apache/beam · error · IllegalArgumentException

Could not encode provided windows with the provided window…

Error message

Could not encode provided windows with the provided window coder

What it means

WindowSupplier.of encodes each provided window with the supplied Coder; if any encoding fails (CoderException) it wraps it in this IllegalArgumentException. The provided coder cannot serialize one of the provided window instances.

Solutions

1. Pass the coder matching the window type, e.g. IntervalWindow.getCoder() or WindowCoder.of()
2. Verify the Iterable<W> windows actually match the Coder<W> type parameter
3. Catch at authoring time by unit-testing encode/decode round-trip of one window

Example fix

// before
WindowSupplier.of(InstantCoder.of(), windows) // wrong coder
// after
WindowSupplier.of(IntervalWindow.getCoder(), windows)

for (W window : windows) {
  CoderUtils.encodeToByteArray(coder, window); // throws CoderException early with better context
}

try { WindowSupplier.of(coder, windows); }
catch (IllegalArgumentException e) { /* coder does not match window type; fix coder */ }

When it happens

Trigger: Calling WindowSupplier.of(coder, windows) with a coder incompatible with the window objects (wrong coder type, coder that throws CoderException for these values).

Common situations: Passing e.g. a GlobalWindow coder for IntervalWindow values, or a coder whose type parameters don't match the windows after a refactor.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• Could not decode provided windows with the provided window…
• The input value cannot be encoded
• The input value cannot be encoded
• ApproximateUnique.PerKey requires its input to use KvCoder
• AvroCoder for GenericRecord requires a schema

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/9f28920dc12f9c5b. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/testing/WindowSupplier.java:55

// Spotbugs thinks synchronization is for the fields, but it is for the act of decoding
@SuppressFBWarnings("IS2_INCONSISTENT_SYNC")
@SuppressWarnings({
  "nullness" // TODO(https://github.com/apache/beam/issues/20497)
})
final class WindowSupplier implements Supplier<Collection<BoundedWindow>>, Serializable {
  private final Coder<? extends BoundedWindow> coder;
  private final Collection<byte[]> encodedWindows;

  /** Access via {@link #get()}. */
  private transient @Nullable Collection<BoundedWindow> windows;

  public static <W extends BoundedWindow> WindowSupplier of(Coder<W> coder, Iterable<W> windows) {
    ImmutableSet.Builder<byte[]> windowsBuilder = ImmutableSet.builder();
    for (W window : windows) {
      try {
        windowsBuilder.add(CoderUtils.encodeToByteArray(coder, window));
      } catch (CoderException e) {
        throw new IllegalArgumentException(
            "Could not encode provided windows with the provided window coder", e);
      }
    }
    return new WindowSupplier(coder, windowsBuilder.build());
  }

  private WindowSupplier(Coder<? extends BoundedWindow> coder, Collection<byte[]> encodedWindows) {
    this.coder = coder;
    this.encodedWindows = encodedWindows;
  }

  @Override
  public Collection<BoundedWindow> get() {
    if (windows == null) {
      decodeWindows();
    }
    return windows;
  }

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