为什么ListChangeBuilder抛出NullPointerException？

Question

I'm trying to figure out why I'm getting this error:

java.lang.NullPointerException: null
    at javafx.collections.ListChangeBuilder.findSubChange(ListChangeBuilder.java:68)
    at javafx.collections.ListChangeBuilder.insertAdd(ListChangeBuilder.java:127)
    at javafx.collections.ListChangeBuilder.nextAdd(ListChangeBuilder.java:254)
    at javafx.collections.ObservableListBase.nextAdd(ObservableListBase.java:179)
    at javafx.collections.ModifiableObservableListBase.add(ModifiableObservableListBase.java:153)
    at java.util.AbstractList.add(AbstractList.java:108)
    at tech.dashman.dashman.RendererApp$1.handle(RendererApp.java:72)
    at tech.dashman.common.pubnub.NewScreenshotsToRender.handle(NewScreenshotsToRender.java:17)
    at tech.dashman.dashman.PubNubMessageHandler.lambda$message$0(PubNubMessageHandler.java:79)
    at java.lang.Thread.run(Thread.java:748)

It happened twice on my application but I don't know how to reproduce it. The line in my code that triggers the error is this:

getActivity().add(new ActivityEntry("..."));

ActivityEntry is a simple class with two fields. I can post the code if necessary but I think it's irrelevant. activity , what getActivity() returns, is a field defined like this:

private ObservableList<ActivityEntry> activity = FXCollections.observableArrayList();

The method in which the actual exception is happening looks like this:

private int findSubChange(int idx, final List<ListChangeBuilder.SubChange<E>> list) {
    int from = 0;
    int to = list.size() - 1;

    while (from <= to) {
        int changeIdx  = (from + to) / 2;
        ListChangeBuilder.SubChange<E> change = list.get(changeIdx);

        if (idx >= change.to) {
            from = changeIdx + 1;
        } else if (idx < change.from) {
            to = changeIdx - 1;
        } else {
            return changeIdx;
        }
    }
    return ~from;
}

Line 68, the one raising the exception is:

if (idx >= change.to) {

The only thing I can think of, other than a bug in ListChangeBuilder , is that it was a race condition and two threads modified the ObservableList at the same time. Does this look like that? Is ObservableList not thread safe?

Answer 1

This looks like a multi-threading issue, so, for now, my solution is to define the list this way:

private ObservableList<ActivityEntry> activity = FXCollections.synchronizedObservableList(FXCollections.observableArrayList());

Since multi-threading issues are so hard to reproduce and debug, I don't know whether this really solves the problem. I'm guessing if it doesn't I'll notice it again in production.