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:
From what I understand, Hashtable is an old implementation (extending the obsolete Dictionaryclass), 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 byCollections.synchronizedMap(Map) and ConcurrentHashMaps? Which one fits which situation?
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.
The "scalability issues" for Hashtable are present in exactly the same way inCollections.synchronizedMap(Map) - they use very simple synchronization, which means that only one thread can access the map at the same time.
This is not much of an issue when you have simple inserts and lookups (unless you do it extremely intensively), but becomes a big problem when you need to iterate over the entire Map, which can take a long time for a large Map - while one thread does that, all others have to wait if they want to insert or lookup anything.
The ConcurrentHashMap uses very sophisticated techniques to reduce the need for synchronization and allow parallel read access by multiple threads without synchronization and, more importantly, provides an Iterator that requires no synchronization and even allows the Map to be modified during interation (though it makes no guarantees whether or not elements that were inserted during iteration will be returned).
ConcurrentHashMap is preferred when you can use it - though it requires at least Java 5.
It is designed to scale well when used by multiple threads. Performance may be marginally poorer when only a single thread accesses the Map at a time, but significantly better when multiple threads access the map concurrently.