[MNG-6906] Whether a core extension can access classes exported by another core extension should not depend on how it is registered

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: Class Loading
• Assignee: Unassigned
• Reporter: Marc Philipp
• Created: 15/May/20 15:33
• Updated: 05/May/23 13:50

Description

We have a Maven core extension (https://search.maven.org/artifact/com.gradle/gradle-enterprise-maven-extension/1.5/jar) that declares two packages as exported in its META-INF/maven/extension.xml:

<?xml version="1.0" encoding="UTF-8"?>
<extension>
  <exportedPackages>
    <exportedPackage>com.gradle.maven.extension.api.scan</exportedPackage>
    <exportedPackage>com.gradle.maven.mojo</exportedPackage>
  </exportedPackages>
  <exportedArtifacts>
    <exportedArtifact>com.gradle:gradle-enterprise-maven-extension</exportedArtifact>
  </exportedArtifacts>
</extension>

The first package (com.gradle.maven.extension.api.scan) contains a BuildScanApi interface for which the extension registers a component in an EventSpy at runtime.

We would now like to consume that component in another core extension (let's call it consuming-extension) that has a provided dependency to com.gradle:gradle-enterprise-maven-extension like this:

BuildScanApi buildScan = (BuildScanApi) session.lookup("com.gradle.maven.extension.api.scan.BuildScanApi");

However, whether that works depends on how the core extensions are registered. Relevant for us are the following registration locations: <maven-home>/lib/ext, -Dmaven.ext.class.path, and .mvn/extensions.xml.

consuming-extension	gradle-enterprise-maven-extension	Result
<maven-home>/lib/ext	<maven-home>/lib/ext
.mvn/extensions.xml	<maven-home>/lib/ext
-Dmaven.ext.class.path	<maven-home>/lib/ext
<maven-home>/lib/ext	.mvn/extensions.xml	NoClassDefFoundError
.mvn/extensions.xml	.mvn/extensions.xml	NoClassDefFoundError
-Dmaven.ext.class.path	.mvn/extensions.xml
<maven-home>/lib/ext	-Dmaven.ext.class.path	NoClassDefFoundError
.mvn/extensions.xml	-Dmaven.ext.class.path	NoClassDefFoundError
-Dmaven.ext.class.path	-Dmaven.ext.class.path

With this workaround, I was able to get it working in all cases:

ClassRealm extensionRealm = (ClassRealm) this.getClass().getClassLoader();
if (!"maven.ext".equals(extensionRealm.getId())) {
  extensionRealm.getWorld().getRealms().stream()
    .filter(realm -> realm.getId().startsWith("coreExtension>com.gradle:gradle-enterprise-maven-extension:") || realm.getId().equals("maven.ext"))
    .max(comparing((ClassRealm realm) -> realm.getId().length()))
    .ifPresent(realm -> {
      try {
        extensionRealm.importFrom(realm.getId(), "com.gradle.maven.extension.api.scan");
      } catch (Exception e) {
        throw new RuntimeException("Could not import package from realm", e);
      }
    });
}

Please let me know if I have misunderstood the intention of declaring exported packages in extension.xml. I was unable to find documentation on it, unfortunately.