Repository: GoogleContainerTools/jib
File: jib-cli/src/main/java/com/google/cloud/tools/jib/cli/jar/JarProcessor.java (Java)

Code before:
            isClassFile,
            APP_ROOT.resolve(RelativeUnixPath.get("explodedJar")));
    FileEntriesLayer resourcesLayer =
        addDirectoryContentsToLayer(
            RESOURCES,
            localExplodedJarRoot,
            isResourceFile,
            APP_ROOT.resolve(RelativeUnixPath.get("explodedJar")));

    layers.add(resourcesLayer);
    layers.add(classesLayer);
    return layers;
  }

  /**
   * Compute the entrypoint for a standard jar in exploded mode.
   *
   * @param jarPath path to jar file
   * @param tempDirPath path to temporary jib local directory
   * @param layers list of {@link FileEntriesLayer}
   * @return list of {@link String} representing entrypoint
   * @throws IOException if I/O error occurs when opening the jar file or if temporary directory
   *     provided doesn't exist
   * @throws IllegalArgumentException if main class is not found in the jar manifest
   */
  public static ImmutableList<String> computeEntrypoint_explodedStandard(
      Path jarPath, Path tempDirPath, List<FileEntriesLayer> layers)
      throws IOException, IllegalStateException {
    Path localExplodedJarRoot = tempDirPath;
    ZipUtil.unzip(jarPath, localExplodedJarRoot);

    String mainClass = null;
    try (JarFile jarFile = new JarFile(jarPath.toFile())) {
      mainClass = jarFile.getManifest().getMainAttributes().getValue(Attributes.Name.MAIN_CLASS);
    }
    if (mainClass == null) {
      throw new IllegalArgumentException("Main-Class not found in jar's manifest.");
    }
    List<String> classpath = new ArrayList<>();
    for (FileEntriesLayer layer : layers) {
      if (layer.getName().equals(DEPENDENCIES) || layer.getName().equals(SNAPSHOT_DEPENDENCIES)) {
        classpath.add(APP_ROOT.resolve("dependencies").toString());
        break;
      }
    }
    classpath.add(APP_ROOT.resolve("explodedJar").toString());
    String classPathString = String.join(":", classpath);
    return ImmutableList.of("java", "-cp", classPathString, mainClass);
  }

  private static FileEntriesLayer addDirectoryContentsToLayer(

Review comment (suggestion):
Using `_` is only accepted in test method names. `computeEntrypointForExplodedStandard()`?

And I don't know who will require this method, but you should always double-checking if a method may have a narrower access scope. (Just don't habitually make everything `public` until necessary.)

Code after:
            APP_ROOT.resolve(RelativeUnixPath.get("explodedJar")));
    FileEntriesLayer resourcesLayer =
        addDirectoryContentsToLayer(
            RESOURCES,
            localExplodedJarRoot,
            isResourceFile,
            APP_ROOT.resolve(RelativeUnixPath.get("explodedJar")));

    layers.add(resourcesLayer);
    layers.add(classesLayer);
    return layers;
  }

  /**
   * Computes the entrypoint for a standard jar in exploded mode.
   *
   * @param jarPath path to jar file
   * @return list of {@link String} representing entrypoint
   * @throws IOException if I/O error occurs when opening the jar file
   */
  static ImmutableList<String> computeEntrypointForExplodedStandard(Path jarPath)
      throws IOException {
    try (JarFile jarFile = new JarFile(jarPath.toFile())) {
      String mainClass =
          jarFile.getManifest().getMainAttributes().getValue(Attributes.Name.MAIN_CLASS);
      if (mainClass == null) {
        throw new IllegalArgumentException(
            "`Main-Class:` attribute for an application main class not defined in the input Jar's manifest (`META-INF/MANIFEST.MF` in the Jar).");
      }
      String classpath = APP_ROOT + "/explodedJar:" + APP_ROOT + "/dependencies/*";
      return ImmutableList.of("java", "-cp", classpath, mainClass);
    }
  }

  private static FileEntriesLayer addDirectoryContentsToLayer(
      String layerName,
      Path sourceRoot,
      Predicate<Path> pathFilter,
      AbsoluteUnixPath basePathInContainer)
      throws IOException {
    FileEntriesLayer.Builder builder = FileEntriesLayer.builder().setName(layerName);
    new DirectoryWalker(sourceRoot)
        .filterRoot()
        .filter(path -> Files.isDirectory(path) || pathFilter.test(path))
        .walk(
            path -> {
              AbsoluteUnixPath pathOnContainer =
                  basePathInContainer.resolve(sourceRoot.relativize(path));
              builder.addEntry(path, pathOnContainer);
            });
    return builder.build();