flowable/flowable-engine · error · FlowableException

Error writing form model response

Error message

Error writing form model response

What it means

getFormModelString serializes a FormModelResponse to JSON with Jackson's ObjectMapper. Any exception during serialization (unwritable property, circular reference, custom serializer failure) is wrapped in a FlowableException with this message.

Source

Thrown at modules/flowable-cmmn-rest/src/main/java/org/flowable/cmmn/rest/service/api/CmmnRestResponseFactory.java:257

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

        // Links to other resources
        response.setDeploymentId(caseDefinition.getDeploymentId());
        response.setDeploymentUrl(urlBuilder.buildUrl(CmmnRestUrls.URL_DEPLOYMENT, caseDefinition.getDeploymentId()));
        response.setResource(urlBuilder.buildUrl(CmmnRestUrls.URL_DEPLOYMENT_RESOURCE, caseDefinition.getDeploymentId(), caseDefinition.getResourceName()));
        if (caseDefinition.getDiagramResourceName() != null) {
            response.setDiagramResource(urlBuilder.buildUrl(CmmnRestUrls.URL_DEPLOYMENT_RESOURCE, caseDefinition.getDeploymentId(), caseDefinition.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) {
        RestUrlBuilder urlBuilder = createUrlBuilder();
        List<RestVariable> result = new ArrayList<>(variables.size());

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

        return result;
    }

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

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Inspect the wrapped cause to identify which property failed serialization
2. Add getters or @JsonIgnore to offending fields in the form model
3. Enable/disable appropriate Jackson features on the ObjectMapper (e.g. SerializationFeature.FAIL_ON_EMPTY_BEANS)
4. Ensure only standard FormModelResponse implementations are passed to this factory

Example fix

// before
class MyFormModel { private Object selfRef; void init(){ selfRef = this; } }
// after
class MyFormModel { @JsonIgnore private Object selfRef; }

new ObjectMapper().writeValueAsString(formModelResponse); // dry-run serialization check

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

When it happens

Trigger: Calling getFormModelString on a FormModelResponse whose properties cannot be serialized by the configured ObjectMapper (e.g. object with no accessible getters, self-referencing structure).

Common situations: Custom form model implementations with non-Jackson-friendly fields;Jackson configuration (FAIL_ON_EMPTY_BEANS) triggered by null/empty beans; version changes in form model classes.

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

• Failed to serialize to a RestVariable instance
• Cannot get variable value for jackson 2
• Failed to parse jase
• An error occurs converting output value as JSon
• Error writing form model response

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/38de3c7df6930810. Report an issue: GitHub.