Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
2026-09-21 15:08:50 +00:00
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+# JEP 500: reflection writing to a final field, same class file behaviour on 25 and on 26
+
+=== JDK 25 (class compiled --release 25)
+    field reads    : 9090
+    port() returns : 8080
+
+=== JDK 26, defaults
+    WARNING: Final field port in class FinalFieldMutation$Config has been mutated reflectively by class FinalFieldMutation in unnamed module @<hash> (file:<repo>/build/recap26/26/)
+    WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
+    WARNING: Mutating final fields will be blocked in a future release unless final field mutation is enabled
+    field reads    : 9090
+    port() returns : 8080
+
+=== JDK 26, --enable-final-field-mutation=ALL-UNNAMED (the opt-in the warning asks for)
+    field reads    : 9090
+    port() returns : 8080
+
+=== JDK 26, --illegal-final-field-mutation=deny (what a future default will look like)
+    setInt refused : class FinalFieldMutation (in unnamed module @<hash>) cannot set final field FinalFieldMutation$Config.port (in unnamed module @<hash>), unnamed module @<hash> is not allowed to mutate final fields
+
+=== JDK 26, --illegal-final-field-mutation=debug (warning plus the offending stack trace)
+    Final field port in class FinalFieldMutation$Config has been mutated reflectively by class FinalFieldMutation in unnamed module @<hash> (file:<repo>/build/recap26/26/)
+    	at java.base/java.lang.reflect.Field.postSetFinal(Field.java:1534)
+    	at java.base/java.lang.reflect.Field.setFinal(Field.java:1449)
+    	at java.base/java.lang.reflect.Field.setInt(Field.java:1134)
+    	at FinalFieldMutation.main(FinalFieldMutation.java:22)
+    field reads    : 9090
+    port() returns : 8080
+
+=== JDK 26, JFR default settings, [no flag]: jfr print --events jdk.FinalFieldMutation
+    jdk.FinalFieldMutation 
+    declaringClass = FinalFieldMutation$Config (classLoader = app)
+    fieldName = "port"
+    FinalFieldMutation.main(String[]) line: 22
+    events recorded: 1
+
+=== JDK 26, JFR default settings, [--enable-final-field-mutation=ALL-UNNAMED]: jfr print --events jdk.FinalFieldMutation
+    jdk.FinalFieldMutation 
+    declaringClass = FinalFieldMutation$Config (classLoader = app)
+    fieldName = "port"
+    FinalFieldMutation.main(String[]) line: 22
+    events recorded: 1
+
+=== JDK 26, JFR default settings, [--illegal-final-field-mutation=deny]: jfr print --events jdk.FinalFieldMutation
+    events recorded: 0