Scoped Values, as proposed under JEP 446, are a preview feature in Java 21 that offers a scoping mechanism for sharing immutable data both within a thread and its child threads. They provide more predictability and efficiency than ThreadLocal variables, especially when many virtual threads are involved.
Some Key Features of Scoped Values:
Immutability: A scoped value is set and cannot be changed thereafter; this makes the data consistent and thread-safe.
Thread-Scoped Lifetime: The lifetime of a scoped value is confined to the execution scope of the thread; it automatically unbinds once the operation finishes, thus preventing memory leaks.
Efficient Inheritance: A child thread can inherit scoped values, especially in the case of structured concurrency, without the overhead that comes along with ThreadLocal inheritance.
Advantages over ThreadLocal:
Concurrency Simplified Reasoning: The immutability and scoped lifetime of these values make it easier to understand data flow within concurrent applications.
Performance Benefits: Due to their design, scoped values have lower space and time overhead compared to ThreadLocal variables; thus, they are better for applications using many virtual threads.
Automatic Cleanup: The value is unbound after the scope ends, avoiding memory leaks.
Use Case: Java 21 Scoped Values
A very basic use case of Scoped Values is to propagate user-specific information in multi-threaded environments, such as logging or request handling in web applications. It was achievable by using ThreadLocal earlier, but it came with baggage in terms of memory leaks and other complex management challenges, especially with virtual threads. With Scoped Values, this would be easier to manage while enforcing immutability and thread safety.
Example: Propagating User Context for Logging
Let's create a simple example where a ScopedValue is used to store and propagate user context (e.g., user ID) for logging in a multi-threaded application.
For detailed information about Scoped Values and how to use them, please refer to JEP 446: Scoped Values
