flowable/flowable-engine · error · FlowableException

Cannot aggregate variable: ${varInstance}

Error message

Cannot aggregate variable: ${varInstance}

What it means

In the same aggregator, when the aggregation context is not OVERVIEW, each collected instance value must still be a JSON node; if not, Flowable throws "Cannot aggregate variable: " + varInstance. The overview and non-overview branches are complementary: whichever context state applies, the value must be JSON-representable as a node.

Source

Thrown at modules/flowable-cmmn-engine/src/main/java/org/flowable/cmmn/engine/impl/delegate/JsonPlanItemVariableAggregator.java:162

                        case JodaDateTimeType.TYPE_NAME:
                        case UUIDType.TYPE_NAME:
                            // For all these types it is OK to use toString as their string representation is what we want to have
                            objectNode.put(targetVarName, varInstance.getValue().toString());
                            break;
                        case ByteArrayType.TYPE_NAME:
                            objectNode.put(targetVarName, (byte[]) varInstance.getValue());
                            break;
                        default:
                            if (PlanItemVariableAggregatorContext.OVERVIEW.equals(context.getState())) {
                                // We can only use the aggregated variable if we are in an overview state
                                Object value = varInstance.getValue();
                                if (jsonMapper.isJsonNode(value)) {
                                    objectNode.set(targetVarName, JsonUtil.asFlowableJsonNode(value));
                                } else {
                                    throw new FlowableException("Cannot aggregate overview variable: " + varInstance);
                                }
                            } else {
                                throw new FlowableException("Cannot aggregate variable: " + varInstance);
                            }
                    }
                }

            }
        }

        return objectNode.getImplementationValue();
    }

    @Override
    public Object aggregateMultiVariables(DelegatePlanItemInstance planItemInstance, List<? extends VariableInstance> instances,
            PlanItemVariableAggregatorContext context) {
        VariableJsonMapper objectMapper = cmmnEngineConfiguration.getVariableJsonMapper();
        FlowableArrayNode arrayNode = objectMapper.createArrayNode();
        for (VariableInstance instance : instances) {
            arrayNode.add(JsonUtil.asFlowableJsonNode(instance.getValue()));
        }

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Store JSON node values for all variables included in repetition aggregation.
2. In the producing delegate/listener, wrap values with JsonUtil/JsonMapper before setVariable.
3. Exclude non-JSON variables from the aggregation configuration's variable list.
4. Migrate/normalize existing case variables to JSON for in-flight cases.

Example fix

// before
execution.setVariable("score", 42);
// after
ObjectNode node = JsonUtil.createObjectNode();
node.put("score", 42);
execution.setVariable("scoreData", node);

Object value = varInstance.getValue();
if (!(value instanceof com.fasterxml.jackson.databind.JsonNode)) {
    throw new IllegalArgumentException("Aggregated variable must hold a JSON node, got: " + value.getClass());
}

boolean canAggregate(VariableInstance v) {
    return v.getValue() instanceof com.fasterxml.jackson.databind.JsonNode;
}

try {
    Object result = aggregator.aggregateSingleVariable(planItemInstance, context);
} catch (org.flowable.common.engine.api.FlowableException e) {
    if (e.getMessage().startsWith("Cannot aggregate variable")) { log.error("Non-JSON value collected: {}", e.getMessage()); }
    throw e;
}

When it happens

Trigger: Repetition aggregation in a non-overview state (e.g. aggregating completed plan item variables into a result list) encounters an instance variable whose value is not a JSON node (plain String, Integer, POJO, byte[]).

Common situations: Delegates or scripts storing primitive/POJO values into variables that the aggregation configuration then collects; mixed variable types across repetitions; upgrading and changing what a delegate stores without migrating existing case variables.

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

• Cannot aggregate overview variable: ${varInstance}
• ${className} does not implement the ${CmmnTriggerableActivit
• ${className} does not implement the ${CmmnActivityBehavior.c
• ${delegateInstance.getClass().getName()} doesn't implement $
• ${delegateInstance.getClass().getName()} doesn't implement $

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/05e6541ad425bca4. Report an issue: GitHub.