flowable/flowable-engine · error · FlowableIllegalArgumentException

Set of taskIds is empty

Error message

Set of taskIds is empty

What it means

GetTasksLocalVariablesCmd throws FlowableIllegalArgumentException("Set of taskIds is empty") when the supplied task id collection is non-null but has no elements. An empty IN-clause query is meaningless and would either fail in SQL or return misleading results, so the engine rejects it eagerly.

Solutions

1. Check !taskIds.isEmpty() before calling the batch API and skip the call when empty.
2. Return an empty variable map/result directly for an empty id set instead of querying.
3. Fix the filtering logic that unexpectedly reduces the id set to zero if a non-empty set was expected.
4. Catch FlowableIllegalArgumentException as a client-side validation failure.

Example fix

// before
List<VariableInstance> vars = taskService.getTasksLocalVariables(taskIds);
// after
Map<String, List<VariableInstance>> varsByTask = taskIds.isEmpty()
    ? Collections.emptyMap()
    : groupVariables(taskService.getTasksLocalVariables(taskIds));

if (taskIds == null || taskIds.isEmpty()) {
    return Collections.emptyMap(); // skip the query entirely
}

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

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

When it happens

Trigger: taskService.getTasksLocalVariables(Collections.emptySet()) or passing an id collection that was filtered down to zero items (e.g. all tasks already completed) before the batch variable fetch.

Common situations: Bulk UI pages loading variables for a task list that happens to be empty; filtering by criteria that matched nothing; passing a freshly created but unpopulated collection.

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

• at least one of userId or groups must be provided
• callbackIds is null or empty
• Could not find a deployment with id
• Could not find an app definition with id
• Empty appsDefinitionIds

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/61cdf08891881d5e. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine/src/main/java/org/flowable/engine/impl/cmd/GetTasksLocalVariablesCmd.java:47

/**
 * @author Daisuke Yoshimoto
 */
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");
        }

        ProcessEngineConfigurationImpl processEngineConfiguration = CommandContextUtil.getProcessEngineConfiguration(commandContext);
        List<VariableInstanceEntity> entities = processEngineConfiguration.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)