Add support for log4j2.

Fixes TLOG-19.

README.md

@@ -1,6 +1,6 @@
 # Logging
 
-Logging macros which delegate to a specific logging implementation. At runtime a specific implementation is selected from, in order, slf4j, Apache commons-logging, log4j, and finally java.util.logging.
+Logging macros which delegate to a specific logging implementation. At runtime a specific implementation is selected from, in order, slf4j, Apache commons-logging, log4j2, log4j, and finally java.util.logging.
 
 Logging levels are specified by clojure keywords, in increasingly severe order:
 

pom.xml

@@ -42,6 +42,18 @@
       <version>1.2.16</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api</artifactId>
+      <version>2.8.2</version>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-core</artifactId>
+      <version>2.8.2</version>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>commons-logging</groupId>
       <artifactId>commons-logging</artifactId>

project.clj

@@ -9,4 +9,6 @@
   :profiles {:test {:dependencies [[org.slf4j/slf4j-api "1.6.2"]
                                    [org.slf4j/slf4j-log4j12 "1.6.2"]
                                    [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"]]}})

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

@@ -12,10 +12,10 @@
 ;; remove this notice, or any other, from this software.
 (ns ^{:author "Alex Taggart"
       :doc "Logging macros which delegate to a specific logging implementation. At
-            runtime a specific implementation is selected from, in order, slf4j,
-            Apache commons-logging, log4j, and finally java.util.logging.
+            runtime a specific implementation is selected by invoking
+            clojure.tools.logging.impl/find-factory.
 
-            The logging implementation can be explicitly determined by using
+            The logging implementation can be explicitly provided by using
             binding or alter-var-root to change the value of *logger-factory* to
             another implementation of clojure.tools.logging.impl/LoggerFactory
             (see also the *-factory functions in the impl namespace)."}

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

@@ -168,6 +168,37 @@
              (org.apache.log4j.Logger/getLogger ^String (str logger-ns#))))))
     (catch Exception e nil)))
 
+(defn log4j2-factory
+  "Returns a Log4j2-based implementation of the LoggerFactory protocol, or nil if
+  not available."
+  []
+  (try
+    (Class/forName "org.apache.logging.log4j.Logger")
+    (eval
+      `(let [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
+                      :error org.apache.logging.log4j.Level/ERROR
+                      :fatal org.apache.logging.log4j.Level/FATAL}]
+         (extend org.apache.logging.log4j.Logger
+           Logger
+           {:enabled?
+            (fn [^org.apache.logging.log4j.Logger logger# level#]
+              (.isEnabled logger# (get levels# level# level#)))
+            :write!
+            (fn [^org.apache.logging.log4j.Logger logger# level# e# msg#]
+              (let [level# (get levels# level# level#)]
+                (if e#
+                  (.log logger# level# msg# e#)
+                  (.log logger# level# msg#))))})
+         (reify LoggerFactory
+           (name [_#]
+             "org.apache.logging.log4j")
+           (get-logger [_# logger-ns#]
+             (org.apache.logging.log4j.LogManager/getLogger ^String (str logger-ns#))))))
+    (catch Exception e nil)))
+
 (defn jul-factory
   "Returns a java.util.logging-based implementation of the LoggerFactory protocol,
   or nil if not available."
@@ -202,10 +233,11 @@
 
 (defn find-factory
   "Returns the first non-nil value from slf4j-factory, cl-factory,
-   log4j-factory, and jul-factory."
+   log4j2-factory, log4j-factory, and jul-factory."
   []
   (or (slf4j-factory)
       (cl-factory)
+      (log4j2-factory)
       (log4j-factory)
       (jul-factory)
       (throw ; this should never happen in 1.5+

src/test/clojure/clojure/tools/logging/test_impl.clj

@@ -2,8 +2,20 @@
   (:use clojure.test)
   (:require [clojure.tools.logging.impl :as impl]))
 
-(def logger-factories [(impl/slf4j-factory)
+(let [[factory level] (try
+                        [(impl/log4j2-factory) (eval 'org.apache.logging.log4j.Level/FATAL)]
+                        (catch UnsupportedClassVersionError _
+                          ; This will happen when not using a 1.7+ jvm, e.g.,
+                          ; during matrix testing. Use the no-op factory
+                          ; to get past testing log4j2.
+                          [impl/disabled-logger-factory nil]))]
+  (def maybe-log4j2-factory factory)
+  (def maybe-log4j2-level level))
+
+(def logger-factories [impl/disabled-logger-factory
+                       (impl/slf4j-factory)
                        (impl/cl-factory)
+                       maybe-log4j2-factory
                        (impl/log4j-factory)
                        (impl/jul-factory)])
 
@@ -18,13 +30,17 @@
 (deftest test-enabled-non-kw-levels
   ; Just checking that the call doesn't blow up.
   (are [lf level] (let [logger (impl/get-logger lf "logger-ns")]
-                    (impl/enabled? logger level))
+                    (impl/enabled? logger level)
+                    true)
+       maybe-log4j2-factory maybe-log4j2-level
        (impl/log4j-factory) org.apache.log4j.Level/FATAL
        (impl/jul-factory)   java.util.logging.Level/SEVERE))
 
 (deftest test-write-non-kw-levels
   ; Just checking that the call doesn't blow up.
   (are [lf level] (let [logger (impl/get-logger lf "logger-ns")]
-                    (nil? (impl/write! logger level nil "message")))
+                    (impl/write! logger level nil "message")
+                    true)
+       maybe-log4j2-factory maybe-log4j2-level
        (impl/log4j-factory) org.apache.log4j.Level/FATAL
        (impl/jul-factory)   java.util.logging.Level/SEVERE))

src/test/clojure/log4j2.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Configuration status="WARN">
+  <Appenders>
+    <Console name="Console" target="SYSTEM_OUT">
+      <PatternLayout pattern="%d %-5p %c: %m%n"/>
+    </Console>
+  </Appenders>
+  <Loggers>
+    <Root level="fatal">
+      <AppenderRef ref="Console"/>
+    </Root>
+  </Loggers>
+</Configuration>