How do you propagate request context (trace id, user) across threads?

Context lives in ThreadLocals (MDC, SecurityContextHolder, tracing spans), and a ThreadLocal does not follow work to another thread. Submitting to an executor loses all of it, which is why async logs have no trace id.

The mechanisms, in increasing order of tidiness: capture the values before the handoff and restore them inside the task, usually via a decorator that wraps every Runnable; use the framework's own bridge (TaskDecorator, ContextSnapshot from Micrometer Context Propagation, Reactor's Context); or use ScopedValue, final since Java 25 and a preview before that, which structured concurrency forks inherit by design.

Whatever you use, the matching finally that clears the value belongs there too. Pooled threads keep whatever you leave behind, and leaked context is worse than missing context: the next request logs someone else's user id.