RFR : 8016446 : (m) Add override forEach/replaceAll to HashMap, Hashtable, IdentityHashMap, WeakHashMap, TreeMap

[Paul Sandoz]

On Jun 19, 2013, at 8:44 AM, Peter Levart <peter.levart at gmail.com> wrote:

> On 06/19/2013 01:13 AM, Mike Duigou wrote:
>> On Jun 18 2013, at 05:19 , Doug Lea wrote:
>> 
>>> On 06/17/13 19:30, Mike Duigou wrote:
>>> 
>>>> I had to add the improved default for ConcurrentMap which was present in the lambda repo in order to have correct behaviour. Since getOrDefault is already in ConcurrentMap I will include this but we have to be careful when we do a jsr 166 syncup to make sure that the defaults don't get lost.
>>>> 
>>> Now synched up on my side.
>>> 
>>> -Doug
>>> 
>> 
>> 
>> Per a suggestion from Remi I updated the ConcurrentMap.replaceAll default to use forEach. This trades off the entrySet iterator overhead for creation of a capturing BiConsumer lambda.
>> 
>> http://hg.openjdk.java.net/jdk8/tl/jdk/raw-diff/1f7cbe4829fe/src/share/classes/java/util/concurrent/ConcurrentMap.java
>> 
>> Mike
> 
> Hi Mike, Remi,
> 
> Since forEach implementation can be taken from default Map.forEach in some implementations of ConcurrentMap, and that implementation is based on entrySet Iterator, isn't it dangerous for this to trigger ConcurrentModificationException in some implementation of ConcurrentMap? I see nothing in the spec. of ConcurrentMap that suggests it's entrySet iterators are never fail-fast. They can be prepared for modifications from other threads (synchronization), but may not tolerate re-entrant calls.
> 
> For example some implementation of (Concurrent)Map could be structurally modified as a result of Map.replace(key, old, new) - imagine a ConcurrentWeakHashMap that expunges stale entries on each call - and forEach iteration may not be prepared to handle such situations.
> 

Or in general when explicitly iterating on the entrySet. 

  for (Map.Entry e : cm.entrySet()) {
    cm.replace(e.getKey(), e.getValue(), newValue);
  }

A concurrent map implementation that provides a fail-fast iterator for in-thread modification is asking for trouble IMHO! Instead i would expect the iterator to be weakly consistent and never throw a CME.

--

This is another little oddity in Map.forEach:

            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch(IllegalStateException ise) {
                throw new ConcurrentModificationException(ise);
            }

I would presume the entries from CconcurrentMap.entrySet would not throw ISEs

Paul.