argghhh I have a CME in my app that I can't quite ...

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[angarron]

argghhh I have a CME in my app that I can't quite understand -- would greatly appreciate any help!

I have this code (simplified a bit):

@Synchronized
fun notify(updates: Set<UpdateSource>) {
  Log.d("findme", "notify START: $updates")
  observers.forEach { observer ->
    /* stuff */
  }
  Log.d("findme", "notify END")
}

@Synchronized
private fun disconnect(observer: Observer) {
  Log.d("findme", "disconnect START")
  observers.remove(observer)
  Log.d("findme", "disconnect END")
}

and I see these logs:

2020-03-19 09:25:16.545 21962-22072/com.discord.debug D/findme: notify START: [com.discord.stores.StoreUser@c7528b5, com.discord.stores.StoreUser$Companion$UsersUpdate$1@91882e5]
2020-03-19 09:25:16.548 21962-22072/com.discord.debug D/findme: disconnect START
2020-03-19 09:25:16.548 21962-22072/com.discord.debug D/findme: disconnect END

Can anyone explain how it's possible that "notify END" never prints? if that forEach was hanging, I would expect a hung thread, but we are able to enter the disconnect method -- which means that I feel like I'm misunderstanding something about @Synchronized

[araqnid]

is notify invoking something that is then invoking disconnect() on the same thread? monitors are re-entrant, i.e. one thread can keep re-entering synchronized methods and it just increases an internal counter and unlocks at the end

[araqnid]

that would explain CME (= ConcurrentModificationException, I assume)

[araqnid]

for something like an observer list, I’d usually use CopyOnWriteArrayList to avoid it

[angarron]

yup im pretty sure I have something calling disconnect, that makes sense. thanks!

[pedro]

I assume something in the for loop inside notify called disconnect.

You never saw the “END” log because the exception was thrown. The stacktrace would be something like 1. notify 2. disconnect so the notify method had not finished yet