RFR: JDK-8005263: Logging APIs takes Supplier for message (original) (raw)

Remi Forax forax at univ-mlv.fr
Mon Dec 24 16:15:34 UTC 2012

On 12/23/2012 07:45 PM, Brian Goetz wrote:

Henry - just a quick comment on the class description. I think it would be better not to include the sentence "Since 1.8 ..." as it that will quickly become a historical note. It would be much better (in my view) to just highlight the methods with something like "Several of the methods take a Supplier function ..." and make the potential performance benefit of using these methods clear.

You should also add a note saying that the supplier can be specified as a lambda and in that case, the lambda must not capture value of local variable, otherwise a supplier object will be created each time you log something. No, don't do this. First of all, just because the performance characteristics of capturing lambdas are one way today on HotSpot compared to non-capturing, does not mean we should burn this into spec.

It's not related to Hotspot. javac translation uses invokedynamic (by the specification) and the metafactory provided by the JDK uses for non-capturing lambda a ConstantCallSite which is guaranteed by the JSR 292 to be considered as a constant. Hotspot just implements the JSR 292. So this is true for all metafactory implementation that uses a ConstantCallSite as the one provided by the JDK.

Also, there's no way to enforce it.

not a lambda problem, but a JSR 292 problem, and it should be enforced by tests, it's JEP 165.

More importantly, a capturing lambda might still be way better than a String, as in this example:

log(DEBUG, () -> "Children of " + name + ": " + getChildrenOf(name)); Collecting getChildren (and turning it into a String) commonly involves something thousands of times as expensive as an object creation.

Yes, true it depends on the cost of getChildrenOf, my point was to add a sentence saying that using a lambda that capture local variables may impact performance.

Rémi

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