hibernate/hibernate-orm · error · UnsupportedOperationException

Unexpected JSON type " + type

Error message

Unexpected JSON type " + type

What it means

While reading the elements of a JSON array (deserializeArray), only VALUE, BOOLEAN_VALUE and OBJECT_START (nested json aggregate embeddables) are handled; every other item type falls to default and throws UnsupportedOperationException('Unexpected JSON type <type>'). In practice this means a structure the parser cannot represent - most notably nested arrays (ARRAY_START inside an array) - inside a JSON-mapped collection.

Source

Thrown at hibernate-core/src/main/java/org/hibernate/type/descriptor/jdbc/JsonHelper.java:298

				case NULL_VALUE:
					arrayList.add( null );
					break;
				case NUMERIC_VALUE:
					arrayList.add( adapter.fromNumericValue(jdbcJavaType, elementJdbcType ,reader, options)  );
					break;
				case BOOLEAN_VALUE:
					arrayList.add( reader.getBooleanValue() ? Boolean.TRUE : Boolean.FALSE );
					break;
				case VALUE:
					arrayList.add( adapter.fromValue(jdbcJavaType, elementJdbcType ,reader, options) );
					break;
				case OBJECT_START:
					assert elementJdbcType instanceof JsonJdbcType;
					final EmbeddableMappingType embeddableMappingType = ((JsonJdbcType) elementJdbcType).getEmbeddableMappingType();
					arrayList.add( consumeJsonDocumentItems(reader, embeddableMappingType, true, options) );
					break;
				default:
					throw new UnsupportedOperationException( "Unexpected JSON type " + type );
			}
		}

		throw new IllegalArgumentException( "Expected JSON array end, but none found." );
	}


	private static class CustomArrayList extends AbstractCollection<Object> implements Collection<Object> {
		Object[] array = ArrayHelper.EMPTY_OBJECT_ARRAY;
		int size;

		public void ensureCapacity(int minCapacity) {
			int oldCapacity = array.length;
			if ( minCapacity > oldCapacity ) {
				int newCapacity = oldCapacity + ( oldCapacity >> 1 );
				newCapacity = Math.max( Math.max( newCapacity, minCapacity ), 10 );
				array = Arrays.copyOf( array, newCapacity );
			}

View on GitHub (pinned to fad1729dce)

Solutions

1. Avoid nested arrays in json aggregate data: model each element as an embeddable object (list of aggregate embeddables) instead of an array of arrays.
2. Or flatten the structure to a single array and unpack positions in code.

Example fix

// before: stored [[1,2],[3,4]] mapped as
@JdbcTypeCode(SqlTypes.JSON) List<int[]> matrix; // parse -> Unexpected JSON type ARRAY_START

// after: store objects the parser understands
@JdbcTypeCode(SqlTypes.JSON) List<Row> matrix; // @Embeddable class Row { int x; int y; }

// Reject nested arrays before persisting (Jackson example)
static void assertNoNestedArrays(com.fasterxml.jackson.databind.JsonNode arrayNode) {
    for (JsonNode element : arrayNode) {
        if (element.isArray()) {
            throw new IllegalArgumentException("Nested arrays are not supported by Hibernate JSON collection parsing");
        }
    }
}

try {
    return session.find(Product.class, id);
} catch (UnsupportedOperationException e) {
    if (e.getMessage() != null && e.getMessage().startsWith("Unexpected JSON type")) {
        // unsupported item structure inside a JSON array (e.g., nested array): remodel as objects
    } else {
        throw e;
    }
}

When it happens

Trigger: Loading a JSON array attribute whose elements are themselves arrays (e.g., [[1,2],[3,4]]); any item type outside the supported set inside an array bound to a plural JSON mapping.

Common situations: Domain data modeled as nested arrays (matrix-like values) stored in a column mapped as List<T>; other services writing richer JSON than the mapping supports.

Related errors

• Can't parse JSON array for selectable [%s] which is not of t
• Malformed JSON. Expected array but got: " + event
• Expected JSON array end, but none found.
• array case should be treated at upper level
• Duplicate collection definition '%s'

AI-assisted analysis of hibernate/hibernate-orm@fad1729dce (2026-08-22). Data as JSON: /api/errors/e676b487005afc12. Report an issue: GitHub.