Java Synchronized List - Single thread iteration

Question

I am using Collections.synchronizedList, I have multiple threads building this list. However after it's built I only have a single thread iterating through it. Is the below still imperative in this case?

It is imperative that the user manually synchronize on the returned list when iterating over it:

    List list = Collections.synchronizedList(new ArrayList());
    ...   
    synchronized (list) {
      Iterator i = list.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
    }

if all the threads are finished building your list, why you need concurrency control for single thread accessing it? — Shailesh Aswal, Oct 04 '19 at 08:50
No. As extra integrity check / measure, you might wrap it in a `Collections.unmodifiableList`, Or for a speedup copy the data to a new list (does not relate to your concern). — Joop Eggen, Oct 04 '19 at 08:51
That's pretty much my point, was just confirming my suspicions — PDStat, Oct 04 '19 at 08:51
If you can ensure single threaded access to the list after it is build, then can't see why iterating without synchronisation would be a problem. Iterator provides direct access to the backing list, just ensure it's not misused — Martin'sRun, Oct 04 '19 at 08:56
The warning just says that the _structure_ cannot really deal with an iterator on a change by another thread: the state of the local iterator is not updated. — Joop Eggen, Oct 04 '19 at 08:57

Basil Bourque · Answer 1 · 2019-10-04T09:02:51.140

After the list is built, and you are certain it no longer needs to be synchronized, create a new unsynchronized list from the old one. No need to continue paying the performance hit.

List< Whatever > list = new ArrayList<>( mySynchList ) ;

If the list will no longer need elements added or dropped or replaced, make it unmodifiable.

List< Whatever > list = 
        Collections.unmodifiableList ( 
            new ArrayList<>( mySynchList ) 
        ) 
;

Java Synchronized List - Single thread iteration

1 Answers1