What causes "List of SerializableFunction must not be null" in apache/beam?

Pass a non-null List , Void>> with one checker per PCollection

apache/beam · error · IllegalArgumentException

List of SerializableFunction must not be null

Error message

List of SerializableFunction must not be null

What it means

PAssert's PCollectionListContentsAssert.satisfies(List<SerializableFunction>) validates each checker function against the corresponding PCollection in the list. It throws this IllegalArgumentException immediately if the checker list is null, since there would be nothing to satisfy.

Solutions

Pass a non-null List<SerializableFunction<Iterable<T>, Void>> with one checker per PCollection
Initialize the list explicitly (e.g. Arrays.asList(checker1, checker2)) before calling satisfies
If no checkers are needed, don't call satisfies at all rather than passing an empty/null list

Example fix

// before
List<SerializableFunction<Iterable<T>, Void>> checkers = null;
assertThat(list).satisfies(checkers);
// after
List<SerializableFunction<Iterable<T>, Void>> checkers =
    Arrays.asList(input -> null, input -> null);
assertThat(list).satisfies(checkers);

Defensive patterns

Strategy: validation

Validate before calling

Objects.requireNonNull(checkerFnList, "checkerFnList must not be null before satisfies(...)");

Type guard

boolean validCheckerList(List<?> checkers, PCollectionList<?> list) {
  return checkers != null && checkers.size() == list.size();
}

Try / catch

try {
  assertThat(pcollectionList).satisfies(checkers);
} catch (IllegalArgumentException e) {
  if (e.getMessage().contains("must not be null")) {
    throw new IllegalStateException("Initialize the checker list before calling satisfies", e);
  }
  throw e;
}

Prevention

Never pass null checker lists; construct them explicitly with Arrays.asList(...)
Validate list contents at the point of building test assertions
Prefer individual PAssert.that(pcollection).satisfies(fn) calls when checkers are optional

When it happens

Trigger: Calling PAssert.that(pCollectionList).satisfies(null) — typically the result of a helper that builds checkers conditionally and returns null.

Common situations: Programmatic test-generation code that passes a null list of assertion functions; refactoring where individual satisfies(...) calls were consolidated and the list was never initialized.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/4b1683f777783f06. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/testing/PAssert.java:620

    public PCollectionListContentsAssert<T> satisfies(
        SerializableFunction<Iterable<T>, Void> checkerFn) {
      for (int i = 0; i < pCollectionList.size(); i++) {
        PAssert.that(pCollectionList.get(i)).satisfies(checkerFn);
      }
      return this;
    }

    /**
     * Takes list of {@link SerializableFunction}s of the same size as {@link #pCollectionList}, and
     * applies each matcher to the {@code PCollection} with the identical index in the {@link
     * #pCollectionList}.
     *
     * <p>Returns this {@code PCollectionListContentsAssert}.
     */
    public PCollectionListContentsAssert<T> satisfies(
        List<SerializableFunction<Iterable<T>, Void>> checkerFnList) {
      if (checkerFnList == null) {
        throw new IllegalArgumentException("List of SerializableFunction must not be null");
      } else if (checkerFnList.size() != pCollectionList.size()) {
        throw new IllegalArgumentException(
            "List of SerializableFunction must be the same size as the PCollectionList");
      }
      for (int i = 0; i < pCollectionList.size(); i++) {
        PAssert.that(pCollectionList.get(i)).satisfies(checkerFnList.get(i));
      }
      return this;
    }
  }

  /**
   * An {@link IterableAssert} about the contents of a {@link PCollection}. This does not require
   * the runner to support side inputs.
   */
  protected static class PCollectionContentsAssert<T> implements IterableAssert<T> {
    private final PCollection<T> actual;
    private final AssertionWindows rewindowingStrategy;

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