flowable/flowable-engine · error · ActivitiIllegalArgumentException

Task assignee list is empty

Error message

Task assignee list is empty

What it means

Thrown by TaskQueryImpl.taskAssigneeIds(List<String>) when the assigneeIds list is non-null but empty. An empty IN () clause is invalid SQL and meaningless as a filter, so the library rejects it explicitly rather than producing a broken query or silently returning all tasks.

Solutions

  1. Check isEmpty() before calling and skip the filter (or return an empty result directly) when the list is empty.
  2. Populate the list with at least one valid assignee id.
  3. If an empty selection must return nothing, short-circuit: return an empty task list without executing the query.
  4. Validate the ids collection in the service layer before building the query.

Example fix

// before
query.taskAssigneeIds(selectedIds); // selectedIds may be empty

// after
if (selectedIds == null || selectedIds.isEmpty()) {
    return Collections.emptyList(); // or omit the filter
}
query.taskAssigneeIds(selectedIds);
Defensive patterns

Strategy: validation

Validate before calling

if (assigneeIds == null || assigneeIds.isEmpty()) {
    return Collections.emptyList(); // nothing selected: no results (or omit filter)
}
query.taskAssigneeIds(assigneeIds);

Type guard

boolean isQueryableIdList(List<String> l) { return l != null && !l.isEmpty(); }

Try / catch

try {
    query.taskAssigneeIds(assigneeIds);
} catch (ActivitiIllegalArgumentException e) {
    throw new BadRequestException("Assignee ids must be non-empty");
}

Prevention

When it happens

Trigger: Calling taskQuery.taskAssigneeIds(new ArrayList<>()) or passing a collection that ended up empty after filtering (e.g. no users selected, empty group).

Common situations: UI bulk-selection with nothing selected; filtering a role's members leaves zero ids; a stream filter reduced the candidate list to empty before query construction.

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/ed714a5ea99d37a8. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable5-engine/src/main/java/org/activiti/engine/impl/TaskQueryImpl.java:381

    public TaskQuery taskAssigneeLikeIgnoreCase(String assigneeLikeIgnoreCase) {
        if (assigneeLikeIgnoreCase == null) {
            throw new ActivitiIllegalArgumentException("assigneeLikeIgnoreCase is null");
        }
        if (orActive) {
            currentOrQueryObject.assigneeLikeIgnoreCase = assigneeLikeIgnoreCase.toLowerCase();
        } else {
            this.assigneeLikeIgnoreCase = assigneeLikeIgnoreCase.toLowerCase();
        }
        return this;
    }

    @Override
    public TaskQuery taskAssigneeIds(List<String> assigneeIds) {
        if (assigneeIds == null) {
            throw new ActivitiIllegalArgumentException("Task assignee list is null");
        }
        if (assigneeIds.isEmpty()) {
            throw new ActivitiIllegalArgumentException("Task assignee list is empty");
        }
        for (String assignee : assigneeIds) {
            if (assignee == null) {
                throw new ActivitiIllegalArgumentException("None of the given task assignees can be null");
            }
        }

        if (assignee != null) {
            throw new ActivitiIllegalArgumentException("Invalid query usage: cannot set both taskAssigneeIds and taskAssignee");
        }
        if (assigneeLike != null) {
            throw new ActivitiIllegalArgumentException("Invalid query usage: cannot set both taskAssigneeIds and taskAssigneeLike");
        }
        if (assigneeLikeIgnoreCase != null) {
            throw new ActivitiIllegalArgumentException("Invalid query usage: cannot set both taskAssigneeIds and taskAssigneeLikeIgnoreCase");
        }

        if (orActive) {

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