RFR: JDK-8242888: Convert dynamic proxy to hidden classes

[liach]

On Sun, 17 Apr 2022 16:17:30 GMT, liach <duke at openjdk.java.net> wrote:

> Convert dynamic proxies to hidden classes. Modifies the serialization of proxies (requires change in "Java Object Serialization Specification"). Makes the proxies hidden in stack traces. Removes duplicate logic in proxy building.
> 
> The main compatibility changes and their rationales are:
> 1. Modification to the serialization specification: In the "An instance of the class is allocated... The contents restored appropriately" section, I propose explicitly state that handling of proxies are unspecified as to allow implementation freedom, though I've seen deliberate attempts for proxies to implement interfaces with `readResolve` in order to control their serialization behavior.
>    - This is for the existing generated constructor accessor is bytecode-based, which cannot refer to hidden classes.
>    - An alternative is to preserve the behavior, where the serialization constructor calls `invokespecial` on the closest serializable superclass' no-arg constructor, like in #1830 by @DasBrain.
>    - My rationale against preservation is such super calls are unsafe and should be discouraged in the long term. Calling the existing constructor with a dummy argument, as in my implementation, would be more safe.
> 2. The disappearance of proxies in stack traces.
>    - Same behavior exists in lambda expressions: For instance, in `((Runnable) () -> { throw new Error(); }).run();`, the `run` method, implemented by the lambda, will not appear in the stack trace, and isn't too problematic.
> 
> A summary of the rest of the changes:
> 1. Merged the two passes of determining module and package of the proxy into one. This reduced a lot of code and allowed anchor class (for hidden class creation) selection be done together as well.
> 2. Exposed internal API for obtaining a full-privileged lookup to the rest of `java.base`. This API is intended for implementation of legacy (pre `MethodHandles.Lookup`) caller sensitive public APIs so they don't need more complex tricks to obtain proper permissions as lookups.
> 3. Implements [8229959](https://bugs.openjdk.java.net/browse/JDK-8229959): passes methods computed by proxy generator as class data to the hidden proxy class to reduce generated proxy class size and improve performance.
> 
> In addition, since this change is somewhat large, should we keep the old proxy generator as well and have it toggled through a command-line flag (like the old v49 proxy generator or the old reflection implementation)?
> 
> Please feel free to comment or review. This change definitely requires a CSR, but I have yet to determine what specifications should be changed.

I now think that this should be split into probably four separate patches:
1. Merge the 2 passes of scanning interfaces, and store the exported/non-exported package within the classloader value info for dynamic modules
2. Use the JLIA access to obtain lookup than spin an accessor in every proxy class
3. Update serialization constructor generation to use method handles (as in #1830) for hidden classes, don't change the serialization specs for now, as it affects security.

And finally, convert proxies to hidden classes like in this pr. There will be an option (most likely command line) to restore old proxies. And in the new proxies, method objects can be passed as class data, like in the current patch. The jshell test change will be in the final conversion patch.

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PR: https://git.openjdk.java.net/jdk/pull/8278