How to initialize log4j properly?

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Log4j by default looks for a file called log4j.properties or log4j.xml on the classpath.

You can control which file it uses to initialize itself by setting system properties as described here (Look for the “Default Initialization Procedure” section).

For example:

java -Dlog4j.configuration=customName ....

Will cause log4j to look for a file called customName on the classpath.

If you are having problems I find it helpful to turn on the log4j.debug:

-Dlog4j.debug

It will print to System.out lots of helpful information about which file it used to initialize itself, which loggers / appenders got configured and how etc.

The configuration file can be a java properties file or an xml file. Here is a sample of the properties file format taken from the log4j intro documentation page:

log4j.rootLogger=debug, stdout, R

log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

# Pattern to output the caller's file name and line number.
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n

log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.File=example.log

log4j.appender.R.MaxFileSize=100KB
# Keep one backup file
log4j.appender.R.MaxBackupIndex=1

log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%p %t %c - %m%n

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