A log4j2 appender for Clojure

This library exposes the appender implementation for log4j2 that calls a user defined function for each log.

Requires JDK >= 11.

Getting Started

Add dependencies:

org.apache.logging.log4j/log4j-api {:mvn/version "2.13.3"}
org.apache.logging.log4j/log4j-core {:mvn/version "2.13.3"}
org.clojure/tools.logging {:mvn/version "1.1.0"}
funcool/log4j2-clojure {:mvn/version "2020.11.23-1"}

Configure logging (log4j2.xml):

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="info">
  <Appenders>
    <Console name="main" target="SYSTEM_OUT">
      <PatternLayout pattern="[%d{YYYY-MM-dd HH:mm:ss.SSS}] [%t] %level{length=1} %logger{36} - %msg%n"/>
    </Console>

    <CljFn name="test" ns="some.ns" fn="print-log">
      <PatternLayout pattern="[%d{YYYY-MM-dd HH:mm:ss.SSS}] [%t] %logger{36} - %msg%n"/>
    </CljFn>
  </Appenders>

  <Loggers>
    <Root level="info">
      <AppenderRef ref="main" />
      <AppenderRef ref="test" />
    </Root>
  </Loggers>
</Configuration>

This is a simple configuration that logs info (or greater levels) to the console and to the CljFn appender. The CljFn appender will try ti import the some.ns/print-log function and call it for each log entry.

Example:

(ns some.ns)

(defn print-log
  [event]
  (println (str event)))

The event is just a wrapper to the log4j2 LogEvent instance and it implementes IDeref for obtain the original LogEvent and .toString() to format the log using the configured layout (in this case the PatternLayout).

user=> (require '[clojure.tools.logging :as log])
user=> (log/info "foobar")
[2020-11-23 22:12:39.648] [main] user - foobar