More Granular Bridging Functionality in Structured Concurrency

[Alan Bateman]

On 18/08/2025 16:53, Peter Eastham wrote:
> Hello Loom Team,
>
> While reading over the other thread concerning Joiners, I finally 
> loaded in the EA for Java 25 and took a look over the Java Doc. While 
> I'm liking what I'm seeing (and need to play around with it more), I'm 
> struggling to figure out how to bridge older approaches with this.
>
> For example, let's say Library A heavily utilizes a ThreadLocal. 
> Because it's a library we can't migrate to a ScopedValue or 
> InheritableThreadLocal. Currently the best bridge I've found is to 
> provide a Configuration with a custom ThreadFactory. This is a little 
> heavy handed, as you're really only wanting to hydrate a ThreadLocal, 
> not manage the entire Thread.

The ThreadLocal value may be mutable and/or otherwise unsafe to access 
from concurrent threads. ThreadLocals that are intended to be 
shared/inherited into other threads must be InheritableThreadLocals. 
InheritableThreadLocal has been mostly shunned by the eco system for a 
number of good reasons (performance being one). At the same 
time, InheritableThreadLocal does do what you want in a structured 
context, albeit with additional overhead when create threads for the 
subtasks, and possible side effects where the ITL is inherited further 
when subtasks happen to execute some code that creates a Thread.

So your opening paragraph on integrating with existing code and 
libraries is important, I don't think it's feasible to be able to treat 
ThreadLocals as inheritable.

-Alan