles.md to the sitedoc that gives an introduction to the MR-Jar topic both in Java (https://openjdk.org/jeps/238) and OSGi (https://docs.osgi.org/specification/osgi.core/8.0.0/framework.module.html#framework.module-multireleasejar) and shows both ways with tycho and linking the both demos, the java+osgispec and the eclipse news article

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[laeubi]

@copilot please look at the review comments some things have to be adjusted here.

[Copilot]

@copilot please look at the review comments some things have to be adjusted here.

All review comments have been addressed in commit 866687c. The changes include:

• Corrected attribute name to release
• Used OptionalInt for return type
• Made Multi-Release header a requirement (not automatic)
• Manifest check is primary trigger
• Updated all documentation and demo files

Warning

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[eclipse-tycho-bot]

💚 All backports created successfully

Status	Branch	Result
✅	tycho-5.0.x

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