apache/beam · error · RuntimeException

Could not extract the Kafka Deserializer type from

Error message

Could not extract the Kafka Deserializer type from %s

What it means

In getNullableCoder, the deserializer class's generic interfaces are scanned for a direct parameterized Deserializer<T> super-interface. If none is found (the deserializer extends Deserializer indirectly or is raw/generic-less), the type argument cannot be extracted and this RuntimeException is thrown — coder inference is impossible for such classes.

Solutions

1. Make the deserializer directly implement Deserializer<T> with a concrete type parameter.
2. Supply the Coder explicitly rather than relying on automatic inference.
3. Wrap the deserializer in a concrete class declaring its target type.

When it happens

Trigger: Thrown at sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/LocalDeserializerProvider.java:101 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/7b0e3c5d2d379b4b. Report an issue: GitHub.

Appendix: source

Thrown at sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/LocalDeserializerProvider.java:101

      if (parameterizedType.getRawType() == Deserializer.class) {
        Type parameter = parameterizedType.getActualTypeArguments()[0];

        @SuppressWarnings("unchecked")
        Class<T> clazz = (Class<T>) parameter;

        try {
          return NullableCoder.of(coderRegistry.getCoder(clazz));
        } catch (CannotProvideCoderException e) {
          throw new RuntimeException(
              String.format(
                  "Unable to automatically infer a Coder for "
                      + "the Kafka Deserializer %s: no coder registered for type %s",
                  deserializer, clazz));
        }
      }
    }
    throw new RuntimeException(
        String.format("Could not extract the Kafka Deserializer type from %s", deserializer));
  }
}

View on GitHub (pinned to 12126d8942)