GoogleContainerTools/jib · error · MainClassInferenceException

'mainClass' configured in ${projectProperties.getPluginName(

Error message

'mainClass' configured in ${projectProperties.getPluginName()} is not a valid Java class: ${configuredMainClass}

What it means

MainClassInferenceException thrown by MainClassResolver.resolveMainClass() when the explicitly configured mainClass value fails isValidJavaClass(): at least one dot-separated segment is not a valid Java identifier. This is a validation guard on the user-supplied input 'configuredMainClass' — e.g. it contains illegal characters, spaces, or is a path rather than a fully qualified class name — so the entrypoint cannot be built from it.

Source

Thrown at jib-plugins-common/src/main/java/com/google/cloud/tools/jib/plugins/common/MainClassResolver.java:56

   *
   * <p>Warns if main class provided by {@code projectProperties} is not valid, or throws an error
   * if no valid main class is found.
   *
   * @param configuredMainClass the explicitly configured main class ({@code null} if not
   *     configured)
   * @param projectProperties properties containing plugin information and help messages
   * @return the name of the main class to be used for the container entrypoint
   * @throws MainClassInferenceException if no valid main class is configured or discovered
   * @throws IOException if getting the class files from {@code projectProperties} fails
   */
  public static String resolveMainClass(
      @Nullable String configuredMainClass, ProjectProperties projectProperties)
      throws MainClassInferenceException, IOException {
    if (configuredMainClass != null) {
      if (isValidJavaClass(configuredMainClass)) {
        return configuredMainClass;
      }
      throw new MainClassInferenceException(
          HelpfulSuggestions.forMainClassNotFound(
              "'mainClass' configured in "
                  + projectProperties.getPluginName()
                  + " is not a valid Java class: "
                  + configuredMainClass,
              projectProperties.getPluginName()));
    }

    projectProperties.log(
        LogEvent.info(
            "Searching for main class... Add a 'mainClass' configuration to '"
                + projectProperties.getPluginName()
                + "' to improve build speed."));

    String mainClassFromJarPlugin = projectProperties.getMainClassFromJarPlugin();
    if (mainClassFromJarPlugin != null && isValidJavaClass(mainClassFromJarPlugin)) {
      return mainClassFromJarPlugin;
    }

View on GitHub (pinned to fb949e2676)

Solutions

  1. Set <mainClass> (Maven) or the jib/mainClass convention (Gradle) to a fully qualified class name like com.example.Main
  2. Remove invalid characters (spaces, slashes, $ quirks) or a file path from the configured value
  3. If the class genuinely exists, confirm the name including package segments is correct
Defensive patterns

Strategy: validation

When it happens

Trigger: Thrown at jib-plugins-common/src/main/java/com/google/cloud/tools/jib/plugins/common/MainClassResolver.java:56 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of GoogleContainerTools/jib@fb949e2676 (2026-09-06). Data as JSON: /api/errors/08780bb96e131208. Report an issue: GitHub.