Collections.synchronizedXXX() and internal mutex (aka SyncRoot)

[Jason Mehrens]

Background on this can be found here: https://bugs.openjdk.java.net/browse/JDK-4335520

Jason

________________________________________
From: core-libs-dev <core-libs-dev-bounces at openjdk.java.net> on behalf of dmytro sheyko <dmytro.sheyko.jdk at gmail.com>
Sent: Wednesday, April 29, 2020 2:58 AM
To: core-libs-dev
Subject: Collections.synchronizedXXX() and internal mutex (aka SyncRoot)

Hello,

Have you ever discussed to make field mutex in synchronized collections
accessible?

Javadoc for Collections#synchronizedSortedSet suggest to iterate collection
this way:

  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
  SortedSet s2 = s.headSet(foo);
      ...
  synchronized (s) {  // Note: s, not s2!!!
      Iterator i = s2.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }

I.e. in order to iterate subset, we also need a reference to the whole set,
which is not really convenient. How about to make it possible to write:

  SortedSet s2 = s.headSet(foo);
      ...
  synchronized (Collections.getSyncRoot(s2)) {  // Note:
Collections.getSyncRoot(s2)
      Iterator i = s2.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }

Also I think it would be convenient to let to provide custom sync root when
synchronized collection is created.
E.g.

  Object customSyncRoot = new Object();
  SortedSet s = Collections.synchronizedSortedSet(new TreeSet(),
customSyncRoot);

What do you think about this?

Regards,
Dmytro