41-concurrent-modification

Example 41 — Concurrent Modification

Demonstrates ConcurrentModificationDetector: an ArrayList of event listeners is iterated by fireEvent() while another thread calls register(), causing ConcurrentModificationException.

The Problem

EventBusService stores listeners in a plain ArrayList. fireEvent() iterates the list with a for-each loop. register() adds to the same list. When both operations run simultaneously, the ArrayList's modCount check fails and ConcurrentModificationException is thrown mid-dispatch, leaving some listeners uncalled.

How to Reproduce

1. Remove @Disabled from testEventBus_concurrent_detectsModification.
2. Run: mvn test or ./gradlew test
3. The test fails with a ConcurrentModificationDetector report showing that the listener list was modified while an iteration was in progress.

Fix: protect both register() and fireEvent() with synchronized(listeners) blocks, or replace ArrayList with CopyOnWriteArrayList (appropriate here because reads dominate writes).