flowable/flowable-engine · error · FlowableException

Error writing form model response

Error message

Error writing form model response

What it means

RestResponseFactory.getFormModelString serializes a FormModelResponse to JSON with the configured ObjectMapper. Any exception during serialization (unwritable property, failing custom serializer, invalid underlying object graph) is wrapped and rethrown as a generic FlowableException with this message.

Source

Thrown at modules/flowable-rest/src/main/java/org/flowable/rest/service/api/RestResponseFactory.java:289

        response.setStartFormDefined(processDefinition.hasStartFormKey());
        response.setGraphicalNotationDefined(processDefinition.hasGraphicalNotation());
        response.setTenantId(processDefinition.getTenantId());

        // Links to other resources
        response.setDeploymentId(processDefinition.getDeploymentId());
        response.setDeploymentUrl(urlBuilder.buildUrl(RestUrls.URL_DEPLOYMENT, processDefinition.getDeploymentId()));
        response.setResource(urlBuilder.buildUrl(RestUrls.URL_DEPLOYMENT_RESOURCE, processDefinition.getDeploymentId(), processDefinition.getResourceName()));
        if (processDefinition.getDiagramResourceName() != null) {
            response.setDiagramResource(urlBuilder.buildUrl(RestUrls.URL_DEPLOYMENT_RESOURCE, processDefinition.getDeploymentId(), processDefinition.getDiagramResourceName()));
        }
        return response;
    }
    
    public String getFormModelString(FormModelResponse formModelResponse) {
        try {
            return objectMapper.writeValueAsString(formModelResponse);
        } catch (Exception e) {
            throw new FlowableException("Error writing form model response", e);
        }
    }

    public List<RestVariable> createRestVariables(Map<String, Object> variables, String id, int variableType, RestVariableScope scope) {
        RestUrlBuilder urlBuilder = createUrlBuilder();
        List<RestVariable> result = new ArrayList<>(variables.size());

        for (Entry<String, Object> pair : variables.entrySet()) {
            result.add(createRestVariable(pair.getKey(), pair.getValue(), scope, id, variableType, false, urlBuilder));
        }

        return result;
    }

    public RestVariable createRestVariable(String name, Object value, RestVariableScope scope, String id, int variableType, boolean includeBinaryValue) {
        return createRestVariable(name, value, scope, id, variableType, includeBinaryValue, createUrlBuilder());
    }

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Inspect the wrapped cause exception (e.getCause()) — it names the exact serialization failure (property/class).
2. Fix the offending field on the FormModelResponse: mark it @JsonIgnore/transient or remove the cyclic reference.
3. Ensure the ObjectMapper used by RestResponseFactory is configured (e.g. FAIL_ON_EMPTY_BEANS disabled) for your custom types.
4. If a Flowable/Jackson upgrade introduced it, align Jackson versions between flowable-rest and your application.

Example fix

// before
class MyFormModel extends FormModelResponse { private ProcessDefinition pd; /* cyclic */ }
// after
class MyFormModel extends FormModelResponse {
  @JsonIgnore
  private ProcessDefinition pd; // or store pd.getId() instead
}

try { json = factory.getFormModelString(response); } catch (FlowableException e) { logger.error("Form model serialization failed", e.getCause()); }

When it happens

Trigger: Calling getFormModelString(formModelResponse) when objectMapper.writeValueAsString throws — e.g. a self-referencing form field model, a getter that throws, or a form data type the registered serializers cannot handle.

Common situations: Custom FormModelResponse subclasses with non-serializable fields, Jackson version mismatches after upgrading Flowable/Spring Boot, or form info containing cyclic references from custom form implementations.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

• Error writing app model to json
• Error writing form model response
• Failed to parse jase
• Could not read json event payload
• Cannot convert event payload ${jsonValue} to type 'json'

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/a559918143fbb1bf. Report an issue: GitHub.