What's the difference between ConcurrentHashMap and Collections.synchronizedMap(Map)?

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.

╔═══════════════╦═══════════════════╦═══════════════════╦═════════════════════╗
║   Property    ║     HashMap       ║    Hashtable      ║  ConcurrentHashMap  ║
╠═══════════════╬═══════════════════╬═══════════════════╩═════════════════════╣ 
║      Null     ║     allowed       ║              not allowed                ║
║  values/keys  ║                   ║                                         ║
╠═══════════════╬═══════════════════╬═════════════════════════════════════════╣
║ Thread-safety ║                   ║                                         ║
║   features    ║       no          ║                  yes                    ║
╠═══════════════╬═══════════════════╬═══════════════════╦═════════════════════╣
║     Lock      ║       not         ║ locks the whole   ║ locks the portion   ║        
║  mechanism    ║    applicable     ║       map         ║                     ║ 
╠═══════════════╬═══════════════════╩═══════════════════╬═════════════════════╣
║   Iterator    ║               fail-fast               ║ weakly consistent   ║ 
╚═══════════════╩═══════════════════════════════════════╩═════════════════════╝

Regarding locking mechanism: Hashtable locks the object, while ConcurrentHashMap locks only the bucket.

The "scalability issues" for Hashtable are present in exactly the same way in Collections.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).