Is iteration via Collections.synchronizedSet(…).forEach() guaranteed to be thread safe?

Question

As we know, iterating over a concurrent collection is not thread safe by default, so one cannot use:

Set set = Collections.synchronizedSet(new HashSet&l         


        

Answer 1

As @Holger said, the doc clearly says user must manually synchronize collections returned by Collections.synchronizedXyz() when tranversing it via Iterator:

It is imperative that the user manually synchronize on the returned collection when traversing it via Iterator, Spliterator or Stream:

Collection c = Collections.synchronizedCollection(myCollection);
 ...
synchronized (c) {
    Iterator i = c.iterator(); // Must be in the synchronized block
    while (i.hasNext())
       foo(i.next());
}

I want to explain a bit more about code.

Consider Collections.synchronizedList() method. It returns Collections.SynchronizedList class instance, which extends SynchronizedCollection which defines iterator() as follows:

public Iterator iterator() {
    return c.iterator(); // Must be manually synched by user!
}

Compare this with other methods of SynchronizedCollections, for example:

public String toString() {
    synchronized (mutex) {return c.toString();}
}

Thus SynchronizedList inherits iterator() from SynchronizedCollection, which needs to be synched manually by user.