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

liach duke at openjdk.java.net
Mon Apr 18 20:01:39 UTC 2022

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 strongly recommend a new API over revamping `Proxy` itself. https://github.com/forax/hidden_proxy would be a good prototype that serves most needs of the current Proxy API (except a few, like invoking default superinterface method). With hidden classes, I don't see much value in leaving the new API's produced instance in separate modules; what we have for hidden classes should be fine for the most part.

Imo the main obstacle is still the handling of serialization. The annotations are serializable, but currently hidden classes do not work with serialization at all and must use `writeReplace` and `readResolve`. And how to migrate annotations off proxies without breaking serialization is still a question as well. Maybe we can upgrade invocation handlers to allow them to declare custom `readResolve` logic for the proxy to facilitate migration away. How the new API will treat serialization is still undetermined.


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

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