How to initialize log4j properly?
log4j
Solution
`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
Problem
After adding log4j to my application I get the following output every time I execute my application: ``` log4j:WARN No appenders could be found for logger (slideselector.facedata.FaceDataParser). log4j:WARN Please initialize the log4j system properly. ``` It seems this means a configuration file is missing. Where should this config file be located and what is a good start content? I'm using plain java for developing a desktop application. So no webserver etc...