RFR: 8353835: Implement JEP 500: Prepare to Make Final Mean Final

[Alan Bateman]

On Fri, 19 Sep 2025 13:13:22 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Implementation changes for [JEP 500: Prepare to Make Final Mean Final](https://openjdk.org/jeps/500).
>> 
>> Field.set (and Lookup.unreflectSetter) are changed to allow/warn/debug/deny when mutating a final instance field. JFR event recorded if final field mutated. Spec updates to Field.set, Field.setAccessible and Module.addOpens to align with the proposal in the JEP.
>> 
>> HotSpot is updated to add support for the new command line options. To aid diagnosability, -Xcheck:jni reports a fatal error when a mutating a final field with JNI, and -Xlog:jni=debug can help identity when JNI code mutates finals. For now, JNI code is allowed to set the "write-protected" fields System.in/out/err, we can re-visit once we change the System.setIn/setOut/setErr methods to not use JNI (I prefer to keep this separate to this PR because there is a small startup regression to address when changing System.setXXX).
>> 
>> There are many new tests. A small number of existing tests are changed to run /othervm as reflectively opening a package isn't sufficient. Changing the tests to /othervm means that jtreg will launch the agent with the command line options to open the package.
>> 
>> Testing: tier1-6
>
> src/java.base/share/classes/java/lang/Module.java line 950:
> 
>> 948:      * <p> Opening a package with this method does not allow the given module to
>> 949:      * {@linkplain Field#set(Object, Object) reflectively set} a final field declared
>> 950:      * in a class in the package, or
> 
> Suggestion:
> 
>      * {@linkplain Field#set(Object, Object) reflectively set} or

Okay, we could make that wording work if there is a comma after ".. write access".

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PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2369665043