RFR: 8280377: MethodHandleProxies does not correctly invoke default methods with varags

[Johannes Kuhn]

On Fri, 21 Jan 2022 22:49:38 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> The MethodHandle of a default method should be made as a fixed arity method handle because it is invoked via Proxy's invocation handle with a non-vararg array of arguments.  On the other hand, the `InvocationHandle::invokeDefault` method  was added in Java 16 to invoke a default method of a proxy instance.  This patch simply converts the implementation to call `InvocationHandle::invokeDefault` instead.

When testing this patch from a named module and not-exported package, I get the following exception:


Exception in thread "main" java.lang.reflect.UndeclaredThrowableException
	at jdk.proxy1/com.sun.proxy.jdk.proxy1.$Proxy0.toString(Unknown Source)
	at test.openjdk/test.openjdk.ProxyTest.main(ProxyTest.java:22)
Caused by: java.lang.IllegalAccessException: class java.lang.invoke.MethodHandleProxies$1 (in module java.base) cannot access interface test.openjdk.ProxyTest$Test (in module test.openjdk) because module test.openjdk does not export test.openjdk to module java.base
	at java.base/jdk.internal.reflect.Reflection.newIllegalAccessException(Reflection.java:394)
	at java.base/java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:674)
	at java.base/java.lang.reflect.InvocationHandler.invokeDefault(InvocationHandler.java:278)
	at java.base/java.lang.invoke.MethodHandleProxies$1.invoke(MethodHandleProxies.java:202)
	... 2 more


[My test code](https://gist.github.com/DasBrain/60dbc1c9075b15635d9c87e8295f1c1a).  
Running without the patch results in the wrong output for default varargs methods, so this is a regression.

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PR: https://git.openjdk.java.net/jdk/pull/7185