What is the difference between a ConcurrentHashMap and a Hashtable in Java?
Which is more efficient for threaded applications?
Summary of ConcurrentHashMap features – It does not allow duplicate keys. – It does not allow null to be used as a key or value.
it can have duplicate values but not keys.
Concurrent hash map applies locks only at bucket level called fragment while adding or updating the map. So, a concurrent hash map allows concurrent read and write operations to the map. HashTable is a thread-safe legacy class introduced in the Jdk1.
You can't add duplicate keys to hashtables, because hashtables by design can only contain unique keys. If you need to store duplicate key/value pairs, use arrays.
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 i.e. 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 (i.e. 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, i.e. divides the data into shards(segments) then 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.
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