Remove log4j2.enable.threadlocals property

[vy]

This PR addresses #2105 and removes log4j2.enable.threadlocals property. While doing so, following major changes are also implemented:

• ThreadLocal-based recycler is removed
• ParametrizedMessageFactory is removed – switched to ReusableMessageFactory everywhere
• ThreadNameCachingStrategy is removed – Java 17 renders it redundant
• DefaultThreadContextMap is removed – Other implementations serve fine with recyclers, which don't have any ThreadLocal issues anymore. Weirdly enough, DefaultThreadContextMap, which is the fallback used when thread locals are disabled, was using thread locals too.

Question: Similar to ParametrizedMessageFactory, I doubt if DefaultLogEventFactory makes sense anymore. Can we make the ReusableLogEventFactory default instead?

[jvz]

Amazing diff!

[ppkarwasz]

There is something I don't understand about this PR: we do want a ThreadLocalRecyclerFactoryProvider out-of-the-box.

But we want to be enabled/disabled using the Recycler.factory = "threadLocal" and Recycler.factory = "dummy" properties.

It's great you removed all the trailing ThreadLocal instances, but I think you should restore the ThreadLocalRecyclerFactory.

I am not sure what recycler factory should be the default one though: Spring Boot applications usually contain a Servlet API, but have only a single app running, so a default of threadLocal sounds good to me. If there is a way to detect the usage of virtual threads, we could switch the default to "dummy".

[vy]

@ppkarwasz, I don't think you want a thread local recycler. 😜 In the face of virtual threads, thread locals are a risky liability. As a matter of fact, they already were actually; hence, several "if not running in a web application", "if only using JDK classes", etc. guards surrounding them. The new default, the queueing recycler, is almost as good as the thread local recycler. Those who need more efficiency, can use log4j-jctools too. All in all, no need for thread locals anymore really.

[vy]

@ppkarwasz, I placed the thread local recycler back. I want to move on with this PR.

[ppkarwasz]

LGTM

[rgoers]

I didn't see any of the doc files listed in the PR. If the property was removed it should be removed from the documentation as well.

[vy]

I didn't see any of the doc files listed in the PR. If the property was removed it should be removed from the documentation as well.

Fixed in e6044c9.