Remove caching from `PropertiesUtil` (+#2849 review)
The `PropertiesUtil` class preemptively caches lookups for all known keys from enumerable property sources.
On my Debian system a JVM has around 60 among environment variables and system properties, which causes `PropertiesUtil` to perform around 60 lookups for **each** of the 3 standard property sources. Most of these properties have nothing to do with Log4j.
On the other hand all Log4j artifacts use no more than 100 configuration properties and the results of most of those calls are cache in static fields.
This PR removes the property value caches used by `PropertiesUtil`.
The change should have no noticeable impact on the loading time of Log4j Core, while at the same time simplifies the system.
9 files changed
