Concurrent read-only HashMap
concurrenthashmap, hashmap, java
Solution
Since the map isn't updated while being used, use a `HashMap`, which gives excellent O(1) lookup performance (at the sacrifice of thread safety).
When it's time to refresh, build a new map and swap references.
Consider using an `AtomicReference` to make the swap thread safe:
private final AtomicReference<Map<K, V>> mapRef = new AtomicReference<>();
To use:
mapRef.get().get(key);
To initialize or swap in a new map:
Map<K, V> newMap = new HashMap<>();
// populate map
mapRef.set(newMap); // all threads will see this change
Problem
I am writing a web-service that heavily relies on a single large Map that is completely updated once every hour. The rest of the time many threads concurrently read the table. My question is: What is the most efficient construct to realize such a Map? The Map can be rather larger (100 - 500 MB). There is only read access except once an hour where the whole map is replaced. I was thinking about just using a Java HashMap and maybe using reflection to set the field final between the updates if this improves performance, but I have no clue how to make the JVM optimize for many concurrent reads.