@Test
  public void testRunPluginExtensions_extensionDefinedConfigurations_emptyConfig()
      throws JibPluginExtensionException {
    FooExtension fooExtension =
        new FooExtension(
            (buildPlan, properties, extraConfig, mavenData, logger) -> {
              Assert.assertEquals(Optional.empty(), extraConfig);
              return buildPlan;
            });
    BarExtension barExtension =
        new BarExtension(
            (buildPlan, properties, extraConfig, mavenData, logger) -> {
              Assert.assertEquals(Optional.empty(), extraConfig);
              return buildPlan;
            });
    loadedExtensions = Arrays.asList(fooExtension, barExtension);

    gradleProjectProperties.runPluginExtensions(
        Arrays.asList(new FooExtensionConfig(), new BarExtensionConfig()), containerBuilder);
  }
Plugin extension test for {@link GradleProjectProperties}.
jib-gradle-plugin/src/test/java/com/google/cloud/tools/jib/gradle/GradleProjectPropertiesExtensionTest.java