ppkarwasz opened a new pull request, #2849:
URL: https://github.com/apache/logging-log4j2/pull/2849

   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.
   
   As remarkable side effect **token-based** matching of property names is no 
longer supported. This mean that, e.g. a system property like 
[`log4j2.asyncLoggerRingBufferSize`](https://logging.staged.apache.org/log4j/2.x/manual/systemproperties.html#log4j2.asyncLoggerRingBufferSize)
 can be written:
   
   - as `AsyncLogger.RingBufferSize` (pre-2.10 legacy form),
   - as `log4j2.asyncLoggerRingBufferSize` (2.10 canonical form).
   
   No other forms (e.g. `log4j.async.logger.ringBuffer.size`) are accepted.
   
   **Note**: We never documented that we support token-based matching. See 
[current Configuration 
Properties](https://logging.apache.org/log4j/2.x/manual/configuration.html#SystemProperties)
 documentation.
   


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