Hi, I would like to still propose this note for 9, we ain’t gonna resolve the underlying issue in 9.
It’s currently an api note but it’s more appropriate as an implementation note (e.g. Ephemerons might help). We could amend the note with Peter’s suggestion as follows: @implNote Care should be taken to ensure that this ClassValue is not strongly reachable from the computed value, more specifically for the case when the class loader of the computed value’s class is not the same as or a descendent of the class loader of this ClassValue’s class. Doing so will prevent classes and their loader from being garbage collected which in turn may induce out of memory errors. Paul. > On 8 Nov 2016, at 15:27, Paul Sandoz <paul.san...@oracle.com> wrote: > > Hi, > > Please review the addition of an api note to ClassValue.computeValue. > > There is some history behind this issue. Another issue was logged [1] related > to Groovy using ClassValue and there being a memory leak with classes/loaders > not being GC’ed, but it turned out the problem was with Groovy's explicit > retention of computed values in a global set. So i closed that issue down. > > But, there is an edge case where it’s possible to induce out of memory errors > with ClassValue, specifically if the computed value holds onto the > corresponding ClassValue instance. I think this is an edge case and does not > warrant a change to the ClassValue implementation to support weak refs to > computed values which is likely to complicate an already intricate > implementation and perturb its performance characteristics. > > So i have opted for an api note. I don’t want to normatively specify this, > nor do i want to allude to various implementation details. (One can argue a > similar note could be written for ThreadLocal.) > > Thanks, > Paul. > > [1] https://bugs.openjdk.java.net/browse/JDK-8136353 > > --- a/src/java.base/share/classes/java/lang/ClassValue.java Tue Nov 08 > 12:36:21 2016 -0800 > +++ b/src/java.base/share/classes/java/lang/ClassValue.java Tue Nov 08 > 15:25:04 2016 -0800 > @@ -62,6 +62,13 @@ > * If this method throws an exception, the corresponding call to {@code > get} > * will terminate abnormally with that exception, and no class value will > be recorded. > * > + * @apiNote > + * Care should be taken to ensure that this {@code ClassValue} is not > + * <a href="../ref/package-summary.html#reachability"><em>strongly > reachable</em></a> > + * from the computed value. Doing so may prevent classes and their > loaders > + * from being garbage collected which in turn may induce out of memory > + * errors. > + * > * @param type the type whose class value must be computed > * @return the newly computed value associated with this {@code > ClassValue}, for the given class or interface > * @see #get
