apache/flink · error · RuntimeException

{e.getMessage()}

Error message

{e.getMessage()}

What it means

Thrown by the private trySerialize method in IndexLookupGeneratorFunction when serializing user-supplied elements fails during construction. The serializeElements IOException is wrapped as RuntimeException(e.getMessage(), e) because the constructor cannot declare checked exceptions. This mirrors the pattern in FromElementsGeneratorFunction.

Source

Thrown at flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java:173

                    "Failed to deserialize an element from the source. "
                            + "If you are using user-defined serialization (Value and Writable "
                            + "types), check the serialization functions.\nSerializer is "
                            + serializer,
                    e);
        }
    }

    private void buildLookup() throws IOException {
        for (long i = 0; i < numElements; i++) {
            lookupMap.put(i, tryDeserialize());
        }
    }

    private void trySerialize(Iterable<OUT> elements) {
        try {
            serializeElements(elements);
        } catch (IOException e) {
            throw new RuntimeException(e.getMessage(), e);
        }
    }
}

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Examine the wrapped cause chain — the RuntimeException wraps an IOException which wraps the original failure.
2. Ensure all element fields are serializable by the Flink TypeSerializer for the declared TypeInformation.
3. Test the TypeSerializer in isolation to find the failing element.
4. For custom Value or Writable types, verify write/readFields round-trip correctly.

Example fix

// before: fails if CustomType.write() is broken
new IndexLookupGeneratorFunction<>(TypeInformation.of(CustomType.class), items);

// after: fix the serializer and verify round-trip
TypeSerializer<CustomType> ser = TypeInformation.of(CustomType.class).createSerializer(new SerializerConfig());
// unit test: serialize then deserialize each element
new IndexLookupGeneratorFunction<>(TypeInformation.of(CustomType.class), items);

// Validate serialization before constructing IndexLookupGeneratorFunction
TypeSerializer<OUT> serializer = typeInfo.createSerializer(config.getSerializerConfig());
DataOutputViewStreamWrapper wrapper =
    new DataOutputViewStreamWrapper(new ByteArrayOutputStream());
for (OUT element : elements) {
    serializer.serialize(element, wrapper);
}

try {
    new IndexLookupGeneratorFunction<>(typeInfo, elements);
} catch (RuntimeException e) {
    if (e.getCause() instanceof IOException) {
        // inspect e.getCause().getCause() for root serialization error
    }
    throw e;
}

When it happens

Trigger: Constructing new IndexLookupGeneratorFunction<>(typeInfo, elements) where the TypeSerializer cannot serialize one or more elements. The underlying serializeElements wraps the original exception as IOException, which trySerialize re-wraps as RuntimeException.

Common situations: POJO fields that are not serializable; custom Value/Writable types with broken write() methods; TypeInformation mismatch with actual element types; elements containing non-serializable nested objects.

Related errors

• Serializing the source elements failed: {e.getMessage()}
• The collection contains a null element
• Reached the end of the collection. This could be caused by i
• Failed to deserialize an element from the source. If you are
• The collection contains a null element

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/d4de62b2b44f9eb4. Report an issue: GitHub.