This JEP proposes preparing the Java platform for a future where final truly means final, eliminating the current capability of dynamically modifying final fields via reflection or other privileged code. The goal is to improve performance, security, and maintainability by enabling further runtime optimizations based on the immutability guarantees of final. This JEP focuses on identifying and mitigating compatibility risks posed by this change, such as existing frameworks and libraries that rely on altering final fields. It outlines an incremental approach involving a new JVM command-line option to enforce final field immutability, allowing developers to test and adapt their code before the restriction becomes the default and eventually permanent. This preparatory work will pave the way for a subsequent JEP to actually finalize the behavior of final.
This Java Enhancement Proposal (JEP) draft, titled "Prepare to make final mean final," meticulously outlines a preparatory phase for a future enhancement to the Java language aimed at strengthening the semantics of the final keyword for classes and methods. Currently, the final keyword, while signifying immutability in certain contexts (like preventing variable reassignment and subclassing), does not fully guarantee the immutability of the runtime behavior of a designated final class or method. Specifically, dynamic language features like reflection and method handles can circumvent the final designation, potentially altering the behavior of a class or method deemed final and impacting maintainability, security, and performance optimization efforts.
This JEP itself does not implement the full restriction of final’s meaning; rather, it focuses on the necessary groundwork to enable such a change in the future. This groundwork comprises two principal actions. Firstly, it proposes the introduction of a command-line option, provisionally named --enable-preview-final-classes, which developers can utilize to activate stricter final semantics on a per-project basis. This "preview" mode will allow developers to experiment with the enhanced final behavior and assess its impact on their codebases before its full enforcement. Secondly, the JEP outlines the development of a migration tool designed to assist developers in identifying and addressing potential compatibility issues stemming from the stricter interpretation of final. This tool will help to smooth the transition by flagging areas where reflection or method handles are currently employed to modify the behavior of final classes or methods, providing developers with the opportunity to adapt their code before the eventual firm enforcement of the strengthened final semantics. This phased approach ensures a more controlled and less disruptive transition for the Java ecosystem, minimizing the potential for widespread compatibility issues. The ultimate goal, after this preparatory phase, is to make final truly "final" and thereby bolster the guarantees and assumptions developers can make regarding the behavior of their code, ultimately contributing to more robust and maintainable Java applications.
Summary of Comments ( 112 )
https://news.ycombinator.com/item?id=43538919
HN commenters largely discuss the implications of making
finalmean truly final in Java. Some express concern about the performance impact, particularly for JIT compilers and escape analysis. Others question the practicality and benefit, given the existing workarounds likesealedclasses and the potential disruption to existing codebases. A few commenters welcome the change, seeing it as a positive step toward stricter immutability and potentially simplifying some aspects of the language. There's also discussion around the nuances of the proposal, such as its impact on method overriding and the interaction with reflection. Several users highlight the complexity of implementing this change in the JVM and the potential for unforeseen consequences.The Hacker News post titled "JEP draft: Prepare to make final mean final" (https://news.ycombinator.com/item?id=43538919) discussing the JEP draft at https://openjdk.org/jeps/8349536, has a moderate number of comments, exploring different facets of the proposed change.
Several commenters discuss the implications for mocking frameworks like Mockito. One user points out the potential difficulties this change poses for testing and mocking, as overriding final methods is a common technique employed by these frameworks. They question the practicality of the proposed solution of using method handles for mocking, expressing concern about the performance overhead and complexity it might introduce. Another user suggests that this change might push developers towards compile-time mocking solutions or different testing strategies altogether. The discussion around Mockito highlights a significant trade-off between stricter language semantics and the flexibility required for testing.
The performance implications of the JEP are also a topic of discussion. One commenter questions whether the potential performance gains from this change are significant enough to justify the disruption it would cause to existing codebases and testing practices. Another user counters this by arguing that while the immediate performance gains might be minimal, it lays the groundwork for future optimizations by the JVM, enabling more aggressive inlining and other optimizations based on the guarantee of finality.
Another thread of discussion revolves around the meaning of "final" in other languages and contexts. A commenter draws parallels to C++, noting the different meanings of "const" in different contexts and expressing a preference for Java's clearer distinction between final fields and final methods. This comparison highlights the nuances of the concept of "finality" in object-oriented programming.
One user brings up the issue of inner classes accessing fields of the enclosing class, a situation where the effective finality of fields is important for performance but not explicitly enforced by the
finalkeyword. They suggest that the JEP could clarify the treatment of such effectively final fields.Some users express general support for the JEP, viewing it as a step towards cleaner and more predictable code. They argue that the current behavior of "final" can be confusing and that enforcing its intended meaning would lead to more robust and maintainable programs.
Finally, there's a short discussion about the practicality of migrating existing codebases to comply with this change. One commenter suggests that automated tooling could help with the transition, mitigating the potential disruption.
Overall, the comments reflect a mixed reception to the proposed JEP. While some appreciate the stricter semantics and potential performance benefits, others express concerns about the impact on testing practices and the effort required for migration. The discussion highlights the complex trade-offs involved in evolving a mature language like Java.