* same class realm is used to load build extensions and load mojos for extensions=true plugins.
     * </p>
     * <strong>Note:</strong> This is part of internal implementation and may be changed or removed without notice
     *
     * @since 3.3.0
     */
    public static final String KEY_EXTENSIONS_REALMS = DefaultMavenPluginManager.class.getName() + "/extensionsRealms";

    }

    private static String getKey(Plugin plugin, boolean extension) {
        String version = ArtifactUtils.toSnapshotVersion(plugin.getVersion());
        return (extension ? "extension>" : "plugin>") + plugin.getGroupId() + ":" + plugin.getArtifactId() + ":"
                + version;
    }

        if (build != null) {
            for (Extension extension : build.getExtensions()) {
                Plugin plugin = new Plugin();
                plugin.setGroupId(extension.getGroupId());
                plugin.setArtifactId(extension.getArtifactId());
                plugin.setVersion(extension.getVersion());
                XmlNode configuration = extension.getDelegate().getConfiguration();

      | provoke linkage errors for wagons contributed by build extensions. We also don't need to exclude the wagons
      | from plugins. Plugins that use wagons directly and declare the corresponding dependency will simply use a
      | wagon from their plugin realm.
     -->
  </exportedArtifacts>
</extension>

     * @param extension the artifact extension, or {@code null} is unspecified
     * @return artifact with the given coordinates
     *
     * @see org.apache.maven.api.services.ArtifactFactory#create(Session, String, String, String, String)
     */
    @Nonnull
    Artifact createArtifact(String groupId, String artifactId, String version, String extension);

    /**

                session.createArtifactCoordinate("org.apache.maven.core.test", "test-extension", "1", "jar"));

        List<Path> paths = session.resolveDependencies(coord);

        assertNotNull(paths);
        assertEquals(10, paths.size());
        assertEquals("test-extension-1.jar", paths.get(0).getFileName().toString());

        String groupId = artifact.getGroupId();
        String artifactId = artifact.getArtifactId();
        String version = artifact.getBaseVersion();
        String classifier = artifact.getClassifier();
        String extension = artifact.getExtension();
        Path repo = getProjectLocalRepo();
        return repo.resolve(groupId)
                .resolve(artifactId)
                .resolve(version)
                .resolve(artifactId

META-INF/MANIFEST.MF Manifest-Version: 1.0 Archiver-Version: Plexus Archiver Created-By: Apache Maven Built-By: Brett Build-Jdk: 1.5.0_04 Extension-Name: maven-plugin-api Specification-Title: Maven is a project development management and com prehension tool. Based on the concept of a project object model: buil ds, dependency management, documentation creation, site publication, and distribution publication are all controlled from the declarative file. Maven can be extended by plugins to utilise...

        }

        for (MavenProject project : projects) {
            // MNG-1911 / MNG-5572: Building plugins with extensions cannot be part of reactor
            for (Plugin plugin : project.getBuildPlugins()) {
                if (plugin.isExtensions()) {
                    String pluginKey =
                            ArtifactUtils.key(plugin.getGroupId(), plugin.getArtifactId(), plugin.getVersion());

            mergePlugin_ArtifactId(builder, target, source, sourceDominant, context);
            mergePlugin_Version(builder, target, source, sourceDominant, context);
            mergePlugin_Extensions(builder, target, source, sourceDominant, context);
            mergePlugin_Executions(builder, target, source, sourceDominant, context);
            mergePlugin_Dependencies(builder, target, source, sourceDominant, context);