Hibernate Concurrent ManyToMany relationship - spring

Question

I am having troubles when running last test against a Kotin Spring Boot application. One critical object has a ManyToMany relationship to another one:
#Entity
data class Subscriber(
#ManyToMany
#JoinTable(name = "subscriber_message",
joinColumns = [JoinColumn(name = "subscriber_id")],
inverseJoinColumns = [JoinColumn(name = "message_id")],
indexes =[
Index(name = "idx_subscriber_message_message_id", columnList = "message_id"),
Index(name = "idx_subscriber_message_subscriber_id", columnList = "subscriber_id")
]
)
val messages: MutableList<Message>
}
#Entity
data class Message(
)
Messages are added to a subscriber like this:
subscriber.messages.add(message)
save(subscriber)
When a new message arrives, the controller asks the repository to add the message to all existing subscribers by calling the following method:
#Synchronized
fun SubscriberRepository.addMessage(message: Message) {
findAll().forEach {
it.messages.add(message)
save(it)
}
}
We currently use MutableList for this property since we need to add new elements to the list. This type is not thread safe, so I tried the good old Java concurrent set java.util.concurrent.ConcurrentSkipListSet, to which Spring complained saying that this type is not supported.
Is there a better way to stay thread safe in a web application regarding such a relationship. The list of messages is consumed by a subscriber who then clears it from messages it is done with. But because of concurrency, the cleaning process doesn't work either!

Answer 1

I don't think JPA is going to allow it to be you to initialise it with any concurrent versions because hibernate has own implementations like PersistentSet etc for each collection type which it supports.
When adding a message, you already have synchronised method so that should be fine.
Now I guess many threads trying to consume the messages from the retrieved subscriber. So why don't you do modify the subscriber like this before giving it to the threads consume it.
retrieve subscriber with messages
subscriber.setMessages(Collections.synchronizedList(subscriber.getMessages())) and give it to threads (I don't know what is the equivalent in kotlin)
So now subscriber.messages.removeAll(confirmedMessages) will be thread safe