flowable/flowable-engine · error · FlowableIllegalArgumentException

Request did not contain a list of variables to create.

Error message

Request did not contain a list of variables to create.

What it means

After successfully deserializing the request, createExecutionVariable checks that the resulting list is non-empty and throws FlowableIllegalArgumentException if inputVariables is null or has zero entries. An empty request body or an empty JSON array carries no variables to create, so the call is rejected.

Solutions

  1. Include at least one variable object in the JSON array
  2. Guard client-side: skip the API call when the variables list is empty
  3. Check the request body actually got populated before sending (log it in debug)

Example fix

// before
POST /runtime/process-instances/5001/variables  body: []
// after
POST /runtime/process-instances/5001/variables  body: [{"name":"x","type":"integer","value":1}]
Defensive patterns

Strategy: validation

Validate before calling

if (!Array.isArray(variables) || variables.length === 0) throw new Error('refusing to POST empty variable list to Flowable');

Type guard

function hasVariables(vars) { return Array.isArray(vars) && vars.length > 0; }

Try / catch

try { await api.post(url, vars); } catch (e) { if (e.message.includes('did not contain a list of variables')) { console.warn('skipped empty variable batch'); } }

Prevention

When it happens

Trigger: POSTing "[]" or an empty body to a variables collection endpoint (e.g. POST /runtime/process-instances/{id}/variables).

Common situations: Client code building the variable list from an empty map/array dynamically; a loop or filter that removed all entries before the request; template code with a placeholder that never got filled.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/a203435e51062f19. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-rest/src/main/java/org/flowable/rest/service/api/runtime/process/BaseVariableCollectionResource.java:105

        } else {

            List<RestVariable> inputVariables = new ArrayList<>();
            List<RestVariable> resultVariables = new ArrayList<>();
            result = resultVariables;

            try {
                @SuppressWarnings("unchecked")
                List<Object> variableObjects = (List<Object>) objectMapper.readValue(request.getInputStream(), List.class);
                for (Object restObject : variableObjects) {
                    RestVariable restVariable = objectMapper.convertValue(restObject, RestVariable.class);
                    inputVariables.add(restVariable);
                }
            } catch (Exception e) {
                throw new FlowableIllegalArgumentException("Failed to serialize to a RestVariable instance", e);
            }

            if (inputVariables == null || inputVariables.size() == 0) {
                throw new FlowableIllegalArgumentException("Request did not contain a list of variables to create.");
            }

            RestVariableScope sharedScope = null;
            RestVariableScope varScope = null;
            Map<String, Object> variablesToSet = new HashMap<>();

            for (RestVariable var : inputVariables) {
                // Validate if scopes match
                varScope = var.getVariableScope();
                if (var.getName() == null) {
                    throw new FlowableIllegalArgumentException("Variable name is required");
                }

                if (varScope == null) {
                    varScope = RestVariableScope.LOCAL;
                }
                if (sharedScope == null) {
                    sharedScope = varScope;

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