flowable/flowable-engine · error · ActivitiIllegalArgumentException

Class does not implement

Error message

Class %s does not implement %s

What it means

DelegateActivitiEventListener lazily instantiates its listener delegate from a class name via reflection. If the instantiated object does not implement FlowableEventListener, it throws ActivitiIllegalArgumentException and forces the dispatch to fail rather than silently skip the event.

Solutions

1. Make the configured class implement org.flowable.engine.delegate.event.FlowableEventListener (with onEvent and isFailOnException)
2. Fix the class name in the listener registration to point at the actual listener class
3. Check for stale copies of the class still implementing an old/other interface
4. Redeploy/restart after correcting the configuration

Example fix

// before
public class MyListener { /* no interface */ }
// after
public class MyListener implements FlowableEventListener {
    public void onEvent(FlowableEvent event) { /* ... */ }
    public boolean isFailOnException() { return true; }
}

if (!FlowableEventListener.class.isAssignableFrom(listenerClass)) throw new IllegalArgumentException(listenerClass + " must implement FlowableEventListener");

When it happens

Trigger: An event listener registered (e.g. via engine config addEventlistener or process XML <flowable:eventListener>) with a class attribute whose class exists and instantiates, but does not implement org.flowable.engine.delegate.event.FlowableEventListener.

Common situations: Registering a plain POJO or wrong listener interface implementation by mistake; migrating from another listener interface (e.g. ActivitiEventListener in old versions vs FlowableEventListener); class name pointing to an unrelated class after refactor.

Related errors

• No execution context active and event is not related to an…
• Delegate expression did not resolve to an implementation of…
• BPMN XSD could not be found
• Cannot create 'script' task listener. Missing ScriptInfo.
• Cannot have more than one message event subscription with…

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/6a11fc1acc43c165. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable5-engine/src/main/java/org/activiti/engine/impl/bpmn/helper/DelegateActivitiEventListener.java:63

    }

    @Override
    public boolean isFailOnException() {
        if (delegateInstance != null) {
            return delegateInstance.isFailOnException();
        }
        return failOnException;
    }

    protected FlowableEventListener getDelegateInstance() {
        if (delegateInstance == null) {
            Object instance = ReflectUtil.instantiate(className);
            if (instance instanceof FlowableEventListener) {
                delegateInstance = (FlowableEventListener) instance;
            } else {
                // Force failing of the listener invocation, since the delegate cannot be created
                failOnException = true;
                throw new ActivitiIllegalArgumentException("Class " + className
                        + " does not implement " + FlowableEventListener.class.getName());
            }
        }
        return delegateInstance;
    }
}

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