有不可变（通常为ConcurrentHashMap）数据结构的正式模式，可以定期替换吗？

Question

I find myself repeating the code of

1. store write only copy of datastructure and read only immutable ConcurrentHashMap
2. readers are reading only from read only immutable HashMap no locks .
3. Writer are writing to write only data structure (another ConcurrentHashMap) with locks.
4. Every few minutes (depending on app needs) I switch the Read only datastructure which is a little old with the write only datastucture. For this reason I use no locks here and I use a volatile only mark on the read only datastructure.

Is there an already published well know pattern with example code in java / official library which does that so that i don't have to create this pattern by myself?

Answer 1

Perhaps I don't fully understand your requirement, but why don't you read and write to the one instance of ConcurrentHashMap ? The whole point of a ConcurrentMap is that reading and writing to the maps between threads occurs in a thread-safe manner. Can you please explain why this can't occur in your case?

Read only immutable ConcurrentHashMap <-- this definitely doesn't make sense. Any Map instance that's safely published (eg a plain old HashMap ), can be safely read by multiple threads. The only danger here is guaranteeing that the Map is only read from and not written to. That guarantee can be made by using either Collections.unmodifiableMap() or Guava's ImmutableMap . It's also worth mentioning that it's best to ensure that the map entries themselves are immutable.