String xml = "<extension></extension>";

        ExtensionDescriptor ed = builder.build(toStream(xml));

        assertNotNull(ed);
        assertNotNull(ed.getExportedPackages());
        assertTrue(ed.getExportedPackages().isEmpty());
        assertNotNull(ed.getExportedArtifacts());
        assertTrue(ed.getExportedArtifacts().isEmpty());
    }

    @Test
    void testCompleteDescriptor() throws Exception {

            CoreExtensionEntry coreEntry, List<CoreExtensionEntry> extensionEntries) {
        Set<String> exportedPackages = new HashSet<>(coreEntry.getExportedPackages());
        for (CoreExtensionEntry extension : extensionEntries) {
            exportedPackages.addAll(extension.getExportedPackages());
        }
        return exportedPackages;
    }

    /**

    private List<String> exportedPackages;

    private List<String> exportedArtifacts;

    ExtensionDescriptor() {
        // hide constructor
    }

    public List<String> getExportedPackages() {
        if (exportedPackages == null) {
            exportedPackages = new ArrayList<>();
        }

        return exportedPackages;
    }

            extensionRealms.add(extensionRealm);
            if (extensionDescriptor != null) {
                exportedPackages.put(extensionRealm, extensionDescriptor.getExportedPackages());
                exportedArtifacts.put(extensionRealm, extensionDescriptor.getExportedArtifacts());
            }

            if (!plugin.isExtensions()
                    && artifacts.size() == 1

        this.world = coreRealm.getClassWorld();
        this.containerRealm = coreRealm.getRealm();
        this.delegates = delegates;

        Map<String, ClassLoader> foreignImports = exports.getExportedPackages();

        this.mavenApiRealm = createRealm(
                API_REALMID,
                RealmType.Core,
                null /* parent */,
                null /* parentImports */,

        Set<String> exportedPackages = new HashSet<>(coreEntry.getExportedPackages());
        for (CoreExtensionEntry extension : extensions) {
            exportedArtifacts.addAll(extension.getExportedArtifacts());
            exportedPackages.addAll(extension.getExportedPackages());
        }