How to use Jenkins with a modern logging framework?

Question

Jenkins uses by default Java Utils Logging. This is a somewhat outdated logging framework that lacks many features of modern logging frameworks like Log4J or Logback.

How can Jenkins be deployed/configured on Tomcat using Logback or Log4J?

Answer 1

Jenkins uses mainly Java Utils Logging (JUL), some dependencies and plugins use SLF4J. JUL cannot be replaced, but there is the possibility to redirect logger output to SLF4J (see http://www.slf4j.org/legacy.html#jul-to-slf4j) and propagate log configuration to JUL when using Logback (http://logback.qos.ch/manual/configuration.html#LevelChangePropagator).

Jenkins bundles the slf4j-jdk14 JUL binding (see https://github.com/jenkinsci/jenkins/blob/jenkins-1.620/war/pom.xml#L144). This binding must be replaced with logback. That can be done either by patching the jenkins-war pom or by manually replacing WEB-INF/lib/slf4j-jdk14-1.7.7.jar with logback-classic-1.1.3.jar and logback-core-1.1.3.jar (Jenkins-1.620).

The JUL bridge can be installed by putting a logging.properties file (JULI configuration, see https://tomcat.apache.org/tomcat-7.0-doc/logging.html#Using_java.util.logging_%28default%29) at WEB-INF/classes/ containing

handlers = org.slf4j.bridge.SLF4JBridgeHandler
.handlers = org.slf4j.bridge.SLF4JBridgeHandler

Following an example logback configuration (to be put at WEB-INF/classes/logback.xml) that enables Logback internal logging (debug="true") and activates the LevelChangePropagator:

<configuration debug="true">
  <contextListener class="ch.qos.logback.classic.jul.LevelChangePropagator" />
  <appender name="WARN" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
      <fileNamePattern>/home/jenkins/work/log/warn-%d{yyyy-MM-dd}.%i.log</fileNamePattern>
      <timeBasedFileNamingAndTriggeringPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP">
        <maxFileSize>10MB</maxFileSize>
      </timeBasedFileNamingAndTriggeringPolicy>
      <maxHistory>50</maxHistory>
    </rollingPolicy>
    <encoder>
      <pattern>%date{HH:mm:ss.SSS} %-5level [%thread] %logger %msg%n</pattern>
    </encoder>
    <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
      <level>WARN</level>
    </filter>
  </appender>   
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%-4relative %-5level [%thread] %logger %msg %n</pattern>
    </encoder>
  </appender>
  <root level="debug">
    <appender-ref ref="WARN" />
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

Tomcat JULI logging is a Tomcat-specific LogManager (http://docs.oracle.com/javase/7/docs/api/java/util/logging/LogManager.html) implementation that allows a ClassLoader-specific JUL configuration. Thus the scope of the configuration above is specific to the Jenkins webapp.