On Wed, May 12, 2021 at 9:43 AM Andrew Haley <aph@redhat.com> wrote:
There's been considerable discussion about scope locals on the loom-dev list, and it's now time to open this to a wider audience. This subject is important because. although scope locals were motivated by the the needs of Loom, they have many potential applications outside that project.
I didn't get a chance to mention earlier, but I think scope locals are shaping up really great so far: simple yet powerful. I know this will be a really useful addition to the JDK. There are several projects both within and without Red Hat that I have authored or have contributed to which use a very compatible pattern (that is, using a lexically constrained binding strategy for context association) and are perfect candidates to switch over to use this feature. That said, I have one minor comment about the API. :-) I think it would be really nice if the snapshot class could hold its run/call method rather than making it a static method of `ScopeLocal`. This reads more naturally to me (and is nicer to write): var snap = ScopeLocal.snapshot(); return executor.submit(() -> snap.call(aCallable)); Than this: var snap = ScopeLocal.snapshot(); return executor.submit(() -> ScopeLocal.callWithSnapshot(aCallable, snap )); This kind of lexically bound contextual pattern can be found in several of our existing projects (including those listed below [1] [2]). In writing these projects, we took it a step further than just Runnable and Callable and added variants which accept `Consumer<T>` plus a `T` to pass in, a `Function<T, R>` plus a `T` to pass in, and `BiConsumer` and `BiFunction` variants as well which accept two arguments to pass in. The practical reason here is that we could then pass in a method handle with explicit arguments rather than relying on lambda capture, which can be expensive in certain circumstances. The overall benefit vs cost of doing this is probably debatable, but I thought the idea might be interesting at any rate. Anyway that's it. I love this feature and am excited to see it get into the JDK! [1] WildFly Elytron "Scoped" API, used by client and server authentication contexts (WildFly's version of JAAS Subject): https://github.com/wildfly-security/wildfly-elytron/blob/1.x/auth/server/bas... [2] WildFly Common "Contextual" API used for transaction, configuration, and other context propagation: https://github.com/wildfly/wildfly-common/blob/master/src/main/java/org/wild... -- - DML • he/him