RFR: JDK-8282798 java.lang.runtime.Carrier

[Jim Laskey]

On Tue, 8 Mar 2022 16:51:22 GMT, John R Rose <jrose at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/runtime/Carrier.java line 48:
>> 
>>> 46: 
>>> 47: /**
>>> 48:  * This  class is used to create objects that have number and types of
>> 
>> The class javadoc seems a bit on the thin side. I would say something on the fact that the shape of the carrier is determined by a MethodType, etc, and add an example to illustrate basic usage.
>
> Agreed.  Also, this class is unusual in that it is not instantiated; like `Arrays` or `Collections` it is a (small) bundle of static factories (or are they algorithms?).  I think as such it should be called `Carriers`.
> 
> I'm slightly surprised the MH factories are not factored through a metaobject of the form
> 
> record CarrierBinding(
>     MethodType methodType,
>     MethodHandle constructor,
>     List<Class<?>> componentTypes,
>     List<MethodHandle> components)
> { … }
> 
> 
> The presupposition here, I suppose, is that carriers will only be used by condy or similar quasi-statically configured clients, so having the multiple lookups through a hidden table is no burden, and the clients can always keep the associations correct (between constructors and various component accessors).
> 
> **But** if I were to use carriers to manage intermediate structures in (say) a serialization package (for instance) I would need to make my own records like the above for my own bookkeeping, and I would be slightly miffed that the Carrier API insisted on doing a de-novo lookup twice on each MT key (to say nothing of me having to create the MT key first).
> 
> **And** if I were to use carriers in that way (away from condy), **then** I would want to skip the step of building the MethodType, and wish for a factory method for CarrierBinding instances that took a plain List<Class>, as well as a factory method that took the MethodType (which is convenient for condy).
> 
> BTW, it would be normal to give the name `Carrier` (which is a good name BTW) to the record type that embodies the group of method handles, so `record Carrier(…constructor…components…) {…factories…}`.
> 
> I suppose stuff like this could be added later.  But it's worth considering now, simply because there is an early decision point between a class named `Carrier` and a static-only class named `Carriers`.

Will do. When I wrote this code, I wasn't expecting the external exposure. Hence...

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PR: https://git.openjdk.java.net/jdk/pull/7744