How to Close() all the instances of the Realm before deleting its file

Hard to answer without some more details, as open instances can come from a lot of places. Perhaps the two links below can provide some guidance in how you work with the Realm instances:

See https://realm.io/docs/java/latest/#closing-realms and https://realm.io/docs/java/latest/#realm-instance-lifecycle

OK, so this won't really solve the problem (but it will at least try to close Realms on main thread), but will allow you to debug why and where you opened a Realm that wasn't closed.

Just get an instance of TraceableRealm.getInstance(realmConfig, "I needed it for that tiny query") and close TraceableRealm instance instead of just Realm.

Inside realmIndex you will get a list of still open Realms together with threads and reasons for their existence...

class TraceableRealm(
        val realm: Realm,
        val thread: String,
        val purpose: String
): Closeable
{
    companion object {
        val realmIndex = mutableListOf<TraceableRealm>()

        fun getInstance(config: RealmConfiguration, purpose: String): TraceableRealm {
            val newRealm = TraceableRealm(Realm.getInstance(config), Thread.currentThread().name, purpose)
            realmIndex.add(newRealm)
            realmIndex.removeAll { !it.isOpen }
            return newRealm
        }

        fun killZombies() {
            runUI {
                realmIndex.filter { it.isOpen && it.thread == Thread.currentThread().name }.forEach {
                    try {
                        it.close()
                    } catch (ex: Exception) {
                        Timber.d("Problem removing zombie realm: $it")
                    }
                }
                realmIndex.removeAll { !it.isOpen }
                realmIndex.forEach {
                    Timber.d("Couldn't close realm: $it")
                }
            }
        }
    }

    var isOpen = true

    override fun toString(): String {
        return "$purpose in $thread open=$isOpen"
    }

    override fun close() {
        realm.close()
        isOpen = false
    }
}