CODEWITHSUNDEEP
javamultithreadingcollectionsconcurrenthashmap
Sam
Sam

Reputation: 2065

Sort concurrentHash Map with threadsafty

I am using 'concurrentHashMap' in my 'multithreaded' application. i was able to sort it as describe here. but since i am converting hashmap to a list i am bit worried about the thred safty. My 'ConcurrentHashMap' is a static variable hence i can guarantee there will be only one instance of it. but when i am going to sort it i convert it to a list, and sort then put it back to a new concurrentHashMap.

Is this a good practice in multi-threading enlivenment?

Please let me know your thoughts and suggestions.

Thank you in advance.

Upvotes: 4

Views: 3707

Answers (3)

Stephen C
Stephen C

Reputation: 718946

My 'ConcurrentHashMap' is a static variable hence i can guarantee there will be only one instance of it. but when i am going to sort it i convert it to a list, and sort then put it back to a new concurrentHashMap.

This is not a simple problem.

I can tell you for a fact, that using a ConcurrentHashMap won't make this thread-safe. Nor will using a synchronizedMap wrapper. The problem is that sorting is not supported as a single atomic operation. Rather it involves a sequence of Map API operations, probably with significant time gaps in between them.

I can think of two approaches to solving this:

  • Avoid the need for sorting in the first place by using a Map that keeps the keys in order; e.g. use ConcurrentSkipListMap.

  • Wrap the Map class in a custom synchronized wrapper class with a synchronized sort method. The problem with this approach is that you are likely to reintroduce the concurrency bottleneck that you avoided by using ConcurrentHashMap.

And it is worth pointing out that it doesn't make any sense to sort a HashMap or a ConcurrentHashMap because these maps will not preserve the order into which you sort the elements. You could use a LinkedHashMap, which preserves the entry insertion order.

Upvotes: 1

John Vint
John Vint

Reputation: 40256

You should use a ConcurrentSkipListMap. It is thread-safe, fast and maintains ordering according to the object's comparable implementation.

Upvotes: 6

Maurício Linhares
Maurício Linhares

Reputation: 40333

If you don't change it a lot and all you want is to have it sorted, you should use a TreeMap ** wrapped by a **Collections.synchronizedMap() call

Your code would be something like this:

public class YourClass {
  public static final Map<Something,Something> MAP = Collections.synchronizedMap( new TreeMap<Something,Something>() );
}

Upvotes: 1

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