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I have a Map which is to be modified by several threads concurrently.
There seem to be three different synchronized Map implementations in the Java API:
HashtableCollections.synchronizedMap(Map)ConcurrentHashMapFrom what I understand, Hashtable is an old implementation (extending the obsolete Dictionary class), which has been adapted later to fit the Map interface. While it is synchronized, it seems to have serious scalability issues and is discouraged for new projects.
But what about the other two? What are the differences between Maps returned by Collections.synchronizedMap(Map) and ConcurrentHashMaps? Which one fits which situation?
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HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously.
The synchronizedMap() method of java. util. Collections class is used to return a synchronized (thread-safe) map backed by the specified map. In order to guarantee serial access, it is critical that all access to the backing map is accomplished through the returned map.
ConcurrentHashMap allows performing concurrent read and write operation. Hence, performance is relatively better than the Synchronized Map. In Synchronized HashMap, multiple threads can not access the map concurrently. Hence, the performance is relatively less than the ConcurrentHashMap.
The main reason for this slowness is locking; synchronized collections lock the whole collection e.g. whole Map or List while concurrent collection never locks the whole Map or List. They achieve thread safety by using advanced and sophisticated techniques like lock stripping.
For your needs, use ConcurrentHashMap. It allows concurrent modification of the Map from several threads without the need to block them. Collections.synchronizedMap(map) creates a blocking Map which will degrade performance, albeit ensure consistency (if used properly).
Use the second option if you need to ensure data consistency, and each thread needs to have an up-to-date view of the map. Use the first if performance is critical, and each thread only inserts data to the map, with reads happening less frequently.
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