Decrease overhead of get-logger

The previously used static methods have baked-in setup overhead. Move that overhead to instantiation.
The change in the mean time to call get-logger on my laptop:
- slf4j-factory: from 10ns to 7ns
- cl-factory: from 198ns to 13ns
- log4j2-factory: from 2587ns to 14ns

Author: ataggart

CHANGELOG.md

@@ -6,6 +6,12 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ([despite its flaws](https://www.youtube.com/watch?v=oyLBGkS5ICk)).
 
 ## [Unreleased]
+### Changed
+- Decreased the per-call overhead when using SLF4J, Commons Logging, and Log4j2.
+  Previously, their associated `logger-factory` implementations were calling
+  library-specific static convenience methods for each call to `get-logger`.
+  These methods have some baked-in setup overhead, which now only occurs during
+  instantiation of the associated `logger-factory`.
 
 ## [1.0.0] - 2020-02-27
 ### Added

project.clj

@@ -15,4 +15,5 @@
                                         [log4j "1.2.16"]
                                         [org.apache.logging.log4j/log4j-api "2.8.2"]
                                         [org.apache.logging.log4j/log4j-core "2.8.2"]
-                                        [commons-logging "1.1.1"]]}})
+                                        [commons-logging "1.1.1"]
+                                        [criterium "0.4.5"]]}})

src/main/clojure/clojure/tools/logging/impl.clj

@@ -61,7 +61,8 @@
   []
   (when (class-found? "org.slf4j.Logger")
     (eval
-      `(do
+      `(let [; Same as is done inside LoggerFactory/getLogger(String).
+             factory# (org.slf4j.LoggerFactory/getILoggerFactory)]
         (extend org.slf4j.Logger
           Logger
           {:enabled?
@@ -98,15 +99,16 @@
           (name [_#]
             "org.slf4j")
           (get-logger [_# logger-ns#]
-            (org.slf4j.LoggerFactory/getLogger ^String (str logger-ns#))))))))
+            (.getLogger factory# ^String (str logger-ns#))))))))
 
 (defn cl-factory
   "Returns a Commons Logging-based implementation of the LoggerFactory protocol, or
   nil if not available."
   []
   (when (class-found? "org.apache.commons.logging.Log")
     (eval
-      `(do
+      `(let [; Same as is done inside LogFactory/getLog(String).
+             factory# (org.apache.commons.logging.LogFactory/getFactory)]
          (extend org.apache.commons.logging.Log
            Logger
            {:enabled?
@@ -142,7 +144,7 @@
            (name [_#]
              "org.apache.commons.logging")
            (get-logger [_# logger-ns#]
-             (org.apache.commons.logging.LogFactory/getLog (str logger-ns#))))))))
+             (.getInstance factory# (str logger-ns#))))))))
 
 (defn log4j-factory
   "Returns a Log4j-based implementation of the LoggerFactory protocol, or nil if
@@ -179,7 +181,9 @@
   []
   (when (class-found? "org.apache.logging.log4j.Logger")
     (eval
-      `(let [levels# {:trace org.apache.logging.log4j.Level/TRACE
+      `(let [; Same as is done inside LogManager/getLogger(String).
+             context# (org.apache.logging.log4j.LogManager/getContext false)
+             levels# {:trace org.apache.logging.log4j.Level/TRACE
                       :debug org.apache.logging.log4j.Level/DEBUG
                       :info  org.apache.logging.log4j.Level/INFO
                       :warn  org.apache.logging.log4j.Level/WARN
@@ -206,7 +210,7 @@
            (name [_#]
              "org.apache.logging.log4j")
            (get-logger [_# logger-ns#]
-             (org.apache.logging.log4j.LogManager/getLogger ^String (str logger-ns#))))))))
+             (.getLogger context# ^String (str logger-ns#))))))))
 
 (defn jul-factory
   "Returns a java.util.logging-based implementation of the LoggerFactory protocol,