RFR: 8254354: Add a withInvokeExactBehavior() VarHandle combinator [v14]

[Chris Hegarty]

On Tue, 10 Nov 2020 15:02:08 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Hi,
>> 
>> This patch adds an asExact() combinator to VarHandle, that will return a new VarHandle that performs exact type checks, similar to MethodHandle::invokeExact, to help developers catch inexact VarHandle usage, which can lead to performance degradation.
>> 
>> This is implemented using a boolean flag in VarForm. If the flag is set, the exact type of the invocation is checked against the exact type in the VarForm. If there is a mismatch, a WrongMethodTypeException is thrown.
>> 
>> Other than that, there is also an asGeneric() combinator added that does the inverse operation (thanks to Rémi for the suggestion). I've also added The `@Hidden` annotation to the VarHandleGuards methods, as well as a type-checking helper method called from the generic invocation lambda form, so that the stack trace we get points at the location where the VarHandle is being used.
>> 
>> Thanks,
>> Jorn
>> 
>> CSR link: https://bugs.openjdk.java.net/browse/JDK-8255375
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Re-order javadoc

LGTM.

The test could be sprinkled with a number of assertions related to the invocation behaviour, a.k.a `hasInvokeExactBehavior`; either the newly specified default behaviour assertFalse(vh. hasInvokeExactBehavior()), or post switching to exact: assertTrue(vh. hasInvokeExactBehavior())

test/jdk/java/lang/invoke/VarHandles/VarHandleTestExact.java line 82:

> 80:             throws NoSuchFieldException, IllegalAccessException {
> 81:         if (ro) throw new SkipException("Can not test setter with read only field");
> 82:         VarHandle vh = MethodHandles.lookup().findVarHandle(Widget.class, fieldBaseName + "_RW", fieldType);

Quite trivially we can assertFalse(vh. hasInvokeExactBehavior())

test/jdk/java/lang/invoke/VarHandles/VarHandleTestExact.java line 92:

> 90:         }
> 91: 
> 92:         vh = vh.withInvokeExactBehavior();

Quite trivially we can assertTrue(vh. hasInvokeExactBehavior())

-------------

Marked as reviewed by chegar (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/843