Java Tightens the Reins: Immutability Enforcement with JEP 500

Final Fields: No More Sneaky Changes?

JEP 500 represents a crucial step towards stricter immutability in Java, addressing a long-standing weakness that has hindered compiler optimizations. The introduction of different runtime modes and JFR integration provides a practical migration path for developers, minimizing disruption while enabling potential performance gains. However, the reliance on warnings in the initial release might lead to some developers overlooking the issue. Furthermore, the article doesn't delve into the specific performance improvements expected, which could enhance the impact of the announcement. A more detailed examination of performance benchmarks would strengthen the article's value.

Key Points

• JEP 500 prepares Java to restrict modification of final fields via reflection, enforcing immutability more strictly.
• JDK 26 introduces runtime modes (--illegal-final-field-mutation) to manage the transition, including warn, deny, and allow.
• JDK Flight Recorder (JFR) integration provides auditing capabilities via the jdk.FinalFieldMutation event to help developers identify and fix reflective modifications.

---

📖 Source: JEP 500: Java to Enforce Strict Final Field Immutability by Restricting Reflection