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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?
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Concurrent. ConcurrentHashMap class achieves thread-safety by dividing the map into segments, the lock is required not for the entire object but for one segment, i.e one thread requires a lock of one segment. In ConcurrentHashap the read operation doesn't require any lock.
ConcurrentHashMap does not throw ConcurrentModificationException if the underlying collection is modified during an iteration is in progress. Iterators may not reflect the exact state of the collection if it is being modified concurrently. It may reflect the state when it was created and at some moment later.
So unlike hashtable, we perform any sort of operation ( update ,delete ,read ,create) without locking on entire map in ConcurrentHashMap. Retrieval operations (including get) generally do not block. It uses the concept of volatile in this case., so may overlap with update operations (including put and remove).
In ConcurrentHashMap, at a time any number of threads can perform retrieval operation but for updated in the object, the thread must lock the particular segment in which the thread wants to operate. This type of locking mechanism is known as Segment locking or bucket locking.
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.
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