For decades, Java developers have relied on the final keyword as a cornerstone of immutability—a promise that a field’s value, once set, remains constant. This contract underpins thread safety, program correctness, and critical JVM optimizations like constant folding. Yet beneath this veneer of certainty lurks a dangerous loophole: deep reflection. Using java.lang.reflect.Field.setAccessible(true) followed by set(), any code can mutate final fields at will, undermining security and performance guarantees across the entire ecosystem.

The Integrity Crisis

Final fields aren’t truly final today. As the OpenJDK JEP 500 proposal starkly admits:

"Final fields are, in reality, as mutable as non-final fields. We cannot rely on them when reasoning about correctness."

This fragility has profound consequences:
1. Security Risks: Safe object initialization in multithreaded environments—guaranteed since Java 5’s memory model—becomes unreliable.
2. Lost Optimizations: The JVM hesitates to apply aggressive optimizations (e.g., inlining constant values) when fields could change unexpectedly.
3. Ecosystem Contamination: Even if your code doesn’t mutate final fields, third-party libraries might—and the JVM can’t trust any field.

The loophole originated in JDK 5 to accommodate serialization libraries, which mutate final fields during deserialization. But what began as a pragmatic exception evolved into a systemic vulnerability.

The Enforcement Plan: Warnings, Then Walls

OpenJDK’s solution is a phased lockdown:

  1. JDK XX (Imminent):

    • Any illegal mutation via reflection triggers runtime warnings.
    • Use --illegal-final-field-mutation=debug to log stack traces or jdk.FinalFieldMutation JFR events for pinpoint diagnosis.
    • Enable exemptions with --enable-final-field-mutation=ALL-UNNAMED (or specific modules).

  2. Future JDK Release:

    • Default shifts to deny mode (IllegalAccessException on mutation attempts).
    • --enable-final-field-mutation becomes mandatory for exempted code.
// Example of risky code that will soon break:
Field field = MyClass.class.getDeclaredField("finalValue");
field.setAccessible(true);
field.set(myObject, 42); // Warnings today; exceptions tomorrow

Critical Implications for Libraries

  • Serialization: Libraries like Jackson or Gson must migrate to sun.reflect.ReflectionFactory—a sanctioned API for mutating final fields only in Serializable classes. This preserves optimization boundaries (non-Serializable fields remain truly immutable).
  • DI/Testing Frameworks: Avoid final-field injection. Prefer constructor injection. Mocking libraries must evolve beyond reflection hacks.
  • Native Code (JNI): Mutating finals via JNI remains undefined behavior. New diagnostics (-Xlog:jni=debug) flag violations, and future JDKs may no-op such calls.

Strategic Shifts for Developers

  • Audit Codebases: Hunt reflection usage with --illegal-final-field-mutation=debug or JFR.
  • Command-Line Configuration: Exempt modules sparingly via:
    java --enable-final-field-mutation=com.module.a,com.module.b -jar app.jar
  • Redesign When Possible: Joshua Bloch’s advice resonates anew: avoid clone() for final fields; use factory methods or constructors.

Why This Matters Beyond Java

This isn’t just about closing a loophole—it’s a philosophical shift toward integrity by default. By treating immutability as a first-class contract, Java:
- Unlocks deeper compiler optimizations for speed gains.
- Hardens security for critical infrastructure.
- Aligns with modern language trends (e.g., records’ implicitly final fields).

The era of “assume nothing is final” ends. For enterprises running Java, proactive adaptation isn’t optional—it’s foundational to safety and performance in tomorrow’s JVM.