What causes "Converter can only convert integers" in flowable/flowable-engine?

Convert the value to Long first: Long.valueOf(((Number) v).longValue()).

flowable/flowable-engine · error · FlowableIllegalArgumentException

Converter can only convert integers

Error message

Converter can only convert integers

What it means

LongRestVariableConverter.convertVariableValue serializes an engine variable to a REST variable and requires the value to be exactly java.lang.Long. Note the message says "integers" but the guard is instanceof Long: Integer, int, BigInteger, or other numeric types fail and throw FlowableIllegalArgumentException.

Solutions

Convert the value to Long first: Long.valueOf(((Number) v).longValue()).
Declare process variables as long/Long end-to-end so the engine stores java.lang.Long.
If you genuinely need Integer variables, use the IntegerRestVariableConverter instead.
Update the misleading message text in a patch if you maintain a fork, to say "Converter can only convert Long".

Example fix

// before
int count = 42;
converter.convertVariableValue(count, restVar); // throws (autoboxed to Integer)

// after
converter.convertVariableValue(Long.valueOf(count), restVar);

Defensive patterns

Strategy: type-guard

Validate before calling

if (value instanceof Long) {
    converter.convertVariableValue(value, restVar);
} else if (value instanceof Number n) {
    converter.convertVariableValue(n.longValue(), restVar);
} else {
    throw new IllegalArgumentException("Cannot serialize as long: " + value.getClass());
}

Type guard

static boolean isJavaLangLong(Object v) {
    return v instanceof Long;
}

Try / catch

try {
    converter.convertVariableValue(value, restVar);
} catch (FlowableIllegalArgumentException e) {
    throw new IllegalStateException("long REST variable requires java.lang.Long, got: "
        + value.getClass().getName(), e);
}

Prevention

Declare variables as long/Long end-to-end; avoid int/Integer for long-typed variables.
Wrap numeric literals explicitly with Long.valueOf before adding to variable maps.
Note the converter's message says 'integers' but the required type is Long — trust the instanceof, not the text.

When it happens

Trigger: Calling convertVariableValue with a non-null value that is not a Long — commonly an Integer, int (autoboxed), or BigInteger — when the variable is declared as a long REST type.

Common situations: Passing int/Integer literals into variable maps typed as long; ORM or JSON layers producing Integer/BigInteger; copy-paste from integer converters despite the misleading message text.

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/3f8e861eb5a17d56. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-common-rest/src/main/java/org/flowable/common/rest/variable/LongRestVariableConverter.java:48

        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);
        }
    }

}

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