flowable/flowable-engine · error · FlowableIllegalArgumentException

Converter can only convert longs

Error message

Converter can only convert longs

What it means

LongRestVariableConverter.getVariableValue converts a REST variable into a java.lang.Long and requires the REST value to be a Number. If the value is any other type it throws FlowableIllegalArgumentException, since only numeric JSON values can be narrowed via Number.longValue().

Solutions

1. Send the value as an unquoted JSON number (e.g. 123, not "123").
2. Parse numeric strings with Long.parseLong client-side before posting.
3. Verify the REST variable type marker matches the payload (integer vs long vs string).
4. Fix client serialization that quotes all values (e.g. Map<String, String> payloads).

Example fix

// before
{ "type": "long", "value": "123" }

// after
{ "type": "long", "value": 123 }

if (!(rawValue instanceof Number)) {
    throw new IllegalArgumentException("long variable must be a JSON number, got: "
        + (rawValue == null ? "null" : rawValue.getClass().getSimpleName()));
}

static boolean isNumeric(Object v) {
    return v instanceof Number;
}

try {
    Object value = converter.getVariableValue(restVar);
} catch (FlowableIllegalArgumentException e) {
    log.error("long variable value is not numeric: {}", restVar.getValue(), e);
}

When it happens

Trigger: Sending a variable of REST type long whose value is a String (e.g. "123"), boolean, array, or object, so result.getValue() fails the Number instanceof check in getVariableValue.

Common situations: Clients posting numeric IDs as quoted strings; form/query inputs kept as text; JSON frameworks that deserialize numbers into Strings or BigDecimal wrappers unexpectedly (BigDecimal is fine, but String is not).

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

• Converter can only convert integers
• Converter can only convert…
• Converter can only convert localDate
• Converter can only convert localDateTime
• Converter can only convert shorts

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/75b45005f8480d86. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-common-rest/src/main/java/org/flowable/common/rest/variable/LongRestVariableConverter.java:37

 * @author Frederik Heremans
 */
public class LongRestVariableConverter implements RestVariableConverter {

    @Override
    public String getRestTypeName() {
        return "long";
    }

    @Override
    public Class<?> getVariableType() {
        return Long.class;
    }

    @Override
    public Object getVariableValue(EngineRestVariable result) {
        if (result.getValue() != null) {
            if (!(result.getValue() instanceof Number)) {
                throw new FlowableIllegalArgumentException("Converter can only convert longs");
            }
            return ((Number) result.getValue()).longValue();
        }
        return null;
    }

    @Override
    public void convertVariableValue(Object variableValue, EngineRestVariable result) {
        if (variableValue != null) {
            if (!(variableValue instanceof Long)) {
                throw new FlowableIllegalArgumentException("Converter can only convert integers");
            }
            result.setValue(variableValue);
        } else {
            result.setValue(null);
        }
    }

View on GitHub (pinned to d6d39ce1c6)