Is Reference.reachabilityFence() needed in Reference constructor? (original) (raw)

Peter Levart peter.levart at gmail.com
Wed Oct 21 12:34:27 UTC 2015

Hi,

I have a question about a use-case for Reference.reachabilityFence().

While reviewing code of new Cleaner API that is being proposed on core-libs-dev, the following java.lang.Reference constructor caught my eye:

 Reference(T referent, ReferenceQueue<? super T> queue) {
     this.referent = referent;
     this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
 }

Say it is being used like that:

ReferenceQueue queue = ...;

WeakReference wr = new WeakReference(new Object(), queue);

Is it possible that the newly constructed Object is found weakly-reachable before the 'queue' is assigned to the Reference.queue field or is there something in the JVM that prevents this happening (like all the constructor parameters are reachable at least until the constructor is finished)? Might the following be needed or not:

 Reference(T referent, ReferenceQueue<? super T> queue) {
     this.referent = referent;
     this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
     reachabilityFence(referent);
 }

Regards, Peter

More information about the core-libs-dev mailing list