On Mar 11, 2012, at 9:45 AM, Rémi Forax wrote:
> Last week, Fredrik found that I've forgotten to implement ClassValue
> in the backport of jsr292.
>
> While struggling to implement it, I've found a bug in the implementation
> of the JDK. This snippet creates an infinite loop :(
>
> java.lang.ClassValue<Integer> cv = new
> java.lang.ClassValue<Integer>() {
> @Override
> protected Integer computeValue(Class<?> type) {
> remove(int.class);
> return 1;
> }
> };
> cv.get(int.class);
>
> while this code is naughty, I think it's a legal one.
Naughty code, nice bug.
The spec. requires that the CV go from state 2N to state 2N+1 *after* the
return from computeValue, not before.
http://cr.openjdk.java.net/~jrose/pres/indy-javadoc-mlvm/java/lang/ClassValue.html#remove(java.lang.Class)
The remove call should therefore be a no-op (assuming no concurrent calls to
get) from state 2N to 2N.
I think the implementation is treating the internal state for a "pending call
to computeValue" as if it were 2N+1, which causes the remove call to advance to
2N+2. The spec. requires CV.get to retry the computeValue call in such cases,
which leads to the infinite loop.
I have filed 7153157 "ClassValue.get does not return if computeValue calls
remove".
Thanks Remi!
— John
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