RFR: 8159746: (proxy) Support for default methods

[Mandy Chung]

On Fri, 20 Nov 2020 20:06:38 GMT, Rémi Forax <github.com+828220+forax at openjdk.org> wrote:

>> I have a fresh look at the different options we have explored (lots of challenges in finding the right API with security, usability and performance issues to consider).   I agree with Remi that we should keep the design and API simple and makes it easier to invoke default methods with today's Proxy API.    We can design a better Proxy API in the future.   
>> 
>> The options we have explored are:
>> 1. static `InvocationHandler::invokeDefault` method
>> 2. abstract `DelegatingInvocationHandler` class with a protected `invokeDefault` method
>> 3. a new `newProxyInstance` factory method taking a function that produces an invocation handler with the ability to invoke a default method via a `superHandler`
>> 
>> (1) is very simple API but caller-sensitive.  No other API change.   Access check done at default method invocation time (which is consistent with the core reflection `Method::invoke`).    It shares the caller class caching in `Method::checkAccess` which helps the performance.     The performance overhead is slightly higher than (2) & (3) which does access check at proxy creation time.
>> 
>> (2) is simple and I like that the `invokeDefault` can be enforced to be invoked only by the proxy's invocation handler.   However this requires more API changes (including `newProxyInstance`, `getInvocationHandler`, and new unchecked exception type).   (3) is clever but a bit over-rotated (how Alan describes it) to allow it to be expressed in a lambda expression.  If an invocation handler wants to save `superHandler`, it can't assign it to a final field in lambda and would need to workaround it writing to an array element.
>> 
>> I will go with option (1) -  static `invokeDefault` method [1] unless there is any objection. 
>> 
>> CSR: https://bugs.openjdk.java.net/browse/JDK-8253870
>> 
>> Here is the specdiff:
>>   http://cr.openjdk.java.net/~mchung/jdk16/webrevs/8159746/specdiff/
>> 
>> [1] http://cr.openjdk.java.net/~mchung/jdk16/webrevs/8159746/api/java.base/java/lang/reflect/InvocationHandler.html#invokeDefault(java.lang.Object,java.lang.reflect.Method,java.lang.Object...)
>
> Hi Mandy,
> thanks for taking the time to explore all the different options,
> 
> The solution 1 is fine for me.

Thanks Remi.

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PR: https://git.openjdk.java.net/jdk/pull/313