The listener could be invoked concurrently by multiple thread, we should make them threadsafe.
https://github.com/spring-projects/spring-framework/blob/f40a391916ed6c1f9e1130638a0bf19479e514dd/spring-context/src/main/java/org/springframework/context/event/SimpleApplicationEventMulticaster.java#L40
By default, all listeners are invoked in the calling thread.
Comment From: jhoeller
While you got a point that it is generally a good idea to make listener implementations thread-safe (in terms of expecting concurrent invocations), two of the three classes affected here are test classes that are used in synchronous test setups. The third one is a ContextRefreshedEvent listener which is guaranteed to be invoked within a strong startup/shutdown monitor, so the listener does not benefit from its own internal synchronization either.
Thanks for the pull request, in any case!