apache/beam · error · IllegalArgumentException

Could not parse the provided YAML stream into a non-trivial…

Error message

Could not parse the provided YAML stream into a non-trivial AllowList

What it means

Thrown when the YAML stream supplied to AllowList.parseFromYamlStream loads to null (empty stream, comments-only file, or a YAML root that is not a mapping), so no allowlist config Map can be constructed.

Solutions

Verify the YAML file/stream is non-empty and its root is a mapping with keys like 'version' and 'allowedClasses'.
Check the file path/resource actually exists and contains content; log the stream contents before parsing.
If using templating/variable substitution, confirm the rendered output is valid non-empty YAML.

Example fix

# before (empty file)

# after
version: 1
allowedClasses:
  - className: org.apache.beam.sdk.transforms.MapElements
    allowedBuilderMethods: ['of']

Defensive patterns

Strategy: validation

Validate before calling

if (yamlText == null || yamlText.trim().isEmpty()) throw new IllegalArgumentException("AllowList YAML is empty");
Object root = new Yaml().load(new ByteArrayInputStream(yamlText.getBytes(StandardCharsets.UTF_8)));
if (!(root instanceof Map)) throw new IllegalArgumentException("AllowList YAML root must be a mapping");

Try / catch

try { AllowList list = AllowList.parseFromYamlStream(stream); } catch (IllegalArgumentException e) { log.error("Invalid allowlist YAML: {}", e.getMessage()); throw new ConfigException(e); }

Prevention

Ship a non-empty allowlist YAML with at least 'version' and 'allowedClasses' keys
Validate YAML in CI before deploying the expansion service
Log the parsed config map after load for auditability
Never point the service at a placeholder/empty file

When it happens

Trigger: Calling parseFromYamlStream with an empty InputStream, a file containing only comments or 'null', or YAML whose root node is a scalar/list so yaml.load returns null or a non-Map treated as null.

Common situations: An empty or misnamed expansion-service allowlist YAML passed via config; a templated config rendered to nothing; wrong file path silently resolved to an empty resource.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/f152dd2d2d8e4c1d. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/expansion-service/src/main/java/org/apache/beam/sdk/expansion/service/JavaClassLookupTransformProvider.java:497

  public abstract static class AllowList {

    public static AllowList nothing() {
      return create(ALLOW_LIST_VERSION, Collections.emptyList());
    }

    public static AllowList everything() {
      return create(
          ALLOW_LIST_VERSION,
          Collections.singletonList(
              AllowedClass.create("*", AllowedClass.WILDCARD, AllowedClass.WILDCARD)));
    }

    static AllowList parseFromYamlStream(InputStream inputStream) {
      Yaml yaml = new Yaml();
      Map<Object, Object> config = yaml.load(inputStream);

      if (config == null) {
        throw new IllegalArgumentException(
            "Could not parse the provided YAML stream into a non-trivial AllowList");
      }

      String version = config.get("version") != null ? (String) config.get("version") : "";
      List<AllowedClass> allowedClasses = new ArrayList<>();
      if (config.get("allowedClasses") != null) {
        allowedClasses =
            ((List<Map<Object, Object>>) config.get("allowedClasses"))
                .stream()
                    .map(
                        data -> {
                          String className = (String) data.get("className");
                          if (className == null) {
                            throw new IllegalArgumentException(
                                "Expected each entry in the allowlist to include the 'className'");
                          }
                          List<String> allowedBuilderMethods =
                              (List<String>) data.get("allowedBuilderMethods");

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