Collections.addAll: remove outdated performance advice and add a note about atomicity (original) (raw)

Martin Buchholz martinrb at google.com
Wed Jan 31 03:07:47 UTC 2018

I tried to tackle this here: http://openjdk.markmail.org/thread/eet2zd6ig3pfpv5g and it's still on my TODO list but not likely to get to top spot soon.

On Tue, Jan 30, 2018 at 7:00 PM, Tagir Valeev <amaembo at gmail.com> wrote:

Hello!

I suggest a patch for java.util.Collections#addAll JavaDoc: --- Collections.java 2018-01-31 09:39:31.599107500 +0700 +++ Collections.java.patched 2018-01-31 09:51:11.929059600 +0700 @@ -5406,4 +5406,8 @@ * The behavior of this convenience method is identical to that of - * {@code c.addAll(Arrays.asList(elements))}, but this method is likely - * to run significantly faster under most implementations. + * {@code c.addAll(Arrays.asList(elements))} except possible + * difference in intermediate state visibility for concurrent or + * synchronized collections. Calling this method does not guarantee + * that the intermediate state (some of elements are added) is invisible, + * even if the collection itself provides such guarantee for its + * {@link Collection#addAll(Collection)} method. * First, currently it says that Collections#addAll is likely to run significantly faster. However it's only marginally faster for collections which delegate their addAll method to standard AbstractCollection#addAll implementation. Also it could be much slower for collections which have optimized addAll (like ArrayList, CopyOnWriteArrayList, ConcurrentLinkedDeque, etc.). I don't know a single example of collection where Collections#addAll is actually significantly faster. Also it says that the behavior is identical, while it's not. If, e.g. c is a collection returned from synchronizedCollection, then intermediate state of c.addAll(Arrays.asList(elements)) would not be visible under synchronized(c) in another thread. On the other hand, replacing such call with Collections.addAll(c, elements) (to make it "significantly faster") will lift this guarantee: now you can see partially added array. What do you think? Should I file an issue? With best regards, Tagir Valeev.

More information about the core-libs-dev mailing list