junit-team/junit5 · error · IllegalStateException

Could not add entry to the queue for task

Error message

Could not add entry to the queue for task: ${entry.task}

What it means

WorkQueue.doAdd throws IllegalStateException when ConcurrentSkipListSet.add(entry) returns false. A set refuses a duplicate (per its comparator), so this means an Entry equal to an existing one was already present. The comparator orders by level/container-flag/index/uniqueId, so two entries with identical (level, isContainer, index, uniqueId) are considered duplicates.

Solutions

1. Check for duplicate UniqueIds in the test descriptor tree (two tests resolving to the same id) and disambiguate them (e.g. @ParameterizedTest indices, custom display names).
2. If you author custom scheduling code, call remove(entry) before reAdd(entry), or never re-add an entry that is still queued.
3. If this occurs with stock engines and no custom code, report it as a JUnit platform bug with the entry.task details.

Example fix

// before - re-add without removing
queue.reAdd(entry); // throws if entry still present
// after - remove first, then re-add
queue.remove(entry);
queue.reAdd(entry);

// Before re-adding, ensure it is absent; this is internal-API level guidance
if (!queue.contains(entry)) queue.reAdd(entry);
// And ensure UniqueIds in the descriptor tree are distinct.

try {
    queue.reAdd(entry);
} catch (IllegalStateException e) {
    // duplicate enqueue - log entry.task and verify no duplicate UniqueId
}

When it happens

Trigger: Calling WorkQueue.add / addAll / reAdd with an Entry whose (level, isContainer, index, uniqueId) tuple equals an Entry already in the queue. This is an internal invariant violation inside WorkerThreadPoolHierarchicalTestExecutorService scheduling, typically from re-enqueuing a task that is still queued, or two forks producing the same uniqueId/index.

Common situations: Internal scheduler bug or race where the same TestTask entry is submitted twice without being removed first; duplicate UniqueIds in the test graph (two tests share a UniqueId) causing the comparator to treat distinct tasks as equal; custom HierarchicalTestExecutorService misuse that double-schedules entries.

Related errors

• Cannot create Launcher for multiple engines with the same ID

AI-assisted analysis of junit-team/junit5@f070c699a0 (2026-08-11). Data as JSON: /api/errors/e3699c8d5d66bfcc. Report an issue: GitHub.

Appendix: source

Thrown at junit-platform-engine/src/main/java/org/junit/platform/engine/support/hierarchical/WorkerThreadPoolHierarchicalTestExecutorService.java:693

		Entry add(TestTask task, int index) {
			Entry entry = new Entry(task, index);
			logger.trace(() -> "forking: " + entry.task);
			return doAdd(entry);
		}

		void addAll(Collection<Entry> entries) {
			entries.forEach(this::doAdd);
		}

		void reAdd(Entry entry) {
			logger.trace(() -> "re-enqueuing: " + entry.task);
			doAdd(entry);
		}

		private Entry doAdd(Entry entry) {
			var added = queue.add(entry);
			if (!added) {
				throw new IllegalStateException("Could not add entry to the queue for task: " + entry.task);
			}
			return entry;
		}

		boolean remove(Entry entry) {
			return queue.remove(entry);
		}

		boolean isEmpty() {
			return queue.isEmpty();
		}

		@Override
		public Iterator<Entry> iterator() {
			return queue.iterator();
		}

		private static final class Entry {

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