apache/beam · error · CannotProvideCoderException

PTransform.getOutputCoder called.

Error message

PTransform.getOutputCoder called.

What it means

PTransform.getDefaultOutputCoder() is a deprecated hook that was meant to be overridden to supply an output coder. The base implementation throws CannotProvideCoderException because the generic PTransform cannot infer a coder. If your transform neither overrides this nor sets a coder on its output PCollection, pipeline construction fails.

Solutions

1. In expand(), explicitly set a coder: PCollection<O> out = input.apply(...); out.setCoder(MyCoder.of()); return out;
2. Override getDefaultOutputCoder(input) (the modern one-argument variant) to return a Coder for your output
3. Ensure the output type is a concrete, coder-inferrable class (avoid raw/generic types)

Example fix

// before
return input.apply("t", new MyTransform());
// after
PCollection<O> out = input.apply("t", new MyTransform());
out.setCoder(MyCoder.of());
return out;

// Verify a coder is available before expanding
Coder<?> c = null;
try {
  java.lang.reflect.Method m = PTransform.class.getDeclaredMethod("getDefaultOutputCoder", PCollection.class);
  m.setAccessible(true);
  c = (Coder<?>) m.invoke(transform, input);
} catch (CannotProvideCoderException | ReflectiveOperationException ignored) { }
if (c == null) { /* set coder explicitly on output */ }

try {
  PCollection<O> out = input.apply(transform);
} catch (CannotProvideCoderException | IllegalStateException e) {
  PCollection<O> out = input.apply(transform).setCoder(MyCoder.of());
}

When it happens

Trigger: Applying a custom PTransform that returns a PCollection without ever calling output.setCoder(...) and without overriding getDefaultOutputCoder, so Beam falls back to the deprecated base method.

Common situations: Custom transforms producing PCollection<Row>/POJOs where type erasure prevents inference; older tutorials still using the deprecated override; transforms whose output type is a raw or generic type.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• ApproximateUnique.PerKey requires its input to use KvCoder
• AvroCoder for GenericRecord requires a schema
• cannot encode a null BitSet
• cannot encode a null byte[]
• cannot encode a null Integer

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/594b98ef34e412bf. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/PTransform.java:331

  private void readObject(ObjectInputStream oos) {
    // We don't really want to be serializing this object, but we
    // often have serializable anonymous DoFns nested within a
    // PTransform.
  }

  /**
   * Returns the default {@code Coder} to use for the output of this single-output {@code
   * PTransform}.
   *
   * <p>By default, always throws
   *
   * @throws CannotProvideCoderException if no coder can be inferred
   * @deprecated Instead, the PTransform should explicitly call {@link PCollection#setCoder} on the
   *     returned PCollection.
   */
  @Deprecated
  protected Coder<?> getDefaultOutputCoder() throws CannotProvideCoderException {
    throw new CannotProvideCoderException("PTransform.getOutputCoder called.");
  }

  /**
   * Returns the default {@code Coder} to use for the output of this single-output {@code
   * PTransform} when applied to the given input.
   *
   * <p>By default, always throws.
   *
   * @throws CannotProvideCoderException if none can be inferred.
   * @deprecated Instead, the PTransform should explicitly call {@link PCollection#setCoder} on the
   *     returned PCollection.
   */
  @Deprecated
  protected Coder<?> getDefaultOutputCoder(@SuppressWarnings("unused") InputT input)
      throws CannotProvideCoderException {
    return getDefaultOutputCoder();
  }

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