A new JDK Enhancement Proposal (JEP) is setting the stage for a significant change in how Java handles final fields. The proposed plan, introduced in early February and updated on March 31, aims to strengthen the integrity of Java programs by restricting the ability to mutate final fields via deep reflection. This initiative will issue warnings to developers who use deep reflection to mutate final fields, preparing the Java ecosystem for a future release that will enforce these restrictions by default. The goal is to ensure that final fields remain truly immutable, which could make Java programs safer and potentially improve performance by allowing more reliable optimizations.
The primary objective of the proposal is to prepare Java developers for a version of the language that will prevent final field mutation unless explicitly enabled. In other words, from that future release onward, developers will have to explicitly configure the JVM to allow mutations of final fields at startup. The plan also seeks to align the behavior of final fields in regular classes with the immutability inherent to record classes, which already cannot be mutated via deep reflection. While this restriction aims to enhance the safety of Java applications, it will not break compatibility with serialization libraries. These libraries, which often need to mutate final fields during deserialization, will continue to work as usual, thanks to a special-purpose API that will allow this specific functionality.
Motivating this change, the proposal highlights the fact that developers often rely on final fields to represent immutable states within their programs. This assumption that a final field cannot be reassigned is critical for reasoning about program correctness. However, the current reality is that Java provides APIs that allow any part of the program to mutate final fields, undermining the expected immutability. This creates a significant gap between what developers expect and what is actually possible, leading to potential bugs and missed optimizations. In this context, final fields are as mutable as non-final fields, which poses a risk to both the safety and performance of Java applications.
Despite the fact that only a small amount of code currently mutates final fields, the existence of such APIs prevents developers and the JVM from trusting the immutability of final fields, which in turn hampers performance and security. The proposed change aims to resolve this by enforcing the immutability of final fields across the board, disallowing deep reflection-based mutations by default. There will be an exception for serialization libraries that need to mutate final fields during deserialization, but this will be handled through a controlled, limited API, ensuring that developers’ reliance on final fields for immutability remains intact while still allowing necessary flexibility for specialized use cases.