What causes "condition script returns non-Boolean: () for " in flowable/flowable-engine?

End the condition script with an explicit boolean comparison, e.g. return approved == 'true' or return count > 0.

flowable/flowable-engine · error · FlowableException

condition script returns non-Boolean: () for

Error message

condition script returns non-Boolean:  () for

What it means

FlowableException thrown by ScriptCondition.evaluate when a condition script returns a value that is not a java.lang.Boolean. Flowable gateway routing requires a strict Boolean; any other type (String 'true', Integer, etc.) is rejected, with the actual value and its class included in the message.

Solutions

End the condition script with an explicit boolean comparison, e.g. return approved == 'true' or return count > 0.
Convert string variables before use: Boolean.parseBoolean(approved) in the script or store variables as Boolean from the start.
If using EL expressions (not scripts), wrap in a comparison: ${approved == true} instead of ${approved}.

Example fix

// before (groovy)
return approved

// after
return Boolean.TRUE.equals(approved) || 'true'.equals(approved)

Defensive patterns

Strategy: validation

Validate before calling

Object flag = execution.getVariable("approved");
if (!(flag instanceof Boolean)) {
    execution.setVariable("approved", Boolean.parseBoolean(String.valueOf(flag)));
}

Type guard

boolean isBooleanResult(Object o) {
    return o instanceof Boolean;
}

Try / catch

try {
    return (Boolean) scriptingEngines.evaluate(request).getResult();
} catch (FlowableException e) {
    log.error("non-boolean condition result: {}", e.getMessage());
    throw e;
}

Prevention

Store boolean process variables as java.lang.Boolean, not String 'true'
End condition scripts with an explicit comparison (x > 0, == 'true')
Prefer EL expressions with explicit comparisons over bare variables
Cover each condition with a script unit test asserting a Boolean result

When it happens

Trigger: A condition script (e.g. groovy returning 'true' as String, or javascript returning 1/0, or returning a java.lang.String from EL ${var} where var is a String) evaluates to a non-Boolean result for a conditional flow.

Common situations: Variables stored as String 'true'/'false' used directly as conditions; JS engine coercing to numbers; scripts returning status codes or objects instead of a boolean comparison; language confusion between expression vs script conditions.

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/6318d0c30159c4e5. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine/src/main/java/org/flowable/engine/impl/scripting/ScriptCondition.java:48

    public ScriptCondition(String expression, String language) {
        this.expression = expression;
        this.language = language;
    }

    @Override
    public boolean evaluate(String elementId, DelegateExecution execution) {
        ScriptingEngines scriptingEngines = CommandContextUtil.getProcessEngineConfiguration().getScriptingEngines();

        ScriptEngineRequest.Builder builder = ScriptEngineRequest.builder()
                .script(expression)
                .language(language)
                .scopeContainer(execution);
        Object result = scriptingEngines.evaluate(builder.build()).getResult();
        if (result == null) {
            throw new FlowableException("condition script returns null: " + expression + " for " + execution);
        }
        if (!(result instanceof Boolean)) {
            throw new FlowableException("condition script returns non-Boolean: " + result + " (" + result.getClass().getName() + ") for " + execution);
        }
        return (Boolean) result;
    }

}

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