Question

I understand that collections like the Hashtable are synchronized, but can someone explain to me how it works, and at what point(s) access is restricted to concurrent calls? For example, let’s say I use some iterators like this:

Hashtable<Integer,Integer> map = new Hashtable<Integer,Integer>();

void dosomething1(){
    for (Iterator<Map.Entry<Integer,Integer>> i = map.entrySet().iterator(); i.hasNext();){
        // do something
    }
}
void dosomething2(){
    for (Iterator<Map.Entry<Integer,Integer>> i = map.entrySet().iterator(); i.hasNext();){
        // do something
        // and remove it
        i.remove();
    }
}
void putsomething(int a, int b){
    map.put(a,b);
}
void removesomething(int a){
    map.remove(a);
}
var clear(){
    map = new Hashtable<Integer,Integer>();
}

Can someone please explain if there are any pitfalls with me calling these functions at random from different threads? How does the iterator, in particular, do its synchronization, especially when it is using entrySet(), which would seem to also require synchronization? What happens if clear() is called while one of the loops is in progress? What if removesomething() removes an item that is not yet processed by a concurrent loop in dosomething1() ?

Thanks for any help!

Answer 1

Iteration over collections in Java is not thread safe, even if you are using one of the synchronized wrappers (Collections.synchronizedMap(...)):

It is imperative that the user manually synchronize on the returned
map when iterating over any of its collection views:

Map m = Collections.synchronizedMap(new HashMap());
...
Set s = m.keySet();  // Needn't be in synchronized block
...
synchronized(m) {  // Synchronizing on m, not s!
    Iterator i = s.iterator(); // Must be in synchronized block
    while (i.hasNext())
        foo(i.next());
}

Java Collection Framework docs

Other calls to synchronized collections are safe, as the wrapper classes surround them with synchronized blocks, which use the wrapper collection as their monitor:

public int size() {
    synchronized( this ) {
        return collection.size();
    }
}

with collection being the original collection. This works for all methods exposed by a collection/map, except for the iteration stuff.

The key set of a map is made synchronized just the same way: the synchronized wrapper does not return the original key set at all. Instead, it returns a special synchronized wrapper of the collection’s original key set. The same applies to the entry set and the value set.