Seeing errors or warnings in catalina.out about log4j

Issue

Warnings appear in tomcat/logs/catalina.out about log4j, such as:

log4j:WARN No appenders could be found for logger (net.bull.javamelody).
log4j:WARN Please initialize the log4j system properly.

Environment

CloudBees Jenkins Enterprise
Apache Tomcat container

Resolution

You just need to properly configure log4j.properties or log4j.xml. One of these files should exist in /WEB-INF/classes. If not create them.

Navigate here to the section Configuration to see what to include in this file: Apache log4j manual

One of the examples from that link for log4j.properties file:

# Set root logger level to DEBUG and its only appender to A1.
log4j.rootLogger=DEBUG, A1

# A1 is set to be a ConsoleAppender.
log4j.appender.A1=org.apache.log4j.ConsoleAppender

# A1 uses PatternLayout.
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n