flowable/flowable-engine · error · FlowableIllegalArgumentException

Involved groups are empty

Error message

Involved groups are empty

What it means

TaskQueryImpl.taskInvolvedGroups() throws this when the passed collection is non-null but empty. An empty collection would yield an invalid IN () clause or match nothing, so Flowable rejects it explicitly. Use null/omission for 'no filter' instead of an empty list.

Solutions

1. Only call taskInvolvedGroups() when the collection has at least one element.
2. Handle the empty case by skipping the filter or returning an intentionally empty result.
3. Fix the logic that produces an empty group list if groups are mandatory.
4. If 'no groups' is valid, model it by not setting the filter rather than passing an empty list.

Example fix

// before
TaskQuery query = taskService.createTaskQuery().taskInvolvedGroups(groups);
// after
TaskQuery query = taskService.createTaskQuery();
if (groups != null && !groups.isEmpty()) {
    query = query.taskInvolvedGroups(groups);
}

if (involvedGroups == null || involvedGroups.isEmpty()) {
    throw new IllegalArgumentException("involvedGroups must contain at least one group");
}
query.taskInvolvedGroups(involvedGroups);

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

try {
    query.taskInvolvedGroups(groups);
} catch (FlowableIllegalArgumentException e) {
    logger.warn("Empty/null involved groups: {}", e.getMessage());
    return Collections.emptyList(); // or skip the filter
}

When it happens

Trigger: Calling taskQuery.taskInvolvedGroups(new ArrayList<>()) or any collection where isEmpty() is true, often the result of filtering a group list to zero entries.

Common situations: Group list computed at runtime (e.g. user's groups) ends up empty because the user belongs to no groups; upstream filter removed all items; default-initialized empty list passed unconditionally.

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

• Process category list is empty
• Process instance id list is empty
• Task assignee 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/d2d406bfc7020e17. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-task-service/src/main/java/org/flowable/task/service/impl/TaskQueryImpl.java:609

    public TaskQueryImpl taskInvolvedUser(String involvedUser) {
        if (involvedUser == null) {
            throw new FlowableIllegalArgumentException("Involved user is null");
        }
        if (orActive) {
            currentOrQueryObject.involvedUser = involvedUser;
        } else {
            this.involvedUser = involvedUser;
        }
        return this;
    }

    @Override
    public TaskQueryImpl taskInvolvedGroups(Collection<String> involvedGroups) {
        if (involvedGroups == null) {
            throw new FlowableIllegalArgumentException("Involved groups are null");
        }
        if (involvedGroups.isEmpty()) {
            throw new FlowableIllegalArgumentException("Involved groups are empty");
        }
        if (orActive) {
            currentOrQueryObject.involvedGroups = involvedGroups;
        } else {
            this.involvedGroups = involvedGroups;
        }
        return this;
    }

    @Override
    public TaskQueryImpl taskCandidateGroup(String candidateGroup) {
        if (candidateGroup == null) {
            throw new FlowableIllegalArgumentException("Candidate group is null");
        }

        if (candidateGroups != null) {
            throw new FlowableIllegalArgumentException("Invalid query usage: cannot set both candidateGroup and candidateGroupIn");
        }

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