In which case would this method be invoked concurrently? The JavaDoc from EventListener states that concurrency is used if the method is annotated with @Async, which isn't the case here.

In which case would this method be invoked concurrently?

spring boot allow developer inject ApplicationAvailabilityBean in multiple custom beans and publish their custom AvailabilityState event. there is a chance that ApplicationAvailabilityBean#onApplicationEvent could be invoked concurrently.

It's not expected that users inject the bean and call onApplicationEvent directly. If you want to change the application availability status, you should emit an AvailabilityChangeEvent. The spring event system will then ensure that these listeners are called. And they are not called concurrently.

I think we should accept this. While onApplicationEvent is being called on one thread by the event multicaster and it mutates the map, another thread could call one of the get… methods that reads from the map.

Ah, yeah, you're right. I missed that case. Will merge the PR.

... they are not called concurrently.

@mhalbritter I trace the code from AvailabilityChangeEvent, and find there is nothing to ensure that listeners are not called concurrently —— I can't find any lock or things like BlockingQueue.

// 1. multiple thread invoke this static method concurrently and publish different state.
 AvailabilityChangeEvent.publish(this.eventPublisher, ex, LivenessState.BROKEN);

// 2.  call multicastEvent in ‘AbstractApplicationContext#publishEvent(Object, ResolvableType)‘
//      still don't have any lock, 
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType)

// 3. multiple thread could invoke 'SimpleApplicationEventMulticaster#multicastEvent' at same time and publish different event concurrently.
protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
                 ...
		doInvokeListener(listener, event);
}

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
	try {
		listener.onApplicationEvent(event);
	}
        ...
}