使用 synchronizedSet 同步两个线程之间的访问

Question

I'm not able to synchronize two threads using a set:

private Set<String> set;
...
set = Collections.synchronizedSet(new HashSet<String>());

and passing it to two threads. One accessing:

synchronized (set) {
    // [1]
    if (set.contains(str)) {
    ...
    } else {
        // [3]
    }
}

and another updating:

synchronized (set) {
    set.add(str);   // [2]
...
}

What happens is that [1], [2], [3] happens in sequence. During [1], it is correct that the set doesn't have yet the item I'm looking for. But then [2] updates it by adding the item. And during [3], I now see the item. How do I fix this? I also have a ConcurrentHashMap being shared by the same threads but it works perfectly fine. What is the set's equivalent to ConcurrentHashMAp?

UPDATE: The code is too long. Anyway, my updated question is - What is the set's equivalent to ConcurrentHAshMap?

Answer 1

You are synchronizing access correctly. Actually, wrapping it in synchronizedSet() is not having any additional effect here. There is no ConcurrentHashSet though you can kind of get the same thing from Collections.newSetFromMap() and ConcurrentHashMap . But this isn't the problem.

The problem is somewhere else in your code. For example: are you sure you're synchronizing on the same set? are your keys implementing hashCode() and equals() correctly? have you made them mutable (bad idea) and something is changing the keys?

Answer 2

Just wanted to correct the accepted answer and note that Collections.synchronizedSet(...) does actually make a difference. Calling it creates a SynchronizedSet object (subclass of SynchronizedCollection ) which wraps around the provided Set and uses a mutex object as a lock to synchronize access to the underlying Set .

With this in mind, the Set returned from Collections.synchronizedSet(...) does not need to be externally synchronized when being accessed - unless you are traversing it via Iterator , Spliterator or Stream . See the documentation .