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

[Alan Bateman]

On Mon, 15 Sep 2025 18:18:02 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/reflect/Field.java line 1439:
> 
>> 1437:         } else {
>> 1438:             // no java caller, only allowed if field is public in exported package
>> 1439:             if (!Reflection.verifyPublicMemberAccess(clazz, modifiers)) {
> 
> Is this sufficient? I know core libraries has APIs as public non-static final fields, like java.lang.constant.DirectMethodHandleDesc$Kind.refKind. Don't think they should be allowed to be modified by native code, for example.

If a JNI attached thread does an upcall to Field.set with no Java frames on the stack, then it is can mutate a public final field in a public class in an exported package when "illegal final field mutation" is allowed. If denied then IAE will be thrown.

> test/langtools/jdk/jshell/CompletionSuggestionTest.java line 35:
> 
>> 33:  * @build toolbox.ToolBox toolbox.JarTask toolbox.JavacTask
>> 34:  * @build KullaTesting TestingInputStream Compiler
>> 35:  * @run junit/othervm/timeout=480 CompletionSuggestionTest
> 
> Why does this need an update?

I mentioned this in the PR description because it is necessary to change a small number of tests ro run in /othervm mode. This is because jtreg reflectively opening a package isn't sufficient, it has to run the test with `--add-opens`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2369566678
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2369572798