A call to CopyOnWriteArrayListMultiMap.putIfAbsent(K, V) or remove(K, V) while another thread is iterating over the list returned via get(...), e.g. [1], will result in a ConcurrentModificationException. Additionally, it is unclear to me why any of the methods of this class are synchronized, since the underlying data structure is already thread safe (excluding the mapped list, which should probably use a CopyOnWriteArrayList or similar).

[1] https://github.com/wildfly/wildfly/blob/main/connector/src/main/java/org/jboss/as/connector/deployers/ra/processors/RaXmlDependencyProcessor.java#L43