Java 21 introduces Scoped Values as a safer alternative to ThreadLocal

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Java's ThreadLocal class, available since version 1.2, allows threads to store independent copies of variables but carries risks such as memory leaks and data cross-contamination when threads are reused in pools. With Java 21's introduction of Virtual Threads, these problems are amplified, as millions of lightweight threads can each hold their own ThreadLocal copies, potentially overwhelming heap memory. To address these issues, Java 21 introduced Scoped Values (JEP 446), which bind data to a specific block of code rather than to the thread itself. Unlike ThreadLocal, Scoped Values are immutable, require no manual cleanup, and automatically release data when the code block exits. Developers are advised to prefer Scoped Values in new code, especially when working with virtual threads, while exercising extra caution if continuing to use ThreadLocal.

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