Does ConcurrentHashMap use Read Write lock?

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).

Is it possible to read & write the same bucket parallelly in ConcurrentHashMap?

Simultaneous Read and Write operations by Multiple Threads on same or different segments of ConcurrentHashMap. But Two threads can’t write data on same segments at the same time. One has to wait for other to complete the operation.

How is ConcurrentHashMap thread-safe?

ConcurrentHashMap class is thread-safe i.e. multiple threads can operate on a single object without any complications. 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.

Is ConcurrentHashMap blocked?

The simple answer is no. Unless there is something else you are not telling us, the get call won’t block for longer than a microsecond or so. The source code for the get method and its helper method are below. As you can see, most of the work is done without any locks whatsoever.

Which lock is used in ConcurrentHashMap?

Reentrant Lock mechanism
ConcurrentHashMap use Reentrant Lock mechanism. ConcurrentHashMap uses segments instead of buckets and when new record get insert lock will get acquire only on segment not the complete list of segments. So here idea makes its clear that multi level lock will get acquire on the same.

Why ConcurrentHashMap is fail safe?

concurrent package such as ConcurrentHashMap, CopyOnWriteArrayList, etc. are Fail-Safe in nature. In the code snippet above, we’re using Fail-Safe Iterator. Hence, even though a new element is added to the Collection during the iteration, it doesn’t throw an exception.

Which is faster HashMap or ConcurrentHashMap?

If you choose a single thread access use HashMap , it is simply faster. For add method it is even as much as 3x more efficient. Only get is faster on ConcurrentHashMap , but not much. When operating on ConcurrentHashMap with many threads it is similarly effective to operating on separate HashMaps for each thread.

Why is there no null key in ConcurrentHashMap?

The main reason that null is not allowed in ConcurrentMaps such as ConcurrentHashMaps, ConcurrentSkipListMaps is to avoid ambiguities. If map. get(key) returns null, you cannot detect whether the key explicitly maps to null or the key itself is not mapped.

How is ConcurrentHashMap different from other concurrent threads?

ConcurrentHashMap works bit different as it acquires lock per Segment which means instead of single map wide lock, it has multiple Segment level locks. So 2 Threads operating on different segments can acquire lock on those segments without interfering each other and can proceed simultaneously as they are working on separate Segment locks.

Why is HashMap not thread safe in Java?

But Hashmap is not thread safe, so what will happen if one thread tries to put data and requires Rehashing and at same time other thread tries to read data from Hashmap, It will go in infinite loop. I think because of above reasons Java introduced ConcurrentHashMap in version 5.0.

What’s the difference between HashMap and hastable write?

HashMap is not synchronized and so doesn’t provide any thread safety also. In contrast Hashtable is synchronized and provides thread safety but on the stake of performance. Hastable write operation uses map wide lock which means it locks the complete map object.

How big is the segment array in ConcurrentHashMap?

By default ConcurrentHashMap has segment array size as 16 so simultaneously 16 Threads can put data in map considering each thread is working on separate Segment array index. ConcurrentHashMap has 3 arguments constructor which helps in tuning segment array size by defining concurrencyLevel.