Looking for Sponsor: JDK-8229959 Convert proxy class to use constant dynamic

[Remi Forax]

----- Mail original -----
> De: "Johannes Kuhn" <info at j-kuhn.de>
> À: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> Envoyé: Vendredi 22 Novembre 2019 04:23:31
> Objet: Looking for Sponsor: JDK-8229959 Convert proxy class to use constant dynamic

> Hi everyone,

Hi Johannes,

> 
> a few hours ago I signed the Oracle Contributor Agreement.

felicitation !

> I'd like to work on JDK-8229959: Convert proxy class to use constant
> dynamic.
> 
> Without further ado, here are some questions and answers you may ask:
> 
> * What are you exactly planing to do?
> I'd like to replace the GETSTATIC instruction with an LDC instruction in
> the current generated bytecode for the proxies.
> 
> * What benefits would such a change have?
> First, it allows us to drop the static final fields and the class
> initializer for the generated proxy classes.
> Second, it allows the code (both generated and generator) to be more
> clear by moving the essential parts together.
> Third, it might bring some performance improvement, because the
> j.l.r.Method lookup is delayed to it's first use - which in some cases
> might never happen.
> 
> * Are you able to write such a patch?
> Yes. I have some experience with the Java bytecode, ASM and ConstantDynamic.
> 
> * But java.lang.reflect.Method is not immutable/persistent?
> That's not a question, but you are right. j.l.r.Method has
> setAccessible, modifying the Method instance. Still, the current
> implementation shares the same Method instance with all proxies with the
> same interfaces in the same ClassLoader.

i wonder if some codes in the wild rely on that ?

> 
> * What should the bootstrap method look like?
> The bootstrap method will take one additional argument, a direct
> MethodHandle to the to be resolved method, and should crack it using
> it's passed Lookup and then reflect it as Method. In other words, it
> should do `return lookup.revealDirect(mh).reflectAs(Method.class, lookup);`
> 
> *  Where should this bootstrap method reside?
> I'm glad you asked. After some thoughts, there are two promising places
> where I could put it:
> a) As private static method in the proxy itself.
> Benefits: No public API changes. Also allows easy migration to option b)
> if pursued later.
> Downsides: The bootstrap method will be duplicated into every proxy class.
> b) As a public static method in the java.* namespace, preferably in
> java.lang.invoke.ConstantBootstraps.
> Benefits: Only one single implementation in the JVM.
> Downside: Changes to the public API. Also leads to other questions like
> "Should j.l.r.Member implement Constable?" and "Should I add a subclass
> of j.l.c.DynamicConstantDesc for this bootstrap method?"
> I prefer option a), but I'm fine with both options. Please let me know
> what you think, or if you think of a better way.

There is perhaps a better way, as part of the branch nestmates of valhalla, Mandy has added a way to transfer a live object to a Class when defining it, with that you can send all the Methods as an array and in the bootstrap method of a condy, access to the right method by extracting it from the array using an index.
It will avoid the whole serialization/deserialization of Methods dance.

Here is an example:
  https://gist.github.com/forax/d0f0034190bc479b86ce977fb94ca176

I believe those changes are planned for Java 14 so will be integrated very soon.

> 
> * What do I expect from my Sponsor?
> I expect my sponsor lo believe that this change will improve the
> OpenJDK. If consensus shows a preference for option b) above (bootstrap
> in j.l.i.ConstantBootstaps), a guidance what requests have to be made
> and where. In that case, I volunteer to write those requests and send
> them to my sponsor, who can (at their sole discretion) forward it to the
> relevant places.
> 
> If you have any additional questions, please don't hesitate to answer to
> this mail. I will try to answer all questions to the best of my ability.
> 

regards,

> With best regards,
> Johannes Kuhn

Rémi