当concurrencyLevel大于ConcurrentHashMap的容量时，会发生什么？

Question

I have been scratching my head around a question for some time now, I have looked around but have failed at finding the answer. I want to know what will happen if the concurrencyLevel will be greater than the capacity of the Map.

By default both are 16 which means that each bucket will have a lock. And if the capacity will be 32 and concurrencyLevel 16 that a lock will be held on 2 buckets. But what happens when concurrencyLevel is 32 and capacity is 16?

Will each bucket be held by 2 locks, then what happens if the distribution of concurrencyLevel and capacity is uneven, like 24 and 16, or something?

Answer 1

Nothing special happens. Did you check the documentation of ConcurrentHashMap ? It says: “Also, for compatibility with previous versions of this class, constructors may optionally specify an expected concurrencyLevel as an additional hint for internal sizing.” It's a hint, not a limitation. Furthermore on concurrencyLevel : “the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.” Note: “may”. So if anything happens, the size becomes different. Maybe.

If you want further implementation details, you may study the source code. It's available. In Java 1.8 it contanis the following two lines:

    if (initialCapacity < concurrencyLevel)   // Use at least as many bins
        initialCapacity = concurrencyLevel;   // as estimated threads