flowable/flowable-engine · error · FlowableIllegalArgumentException

Converter can only convert doubles

Error message

Converter can only convert doubles

What it means

DoubleRestVariableConverter.getVariableValue converts an engine variable value into a Double for REST output. If the stored value is non-null but not a Number, the converter cannot produce a double and throws this FlowableIllegalArgumentException.

Source

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

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

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

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

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

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

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Request the variable with the REST type matching the stored value (e.g. 'string').
2. Re-set the variable in the engine as a numeric type (Double, Integer, Long).
3. Parse the value to a number before storing it if it arrives as a string from an upstream source.
4. Verify no custom variable converter collides with the double converter's type.

Example fix

// before
runtimeService.setVariable(executionId, "amount", "3.14");
// after
runtimeService.setVariable(executionId, "amount", 3.14);

public static boolean isNumericValue(Object v) { return v == null || v instanceof Number; }

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

try {
    Object d = restClient.getVariable(id, "amount");
} catch (FlowableIllegalArgumentException e) {
    if (e.getMessage().equals("Converter can only convert doubles")) {
        // request the variable as its actual type or fix the stored value
    } else { throw e; }
}

When it happens

Trigger: Reading a REST variable with type 'double' when the underlying engine value is a non-numeric object (String, Boolean, arbitrary Serializable) instead of a numeric type.

Common situations: Variable declared 'double' in the REST request while the engine holds a string; manual DB edits to variable tables; a custom converter wrote a non-numeric value.

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 booleans
• Converter can only convert string to instant
• Converter can only convert instant
• Converter can only convert integers
• Variable '${restVariable.getName()}' has unsupported type: '

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/55ece4d3069dd6ab. Report an issue: GitHub.