OpenJDK Proposal for Immutable Data Sharing Across Threads
Java is considering a significant enhancement for managing data in multi-threaded environments, with a new OpenJDK proposal that could enable the safe sharing of immutable data across threads. This initiative, focused on “extent-local variables,” is an incubating API currently being discussed for potential inclusion in future Java releases. The goal of this proposal is to improve the management of shared data, particularly in the context of applications that use virtual threads, which are becoming increasingly prevalent due to their ability to efficiently handle large-scale concurrency.
One of the key benefits of extent-local variables, as proposed, is their ability to facilitate data sharing within a single thread and with child threads. This is especially valuable when dealing with virtual threads, which are lightweight and can be spawned in large numbers. The proposal suggests that extent-local variables offer a more efficient and easier-to-use alternative to thread-local variables, which can become complex and inefficient when managing data across many threads. The design is intended to simplify reasoning about data flow and ensure that data sharing is both intuitive and maintainable.
The proposal emphasizes four core principles that drive the design of the extent-local variables API: ease of use, comprehensibility, robustness, and performance. By treating shared data as immutable, the API would allow multiple threads to safely access and share data without the risk of modification, significantly reducing the need for synchronization mechanisms like locks. This immutability also enables the runtime to optimize performance by minimizing contention and facilitating more efficient memory usage. Moreover, the syntactic structure of the code would make the lifetime of shared data explicitly clear, making it easier for developers to track and manage.
While the extent-local variables API is still in its early stages and is not slated for any specific Java version, it is possible that it could be included in Java 20, which is expected to release in March 2023. However, Java 19, which is set to arrive in September 2022, will not feature this new capability, as it is closed to new features at this stage. If adopted, this proposal could mark a significant step forward in Java’s concurrency model, enabling developers to handle shared data across threads in a more efficient, safe, and performant way.