flowable/flowable-engine · error · FlowableIllegalArgumentException

Cannot convert value of type

Error message

Cannot convert value of type ${value.getClass()} to Jackson 3 JsonNode

What it means

JsonUtil.asJsonNode converts a Java value into a Jackson 3 JsonNode. When the value's type is not convertible (and allowOtherTypes is false, meaning null is not an acceptable outcome), it throws this FlowableIllegalArgumentException naming the offending runtime type.

Solutions

  1. Check the reported type in the message and convert it to a supported JSON type (String, Number, Boolean, Map, List, JsonNode) before calling asJsonNode.
  2. For POJOs, serialize with an ObjectMapper first (mapper.valueToTree(pojo)) and pass the resulting JsonNode.
  3. Convert java.util.Date/Calendar values to ISO-8601 strings or epoch millis.
  4. If null is acceptable for non-convertible types, use the allowOtherTypes=true variant.
  5. Audit process variables for custom classes that no longer convert under Jackson 3.

Example fix

// before
JsonNode node = JsonUtil.asJsonNode(myDate);
// after
JsonNode node = JsonUtil.asJsonNode(new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss").format(myDate));
Defensive patterns

Strategy: type-guard

Validate before calling

if (value == null || value instanceof JsonNode || value instanceof String || value instanceof Number || value instanceof Boolean || value instanceof Map || value instanceof Collection) {
    // safe to convert
}

Type guard

static boolean isJsonConvertible(Object v) {
    return v instanceof JsonNode || v instanceof String || v instanceof Number
        || v instanceof Boolean || v instanceof Map || v instanceof Collection;
}

Try / catch

try {
    JsonNode node = JsonUtil.asJsonNode(value);
} catch (FlowableIllegalArgumentException e) {
    JsonNode node = objectMapper.valueToTree(value); // fallback to full serialization
}

Prevention

When it happens

Trigger: Passing a non-JSON-convertible object (e.g. an arbitrary POJO, Date, or custom type) to JsonUtil.asJsonNode(value) or to an overload where allowOtherTypes is false and the value is not a JsonNode, primitive wrapper, String, Map, Collection, or similar supported type.

Common situations: Variable values stored in process variables (e.g. a Date or custom bean) being converted to JSON for REST/serialization; upgrading from Jackson 2 to Jackson 3 where type support changed; passing untyped Object variables.

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

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/8dbc9d9a20644b6b. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine-common/src/main/java/org/flowable/common/engine/impl/util/JsonUtil.java:134

                } else if (jackson2JsonNode.isTextual()) {
                    return StringNode.valueOf(jackson2JsonNode.textValue());
                } else if (jackson2JsonNode.isNumber()) {
                    return objectMapper.valueToTree(jackson2JsonNode.numberValue());
                } else if (jackson2JsonNode.isBoolean()) {
                    return objectMapper.valueToTree(jackson2JsonNode.booleanValue());
                } else if (jackson2JsonNode.isMissingNode()) {
                    return MissingNode.getInstance();
                }

                String jsonString = jackson2JsonNode.toString();
                return objectMapper.readTree(jsonString);
            }
        }
        if (allowOtherTypes) {
            return null;
        }

        throw new FlowableIllegalArgumentException("Cannot convert value of type " + value.getClass() + " to Jackson 3 JsonNode");
    }

    public static Object deepCopyIfJson(Object value) {
        if (value instanceof JsonNode jsonNode) {
            return jsonNode.deepCopy();
        } else if (JACKSON_2_PRESENT) {
            if (value instanceof com.fasterxml.jackson.databind.JsonNode jsonNode) {
                return jsonNode.deepCopy();
            }
        }

        return value;
    }

    public static boolean isArrayNode(Object value) {
        if (value instanceof ArrayNode) {
            return true;
        }

View on GitHub (pinned to d6d39ce1c6)