flowable/flowable-engine · error · FlowableIllegalArgumentException

Converter can only convert string to date

Error message

Converter can only convert string to date

What it means

DateRestVariableConverter expects the REST variable's value to be a String containing a date in the long/timestamp format parsed by RequestUtil.parseLongDate. If the value is not a String, the converter cannot even attempt parsing, so FlowableIllegalArgumentException 'Converter can only convert string to date' is thrown before parsing.

Source

Thrown at modules/flowable-common-rest/src/main/java/org/flowable/common/rest/variable/DateRestVariableConverter.java:41

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

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

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

    @Override
    public Object getVariableValue(EngineRestVariable result) {
        if (result.getValue() != null) {
            if (!(result.getValue() instanceof String)) {
                throw new FlowableIllegalArgumentException("Converter can only convert string to date");
            }
            try {
                return RequestUtil.parseLongDate((String) result.getValue());
            } catch (DateTimeParseException e) {
                throw new FlowableIllegalArgumentException("The given variable value is not a date: '" + result.getValue() + "'", e);
            }
        }
        return null;
    }

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

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Send date variable values as strings, e.g. "1694044800000" (long millis as string), matching the converter's expected format.
2. Check the REST variable 'type' so numeric values are handled by the appropriate converter rather than the date one.
3. If you build EngineRestVariable yourself, set value to a String: String.valueOf(timestamp).
4. For other date formats, convert client-side to epoch millis string before sending (parseLongDate accepts epoch millis).

Example fix

// before
restVariable.setValue(1694044800000L); // Long -> throws
// after
restVariable.setValue(String.valueOf(1694044800000L)); // epoch millis as string

if (value != null && !(value instanceof String)) {
    value = String.valueOf(value); // epoch millis number -> string
}

String asDateString(Object v) {
    if (v instanceof String) return (String) v;
    if (v instanceof Number) return String.valueOf(v.longValue());
    return null;
}

try {
    Object v = converter.getVariableValue(restVariable);
} catch (FlowableIllegalArgumentException e) {
    if (e.getMessage() != null && e.getMessage().contains("convert string to date")) {
        throw new IllegalArgumentException("Date variables must be epoch-millis strings, got: " + restVariable.getValue(), e);
    }
    throw e;
}

When it happens

Trigger: getVariableValue invoked on an EngineRestVariable whose value is a non-String (e.g. a Number/Long object or JSON number deserialized as Integer) while this date converter was selected for the variable.

Common situations: Clients sending epoch-millis as a JSON number instead of a string for date variables; type mapping selecting the date converter for non-string payloads; custom code stuffing pre-parsed Date objects into EngineRestVariable.

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

• Variable '${restVariable.getName()}' has unsupported type: '
• Converter can only convert big decimal values
• Converter can only convert big integer values
• Converter can only convert booleans
• Converter can only convert booleans

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/9fef0e70d3b4f869. Report an issue: GitHub.