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

[Chen Liang]

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

src/hotspot/share/prims/jni.cpp line 1931:

> 1929:   } \
> 1930:   o->Fieldname##_field_put(offset, value); \
> 1931:   log_debug_if_final_instance_field(thread, "Set<Type>Field", k, offset); \

Just curious, can we use macros to convert generic `Set<Type>Field` to specific ones, such as using ##Result## in the string?

src/hotspot/share/prims/jniCheck.cpp line 1252:

> 1250:     IN_VM( \
> 1251:       checkInstanceFieldID(thr, fieldID, obj, FieldType); \
> 1252:       checkCanSetInstanceField(thr, fieldID, obj); \

Same remark, could we pass ##Result## so we avoid `Set<Type>Field`?

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

src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 3449:

> 3447:                 }
> 3448:                 // check if write access to final field allowed
> 3449:                 if (!field.isStatic() && isAccessible && allowedModes != TRUSTED) {

I don't think we need this allowedModes special permission - I see no scenario in which the core libraries implementation needs to perform such a reflective operation.

src/java.base/share/classes/java/lang/reflect/AccessibleObject.java line 173:

> 171:      * <p> The {@code accessible} flag when {@code true} suppresses Java language access
> 172:      * control checks to only enable {@linkplain Field#get <em>read</em>} access to
> 173:      * these non-modifiable final fields.

Note to reviewers: this is moved to Field.setAccessible.

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.

src/java.base/share/classes/java/lang/reflect/Field.java line 1461:

> 1459:         return Modifier.isFinal(modifiers)
> 1460:                 && !Modifier.isStatic(modifiers)
> 1461:                 && !clazz.isRecord()

This check cannot constant fold in the edge case where the declaring class extends `java.lang.Record` but does not have a `Record` attribute. I have seen such classes in proguard-stripped code.

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?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2349051829
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2349061108
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2362845215
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2349087370
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2349097202
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2349771906
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2369472297
PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2369489143