apache/seatunnel · error · java.lang.IllegalArgumentException

Map type requires Map value, got: ${value.getClass()}

Error message

Map type requires Map value, got: ${value.getClass()}

What it means

FragmentConverter.writeMapToVector throws this IllegalArgumentException when writing a SeaTunnel MAP column into an Arrow MapVector but the runtime value is not a java.util.Map. The converter requires an exact Map instance to iterate key/value pairs; anything else (String, List, POJO) cannot be written as map entries. It is a data/type contract violation between the SeaTunnel row and the Lance/Arrow schema.

Source

Thrown at seatunnel-connectors-v2/connector-lance/src/main/java/org/apache/seatunnel/connectors/seatunnel/lance/utils/FragmentConverter.java:139

        Field elementField = children.get(0);
        ArrowType elementType = elementField.getType();

        for (Object element : listValue) {
            writeListElement(writer, elementType, element, allocator);
        }

        writer.setValueCount(listValue.size());
        writer.endList();
    }

    private static void writeMapToVector(
            MapVector mapVector,
            Field field,
            Object value,
            int rowIndex,
            BufferAllocator allocator) {
        if (!(value instanceof java.util.Map)) {
            throw new IllegalArgumentException(
                    "Map type requires Map value, got: " + value.getClass());
        }

        UnionMapWriter writer = mapVector.getWriter();
        writer.setPosition(rowIndex);
        writer.startMap();

        java.util.Map<?, ?> mapValue = (java.util.Map<?, ?>) value;
        List<Field> children = field.getChildren();
        if (children.size() < 2) {
            throw new IllegalArgumentException("Map field must have key and value child fields");
        }
        Field keyField = children.get(0);
        Field valueField = children.get(1);
        ArrowType keyType = keyField.getType();
        ArrowType valueType = valueField.getType();

        for (java.util.Map.Entry<?, ?> entry : mapValue.entrySet()) {

View on GitHub (pinned to cf67b549a7)

Solutions

1. Fix the upstream source/transform so the MAP column carries an actual java.util.Map value
2. Cast or convert the value (e.g. parse JSON string to Map) before writing
3. Verify the SeaTunnel catalog schema declares the column as MAP and matches the data
4. Add a pre-write type check/logging to identify which column and row carries the bad value

Example fix

// before
Object value = row.getField(mapColIndex); // value is a JSON string
converter.setVectorValue(mapVector, field, value, i, allocator); // throws
// after
Object value = row.getField(mapColIndex);
if (!(value instanceof java.util.Map) && value instanceof String) {
    value = new com.fasterxml.jackson.databind.ObjectMapper()
            .readValue((String) value, java.util.Map.class);
}
converter.setVectorValue(mapVector, field, value, i, allocator);

if (!(value instanceof java.util.Map)) {
    throw new IllegalArgumentException("Column '" + field.getName() + "' expects a java.util.Map, got: "
        + (value == null ? "null" : value.getClass().getName()));
}

boolean isWritableMap(Object v) {
    return v instanceof java.util.Map;
}

try {
    converter.setVectorValue(mapVector, field, value, rowIndex, allocator);
} catch (IllegalArgumentException e) {
    if (e.getMessage() != null && e.getMessage().startsWith("Map type requires Map value")) {
        LOG.warn("Skipping non-map value {} at row {} column {}", value, rowIndex, field.getName());
    } else {
        throw e;
    }
}

When it happens

Trigger: setVectorValue dispatches a row field whose SeaTunnel type is MAP_ARRAY/MAP to writeMapToVector, but the Object value at rowIndex is not a java.util.Map (e.g. it is a String, List, or null-wrapped wrapper).

Common situations: Custom source connectors emitting raw JSON strings for map columns; data read from formats that deserialize maps into LinkedHashMap alternatives or Lists of pairs; upstream transforms producing wrong column types; schema mismatch after table definition changes.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Unsupported timestamp value type:
• Unsupported date value type:
• Map type should be handled via FragmentConverter.writeMapToV
• List type requires List value, got: ${value.getClass()}
• TABLE_DATASET_WRITE_ST_ROW_EXCEPTION

AI-assisted analysis of apache/seatunnel@cf67b549a7 (2026-09-10). Data as JSON: /api/errors/2a794e6881b3fc98. Report an issue: GitHub.