CODEWITHSUNDEEP
javamultithreading
user9497425
user9497425

Reputation: 11

Nested synchronized map

How can I safely put elements in

ConcurrentMap <String, Set<Integer>> cmap = new ConcurrentMap<String, Set<Integer>>();

I operate on my concurrent map as follows:

Upvotes: 1

Views: 357

Answers (2)

Liviu Stirb
Liviu Stirb

Reputation: 6075

ConcurrentMap has a method to initialize the map value if missing. I assume also the set inside should be thread safe:

Set<Integer> set = cmap.computeIfAbsent(key,(k)->  Collections.newSetFromMap(new ConcurrentHashMap<>()));

set.add(value);

Before lambda:

    Set<Integer> set = cmap.get(key);
    if (set == null) {
        set = Collections.newSetFromMap(new ConcurrentHashMap<>());
        Set<Integer> prev = cmap.putIfAbsent(key, set);
        if (prev != null) {
            set = prev;
        }
    }
    set.add(value);

Upvotes: 3

John
John

Reputation: 1704

In concurrent environment you probably should use concurrent map and concurrent set. For example ConcurrentHashMap, and Collections.newSetFromMap(new ConcurrentHashMap<>()). The ConcurrentHashMap.putIfAbsent() method prevent race condition of if(!containsKey) put mechanic.

So the code will looks like

public void add(String s, Integer i) {
    cmap.putIfAbsent(s, Collections.newSetFromMap(new ConcurrentHashMap<>()));
    Set<Integer> set = cmap.get(s);
    set.add(i);
}

Upvotes: 0

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