Maybe InheritableThreadLocal is what you’re looking for?
I’m not sure if it’s exactly the same, but as far as I understand it meets this requirement:
a logical request can spawn a hierarchy of new threads – and each of those threads will have access to the same TLS of the original thread.
From the docs
This class extends ThreadLocal to provide inheritance of values from parent thread to child thread: when a child thread is created, the child receives initial values for all inheritable thread-local variables for which the parent has values. Normally the child’s values will be identical to the parent’s; however, the child’s value can be made an arbitrary function of the parent’s by overriding the childValue method in this class.
Inheritable thread-local variables are used in preference to ordinary thread-local variables when the per-thread-attribute being maintained in the variable (e.g., User ID, Transaction ID) must be automatically transmitted to any child threads that are created.
I don’t know about the “copy on write” capability you mentioned, but I guess you can override InheritableThreadLocal.childValue(T) to proxy the parent’s value so that writes don’t go through to the parent and modify the current thread’s local storage