flowable/flowable-engine · error · FlowableException

expected expression ${expression} to resolve to ${type} but

Error message

expected expression ${expression} to resolve to ${type} but it did not. Resolved value is ${value}

What it means

InboundChannelModelProcessor.resolveExpression evaluates a delegate expression (e.g. a Spring/el bean reference) and verifies the resolved value is an instance of the requested type (such as InboundEventKeyDetector, InboundEventSerializer, or a payload extractor). If the expression resolves to something of a different type (or null), Flowable throws this FlowableException instead of returning a bad cast.

Source

Thrown at modules/flowable-event-registry/src/main/java/org/flowable/eventregistry/impl/pipeline/InboundChannelModelProcessor.java:379

                        + ". One of fixedValue, jsonField, jsonPointerExpression, xmlXPathExpression, delegateExpression should be set.");
            }
        }

        //noinspection unchecked
        return new DefaultInboundEventProcessingPipeline(eventRepositoryService, eventDeserializer, eventFilter,
            eventKeyDetector, eventTenantDetector, eventPayloadExtractor, eventTransformer, engineConfiguration);
    }

    protected <T> T resolveExpression(String expression, Class<T> type) {
        Object value = CommandContextUtil.getEventRegistryConfiguration().getExpressionManager()
            .createExpression(expression)
            .getValue(new VariableContainerWrapper(Collections.emptyMap()));

        if (type.isInstance(value)) {
            return type.cast(value);
        }

        throw new FlowableException("expected expression " + expression + " to resolve to " + type + " but it did not. Resolved value is " + value);

    }

    @Override
    public void unregisterChannelModel(ChannelModel channelModel, String tenantId, EventRepositoryService eventRepositoryService) {
        // nothing to do
    }
}

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Make the referenced bean implement the exact expected interface (e.g. implements InboundEventKeyDetector) and redeploy
2. Check the expression string for typos and confirm it points at the intended bean; verify the bean exists in the application context and is of the expected type
3. Log the resolved value's class at that expression to diagnose the mismatch, then correct the configuration or the bean

Example fix

// before
@Bean
public Object orderKeyDetector() { return new OrderKeyResolver(); } // wrong type
// after
@Bean
public InboundEventKeyDetector orderKeyDetector() { return new OrderKeyResolver(); } // implements InboundEventKeyDetector

Object resolved = applicationContext.getBean(beanName);
if (!(resolved instanceof InboundEventKeyDetector)) {
    throw new IllegalArgumentException(beanName + " must implement InboundEventKeyDetector, got " + resolved.getClass());
}

boolean validDelegate = obj instanceof InboundEventKeyDetector;

try {
    registerChannelModel(channelModel, tenantId, repoService);
} catch (FlowableException e) {
    if (e.getMessage().startsWith("expected expression")) {
        // inspect bean type behind the delegateExpression
    } else { throw e; }
}

When it happens

Trigger: Any channel registration/pipeline creation that resolves a delegateExpression — e.g. serializerDelegateExpression, keyDetector delegateExpression, payload extractor expression — where the bean behind the expression does not implement/extend the expected interface, or the expression evaluates to null.

Common situations: Pointing a delegateExpression at the wrong Spring bean (right name, wrong type); bean of a compatible-looking custom class that does not implement Flowable's interface; expression typo resolving to a String; bean not yet created so expression resolves null.

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

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• Unable to resolve formFieldValidationExpression to boolean v
• DelegateExpression inbound channel model with key ${channelM

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/351f5df7b6e8ee95. Report an issue: GitHub.