Most Java devs feel that final means “this field will never change”. Reality: for years, deep reflection (Field#setAccessible(true); field.set(...)) has happily mutated final fields, breaking encapsulation, reasoning, and JVM optimizations.
Java 26 (JEP 500 “Prepare to Make Final Mean Final”) starts closing that door. 👀
🔸 TL;DR
- ▪️ final fields are currently mutable via deep reflection, which hurts safety and performance.
- ▪️ Java 26 emits warnings when code mutates final fields reflectively (new default: --illegal-final-field-mutation=warn).
- ▪️ In a future Java release, illegal final-field mutation will throw exceptions by default (deny mode), unless you explicitly opt in.
- ▪️ You can temporarily allow this behavior via --enable-final-field-mutation while you migrate frameworks & libraries.
- ▪️ Stronger guarantees on final = safer code + better JVM optimizations (constant folding, more aggressive inlining, etc.).
🔸 WHAT’S THE PROBLEM WITH FINAL TODAY?
▪️ Deep reflection lets you do things like:
Field f = Box.class.getDeclaredField("value");
f.setAccessible(true);
f.set(box, "New value");
…even though value is private final. This bypasses both encapsulation and immutability, and it can happen inside third-party libraries you don’t control.
▪️ If any library can mutate final fields, the JVM cannot fully trust them:
▪️ Harder to reason about correctness (especially with concurrency).
▪️ Limits optimizations like constant folding that rely on true immutability.
🔸 WHAT JAVA 26 CHANGES (JEP 500)
Starting with Java 26:
▪️ When code mutates a final field via deep reflection, the JVM:
▪️ Allows the mutation (for now)
▪️ But emits a runtime warning pointing to the module doing it and telling you how to enable it explicitly.
▪️ This behavior is controlled by a new launcher option:
--illegal-final-field-mutation=...
Default in Java 26:
--illegal-final-field-mutation=warn
▪️ A future Java version will flip the default to:
--illegal-final-field-mutation=deny
…which means reflective mutation of final fields will throw IllegalAccessException by default, unless you explicitly enabled it. OpenJDK
🔸 FLAGS YOU SHOULD KNOW ⚙️
▪️ Enable mutation for classpath code (unnamed modules):
java --enable-final-field-mutation=ALL-UNNAMED ...
▪️ Enable mutation for specific modules:
java --enable-final-field-mutation=M1,M2 ...
▪️ Control what happens when illegal mutation is attempted:
--illegal-final-field-mutation=allow # Old behavior, no warnings (pre-26)
--illegal-final-field-mutation=warn # One warning per module (default in 26)
--illegal-final-field-mutation=debug # Warning + stack trace every time
--illegal-final-field-mutation=deny # Throw IllegalAccessException
🔸 HOW TO PREPARE YOUR CODEBASE 🧪
▪️ Run your tests on Java 26 EA with:
```bash
--illegal-final-field-mutation=debug
…to see who is mutating final fields (DI frameworks, mocking libs, serialization, etc.).
▪️ Once you’ve identified dependencies, you can:
▪️ Upgrade to versions that no longer rely on deep reflection.
▪️ For your own code, avoid patterns that need to mutate final:
▪️ Prefer constructor injection over field injection.
▪️ Don’t rely on mutating final state for tests/mocking – use better seams.
▪️ For serialization libraries:
▪️ Move away from deep reflection and use sun.reflect.ReflectionFactory as recommended by the JEP, so you don’t need --enable-final-field-mutation at all.
▪️ To future-proof:
▪️ Try running with:
--illegal-final-field-mutation=deny
…and fix everything that breaks. That’s roughly how your app will behave in a future Java release.
🔸 WHY THIS IS GOOD NEWS 🚀
▪️ Integrity by default: if you see final, you can trust it much more than today.
▪️ Better performance: JVM can safely apply constant folding and other optimizations on immutable graphs of objects.
▪️ Aligns semantics: normal classes move closer to records & hidden classes, which already forbid reflective mutation of their implicit finals.
▪️ Clear contract between app developers and library authors:
▪️ If you really need to mutate final fields, you must opt in explicitly and document it.
Java 26 is not just “more syntax” – it’s tightening the language guarantees we thought we had all along.
#Java26 #JEP500 #Java #JVM #FinalKeyword #Reflection #CleanCode #Performance #Security #JavaDev #JavaDeveloper
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