ConcurrentHashMap serialization fix (original) (raw)

Peter Levart peter.levart at gmail.com
Thu Apr 25 07:47:55 PDT 2013

Hello,

I propose a fix for new segment-less ConcurrentHashMap serialization that is found in lambda hg repository:

The fix is 3-fold:

If new segment-less CHM is serialized and de-serialization is attempted on JDK7, NullPointerException is thrown. That's because CHM.Segment nested class is a subclass of ReentrantLock in JDK7 which contains a non-transient field "sync" which is initialized to null upon de-serialization from new format. The fix adds back "extends ReentrantLock" to CHM.Segment to fix the serialization format of new CHM.
I think that new CHM's writeObject has a data race. It temporarily sets the CHM.segments field to a structure, calls defaultWriteObject and then resets the field back to null. If the same instance of CHM is concurrently serialized by 2 or more threads the defaultWriteObject can see either null or half-initialized segments structure in some thread. The fix gets rid of CHM.segments field and uses serialPersistentFields array to simulate the field and changes the writeObject to use local instance of segments array for populating the stream.
The removal of CHM.segments field saves 8 bytes with 64 bit OOPS (no difference with 32 bit OOPS unfortunately).

Regards, Peter