@Test
  public void testGetMainClassFromJar_missingMainClass() {
    when(mockMavenProject.getPlugin("org.apache.maven.plugins:maven-jar-plugin"))
        .thenReturn(mockPlugin);
    when(mockPlugin.getConfiguration()).thenReturn(pluginConfiguration);
    Xpp3Dom archive = new Xpp3Dom("archive");
    archive.addChild(new Xpp3Dom("manifest"));
    pluginConfiguration.addChild(archive);

    assertThat(mavenProjectProperties.getMainClassFromJarPlugin()).isNull();
  }
Helper for reading back layers in a {@link BuildContext}.
jib-maven-plugin/src/test/java/com/google/cloud/tools/jib/maven/MavenProjectPropertiesTest.java