flowable/flowable-engine · error · ActivitiIllegalArgumentException

Delegate expression did not resolve to an implementation of…

Error message

Delegate expression ${expression} did not resolve to an implementation of org.flowable.engine.delegate.event.FlowableEventListener

What it means

DelegateExpressionActivitiEventListener resolves its delegate from an expression (e.g. a Spring bean). If the resolved object is null or not an instance of FlowableEventListener, it throws ActivitiIllegalArgumentException and forces failure of event dispatch.

Solutions

1. Ensure the referenced bean implements org.flowable.engine.delegate.event.FlowableEventListener
2. Verify the expression (e.g. ${myListener}) names the correct bean in the application context
3. Check the bean is a plain listener implementation, not a proxy/wrapper hiding the interface
4. Fix the delegateExpression value in the listener registration config or BPMN XML

Example fix

// before
<flowable:eventListener events="JOB_EXECUTION_SUCCESS" delegateExpression="${myPojo}"/>
// after
<flowable:eventListener events="JOB_EXECUTION_SUCCESS" delegateExpression="${myEventListener}"/> <!-- implements FlowableEventListener -->

Object bean = applicationContext.getBean("myEventListener");
if (!(bean instanceof FlowableEventListener)) throw new IllegalArgumentException("bean must implement FlowableEventListener");

When it happens

Trigger: A listener registered with a delegateExpression whose expression resolves to a bean that is not a FlowableEventListener (or does not resolve at all to the right type).

Common situations: Spring bean of the wrong type wired into the process engine config; bean defined but wrong interface implemented; expression typo resolving to an unrelated bean; bean scope/proxy returning an unexpected wrapper.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Class does not implement
• No execution context active and event is not related to an…
• doesn't implement
• doesn't implement
• Error while evaluating expression:

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/c52110a43307dbd0. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable5-engine/src/main/java/org/activiti/engine/impl/bpmn/helper/DelegateExpressionActivitiEventListener.java:56

    @Override
    public void onEvent(FlowableEvent event) {
        if (isValidEvent(event)) {
            Object delegate = DelegateExpressionUtil.resolveDelegateExpression(expression, new NoExecutionVariableScope());
            if (delegate instanceof FlowableEventListener) {
                // Cache result of isFailOnException() from delegate-instance until next
                // event is received. This prevents us from having to resolve the expression twice when
                // an error occurs.
                failOnException = ((FlowableEventListener) delegate).isFailOnException();

                // Call the delegate
                ((FlowableEventListener) delegate).onEvent(event);
            } else {

                // Force failing, since the exception we're about to throw cannot be ignored, because it
                // did not originate from the listener itself
                failOnException = true;
                throw new ActivitiIllegalArgumentException("Delegate expression " + expression
                        + " did not resolve to an implementation of " + FlowableEventListener.class.getName());
            }
        }
    }

    @Override
    public boolean isFailOnException() {
        return failOnException;
    }

}

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