List.addAll生成的遍历List是否在Java中产生ConcurrentModificationException？

Question

Code post below produced ConcurrentModificationException , newMR is ensured not modified in the loop, mkrs is thread unsafely in concurrent environment. It is known that List.addAll is implemented by System.arraycopy which is intrinsic method in JVM. Does traverse List generated by List.addAll produce ConcurrentModificationException in Java?

ArrayList<Foo> mkrs = mrm.getFooPush();
ArrayList<Foo> newMR = new ArrayList<Foo>(mkrs.size());
newMR.addAll(mkrs);
for (Foo mkr : newMR) {
    if (mkr != null && mkr instanceof PushRecommend) {
        PushRecommend pr = (PushRecommend) mkr;
        recommendList.add(new MayKnowMessage(pr));
    }
}

Post the exception stack trace happened on an Android machine. Note that (ProGuard:433) is the line where for (Foo mkr : newMR) written.

java.util.ConcurrentModificationException:
java.util.ArrayList$Itr.next(ArrayList.java:837)
NewFriendManager.void loadNewFriendMsg(boolean)(ProGuard:433)
NewFriendManager.void reloadNewFriendMsg()(ProGuard:308)
NewFriendManager.boolean handleMessage(android.os.Message)(ProGuard:144)
android.os.Handler.dispatchMessage(Handler.java:106)
android.os.Looper.loop(Looper.java:232)
android.os.HandlerThread.run(HandlerThread.java:61)

Answer 1

The javadoc for ArrayList.addAll states:

"The behavior of this operation is undefined if the specified collection is modified while the operation is in progress."

So consider this:

newMR.addAll(mkrs);

The addAll method is going to have to iterate the mkrs collection.

If mkrs is modified by another thread while you are calling addAll(mkrs) , then a CCME is possible if the type of mkrs is not a concurrent collection that allows simultaneous iteration and updates.

Answer 2

newMR.addAll(mkrs); iterates over mkrs, mrm.getFooPush() . If that list is changed during the addAll, you get that message.

You need to have a concurrency safe list (inside getFooPush) or a better data structure. Like a queue if the messages are added at the end, and you are filtered (on PushRecommend) consuming from the front.

Then there is no need to make a copy.

As already mentioned, the original data structure could be made safe by Collections.synchronizedList(list) .