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

[Vladimir Ivanov]

On Thu, 8 May 2025 11:22:30 GMT, Alan Bateman <alanb 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

Hotspot changes look good.

src/hotspot/share/runtime/fieldDescriptor.cpp line 49:

> 47: }
> 48: 
> 49: bool fieldDescriptor::is_mutable_static_final() const {

Please, migrate `ciField::initialize_from()` from ad-hoc checks to `fieldDescriptor::is_mutable_static_final()`.


diff --git a/src/hotspot/share/ci/ciField.cpp b/src/hotspot/share/ci/ciField.cpp
index cbe0cadbc93..dffc387612d 100644
--- a/src/hotspot/share/ci/ciField.cpp
+++ b/src/hotspot/share/ci/ciField.cpp
@@ -267,17 +267,7 @@ void ciField::initialize_from(fieldDescriptor* fd) {
       // not be constant is when the field is a *special* static & final field
       // whose value may change.  The three examples are java.lang.System.in,
       // java.lang.System.out, and java.lang.System.err.
-      assert(vmClasses::System_klass() != nullptr, "Check once per vm");
-      if (k == vmClasses::System_klass()) {
-        // Check offsets for case 2: System.in, System.out, or System.err
-        if (_offset == java_lang_System::in_offset()  ||
-            _offset == java_lang_System::out_offset() ||
-            _offset == java_lang_System::err_offset()) {
-          _is_constant = false;
-          return;
-        }
-      }
-      _is_constant = true;
+      _is_constant = !fd->is_mutable_static_final();
     } else {
       // An instance field can be constant if it's a final static field or if
       // it's a final non-static field of a trusted class (classes in

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

Marked as reviewed by vlivanov (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25115#pullrequestreview-3254635791
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2369888932