I'm using Java 8 and would like to know if the computeIfPresent operation of the ConcurrentHashMap does lock the whole table/map or just the bin containing the key.
From the documentation of the computeIfPresent method:
Some attempted update operations on this map by other threads may be blocked while computation is in progress, so the computation should be short and simple, and must not attempt to update any other mappings of this map
This looks like the whole map is locked when invoking this method for a key. Why does the whole map have to be locked if a value of a certain key is updated? Wouldn't it be better to just lock the bin containing the key/value pair?
Judging by implementation (Oracle JDK 1.8.0_101), just the corresponding bin is locked. This does not contradict the documentation snippet you've cited, since it mentions that some update operations may be blocked, not necessarily all. Of course, it'd be clearer if the docs stated explicitly what gets locked, but that'd be leaking implementation details to what is de facto a part of the interface.