How to log to multiple log files with log4j

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We have a Java application that calls an external application and since it gets called quite often we figured we should probably log its standard output and standard error streams to a separate log file so we could easily locate issues with it later.  This would also allow us to create a different message format for it and different handlers.

Luckily, it turned out that this is pretty easy to do with log4j, simply edit your log4j.properties file and add a category (which is basically another logger).

For example:

# Add a new category/logger called "ExternalAppLogger" and specify
# the log level ("INFO") and the appender name ("extAppLogFile").
log4j.category.ExternalAppLogger=INFO, extAppLogFile

# Set the appender settings
log4j.appender.extAppLogFile=org.apache.log4j.RollingFileAppender
log4j.appender.extAppLogFile.File=D:/MyCoolApp/logs/external-app.log
log4j.appender.extAppLogFile.MaxFileSize=20000KB
log4j.appender.extAppLogFile.MaxBackupIndex=10
log4j.appender.extAppLogFile.layout=org.apache.log4j.PatternLayout
log4j.appender.extAppLogFile.layout.ConversionPattern=%d %5p %C{1}: %n%m%n

Now in your Java code, you can create a separate Logger object for the above:

Logger extAppLogger = Logger.getLogger("ExternalAppLogger")

extAppLogger.info("This message will be written to D:/MyCoolApp/logs/external-app.log.")

Source: http://www.jguru.com/faq/view.jsp?EID=1311014