Replace simple iterations of Map.entrySet with Map.forEach calls

[Stuart Marks]

Hi, I have to say I'm pretty skeptical of this change overall.

It sounds like the main issue is iterating over the entrySet of a map that might be 
modified concurrently. The bug report says

 > for concurrent maps, regular iteration of entrySet may fail spontaneously

This isn't true for ConcurrentHashMap or ConcurrentSkipListMap, which have weakly 
consistent iterators that won't error out. It *is* true for synchronizedMap, which 
can throw ConcurrentModificationException if the map is modified during iteration. I 
guess that's mostly what you're referring to.

The problem with synchronizedMap (and the other synchronized wrappers) is that they 
require external synchronization around iteration, and it's pretty much impossible 
to cover all the cases where iteration occurs. It's used everywhere outside the JDK. 
Even within the JDK, it appears quite frequently and is hard to avoid. For example, 
consider

     collection.addAll(synchronizedMap.keySet())

AbstractCollection::addAll iterates its argument, and this is inherited in a bunch 
of places.

The root of the problem is that the synchronized wrappers support concurrent 
updating only in a very narrow range of use cases. If people have trouble with 
concurrent map updating, they need to use a real concurrent data structure, or they 
need to put everything inside their own class and have it do locking around 
higher-level operations. Replacing a few entrySet iteration cases with Map::forEach 
doesn't help anything.

s'marks



On 2/22/22 3:09 PM, - wrote:
> Hello,
> In the patch for 8281631 <https://bugs.openjdk.java.net/browse/JDK-8281631>,
> xenoamess intentionally avoided
> <https://github.com/openjdk/jdk/pull/7431#discussion_r810666916> repeatedly
> calling Map::size, for it may not be constant-timed due to being
> concurrent. This alerted me that the for loops on Map::entrySet may be
> similarly not thread-safe.
> 
> I believe that we should migrate for loop on Map::entrySet to Map::forEach
> calls; concurrent map callees usually have dedicated logic
> <https://github.com/openjdk/jdk/blob/2557ef8a02fe19784bd5e605b11d6bd574cde2c2/src/java.base/share/classes/java/util/concurrent/ConcurrentMap.java#L119>
> to mitigate such thread-safety issues. Synchronized map
> <https://github.com/openjdk/jdk/blob/2557ef8a02fe19784bd5e605b11d6bd574cde2c2/src/java.base/share/classes/java/util/Collections.java#L2735>
> callees are benefitted too.
> 
> Another lesser benefit is reduced object allocation for iteration. While
> the most popular implementations (HashMap, LinkedHashMap, WeakHashMap,
> TreeMap) don't need to allocate entries for iteration, many other (EnumMap,
> IdentityHashMap, concurrent maps) do, and iterating those maps with forEach
> is less costly. (Note that 8170826
> <https://bugs.openjdk.java.net/browse/JDK-8170826> takes care of
> implementing proper forEach in EnumMap)
> 
> A JBS issue has already been submitted at
> https://bugs.openjdk.java.net/browse/JDK-8282178, and I have prepared a
> patch. This patch modifies the putAll implementations of a few JDK maps to
> let the callee feed entries through Map::forEach to be put into the maps.
> Two places of Map::entrySet calls in Collectors has also been updated.
> 
> For most other existing usages in the JDK, I don't think they will benefit
> from such a migration: They mostly iterate on the entryset of the popular
> implementations that already host entries and are single-threaded, and I
> don't think it's in our best interest to touch those use cases.
> 
> So, here are my questions:
> 1. Is such a concept of replacing Map::entrySet calls with Map::forEach
> calls reasonable, or is there any fundamental flaw?
> If the concept sounds good, I will submit a patch, so we can answer
> 2. Is the changes implemented correct for this concept, i.e. targeting code
> where users supply callee maps?
> 
> Best regards
>