ConcurrentHashMap and Hashtable in Java
This question already has an answer here:
ConcurrentHashMap
uses multiple buckets to store data. This avoids read locks and greatly improves performance over a HashTable
. Both are thread safe, but there are obvious performance wins with ConcurrentHashMap
.
When you read from a ConcurrentHashMap
using get()
, there are no locks, contrary to the HashTable
for which all operations are simply synchronized. HashTable
was released in old versions of Java whereas ConcurrentHashMap
is a java 5+ thing.
HashMap
is the best thing to use in a single threaded application.
ConcurrentHashMap and Hashtable locking mechanism
Hashtable
is belongs to the Collection framework; ConcurrentHashMap
belongs to the Executor framework. Hashtable
uses single lock for whole data. ConcurrentHashMap
uses multiple locks on segment level (16 by default) instead of object level ie whole Map
. ConcurrentHashMap
locking is applied only for updates. In case of retrievals, it allows full concurrency, retrievals reflect the results of the most recently completed update operations. So reads can happen very fast while writes are done with a lock. ConcurrentHashMap
doesn't throw a ConcurrentModificationException
if one thread tries to modify it while another is iterating over it and does not allow null values. ConcurrentHashMap
returns Iterator
, which fails-safe (ie iterator will make a copy of the internal data structure) on concurrent modification. ConcurrentHashMap
uses a database shards logic ( Segment<K, V>[] segments
) is known as Concurrency-Level , ie divides the data into shards(segments) than puts locks on each shard (segment) instead of putting a single lock for whole data ( Map
). The default value is 16. To understand the ConcurrentHashMap more technically please look at this link
The following analogy helps you get understand the concept only(not logic)
Hashtable
and ConcurrentHashMap
are two types of Homes. Hashtable
locks home's main door. ConcurrentHashMap
locks specific room door instead of main door. Which is more efficient for threaded applications?
ConcurrentHashMap
is more efficient for threaded applications.