flowable/flowable-engine · error · FlowableIllegalArgumentException

Task assignee list is empty

Error message

Task assignee list is empty

What it means

taskAssigneeIds(Collection<String>) rejects an empty collection with this message. Flowable treats an empty assignee list as a query-usage mistake rather than an innocuous no-op filter, since it almost always indicates uninitialized input and would silently match nothing. Callers must pass at least one assignee ID.

Solutions

1. Only call taskAssigneeIds when the collection is non-empty; omit the filter otherwise.
2. If all tasks should match, construct the query without an assignee constraint.
3. Populate the list with the intended assignee IDs before querying.

Example fix

// before
List<String> ids = getSelectedAssignees();
taskQuery.taskAssigneeIds(ids);

// after
List<String> ids = getSelectedAssignees();
if (ids != null && !ids.isEmpty()) {
    taskQuery.taskAssigneeIds(ids);
}

if (assigneeIds == null || assigneeIds.isEmpty()) { return null; /* skip filter */ }
taskQuery.taskAssigneeIds(assigneeIds);

boolean usable = assigneeIds != null && !assigneeIds.isEmpty();

try {
    taskQuery.taskAssigneeIds(assigneeIds);
} catch (FlowableIllegalArgumentException e) {
    if (!"Task assignee list is empty".equals(e.getMessage())) throw e;
    // fall back to query without assignee filter
}

When it happens

Trigger: Calling taskQuery.taskAssigneeIds(new ArrayList<>()) or taskAssigneeIds(Collections.emptyList()) — e.g. when a filter UI produced no selections or a group lookup returned no members.

Common situations: Building a query from user-selected filters where nothing was selected; a lookup of team members returning an empty set; passing a default-initialized empty list instead of omitting the filter.

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

• Involved groups are empty
• Process category list is empty
• Process instance id list is empty
• Task category list is empty
• at least one of userId or groups must be provided

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/25f4cfcbba9bd3da. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-task-service/src/main/java/org/flowable/task/service/impl/TaskQueryImpl.java:472

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

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

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

        if (orActive) {

View on GitHub (pinned to d6d39ce1c6)