flowable/flowable-engine · error · FlowableIllegalArgumentException

Set of taskIds is empty

Error message

Set of taskIds is empty

What it means

After rejecting a null collection, GetTasksLocalVariablesCmd also rejects an empty one: taskIds.isEmpty() triggers FlowableIllegalArgumentException('Set of taskIds is empty'). The engine refuses to issue a query with an empty IN clause, which would be invalid or would return nothing useful.

Source

Thrown at modules/flowable-cmmn-engine/src/main/java/org/flowable/cmmn/engine/impl/cmd/GetTasksLocalVariablesCmd.java:48

 * @author Tijs Rademakers
 */
public class GetTasksLocalVariablesCmd implements Command<List<VariableInstance>>, Serializable {

    private static final long serialVersionUID = 1L;
    
    protected Set<String> taskIds;

    public GetTasksLocalVariablesCmd(Set<String> taskIds) {
        this.taskIds = taskIds;
    }

    @Override
    public List<VariableInstance> execute(CommandContext commandContext) {
        if (taskIds == null) {
            throw new FlowableIllegalArgumentException("taskIds is null");
        }
        if (taskIds.isEmpty()) {
            throw new FlowableIllegalArgumentException("Set of taskIds is empty");
        }

        CmmnEngineConfiguration cmmnEngineConfiguration = CommandContextUtil.getCmmnEngineConfiguration(commandContext);
        List<VariableInstanceEntity> entities = cmmnEngineConfiguration.getVariableServiceConfiguration()
                .getVariableService().createInternalVariableInstanceQuery().taskIds(taskIds).list();
        List<VariableInstance> instances = new ArrayList<>(entities.size());
        for (VariableInstanceEntity entity : entities) {
            entity.getValue();
            instances.add(entity);
        }

        return instances;
    }

}

View on GitHub (pinned to d6d39ce1c6)

Solutions

1. Guard with !taskIds.isEmpty() before calling the bulk API and return an empty result instead.
2. Check why the upstream task query produced no ids if an empty set is unexpected.
3. Skip the variable fetch entirely when the id set is empty - there is nothing to fetch.

Example fix

// before
List<VariableInstance> vars = cmmnTaskService.getVariableInstancesLocalByTaskIds(taskIds);
// after
List<VariableInstance> vars = (taskIds == null || taskIds.isEmpty())
        ? Collections.emptyList()
        : cmmnTaskService.getVariableInstancesLocalByTaskIds(taskIds);

if (taskIds == null || taskIds.isEmpty()) { return Collections.emptyList(); }

boolean isFetchable(Collection<String> taskIds) { return taskIds != null && !taskIds.isEmpty(); }

try { return taskService.getVariableInstancesLocalByTaskIds(taskIds); } catch (FlowableIllegalArgumentException e) { return Collections.emptyList(); }

When it happens

Trigger: Calling the bulk local-variables API with an empty Set/Collection of task ids, e.g. after filtering out all candidate tasks.

Common situations: A preceding task query matched nothing, so the id set is empty; filters removed every id before the bulk fetch; loop where the first batch legitimately has no tasks.

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

• historic case instanceIds are empty
• taskIds is null
• variables is empty
• rootScopeIds is null or empty
• groupIds are empty

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/e2ec1f0d8ef98d8c. Report an issue: GitHub.