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

"JEP 500, Prepare to Make Final Mean Final, has been completed for JDK 26. This JEP proposes to prepare the Java ecosystem so that it will not allow mutation of fields declared as final via deep reflection, which is generally achieved using the setAccessible() method defined in the class. This marks the beginning of a deprecation path that will eventually lead to IllegalAccessException being thrown by default in future versions, effectively closing a long-standing loophole in Java's encapsulation model."

"Although fields have long represented a contract of immutability, Java has historically allowed this contract to be violated through reflection. This long-standing loophole, originally preserved to support object serialization, has accumulated what the OpenJDK team describes as " integrity debt." Over time, widespread reliance on reflective field mutation by dependency injection, serialization, and mocking frameworks has forced the JVM's Just-In-Time (JIT) compiler to make conservative assumptions."

JEP 500 in JDK 26 prepares the Java platform to prevent mutation of fields declared final via deep reflection. Historical allowances for reflective final-field mutation, preserved for uses like serialization, created "integrity debt" that forced conservative JIT compiler assumptions and limited optimizations such as constant folding. The new behavior preserves compatibility by allowing mutation but emits a per-module warning the first time reflective final mutation occurs. The deprecation path will ultimately make reflective mutation throw IllegalAccessException by default, restoring stronger immutability guarantees and enabling more aggressive optimizations and more reliable concurrent semantics.

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