Java 26 introduces JEP 500, which begins the process of making `final` fields truly immutable by issuing runtime warnings when they are mutated via reflection. The default behavior in JDK 26 is `--illegal-final-field-mutation=warn`, printing a warning to stderr when a final field is mutated reflectively. Developers can use `--enable-final-field-mutation` to explicitly permit this behavior per module, or `--illegal-final-field-mutation=deny` to proactively catch all violations. The long-term goal is 'integrity by default', where final fields cannot be mutated through any API unless explicitly unlocked via command-line flags. Library authors relying on deep reflection to mutate final fields are encouraged to migrate to alternative approaches. The change is part of a broader Java initiative to improve maintainability, security, and JIT optimization by enforcing language-level guarantees at runtime.
Sort: