Advantage of log4j

Question

What\'s the advantage of log4j over set System.out and System.err to output to a log file?

Answer 1

Take a look and you will understand the power of log4j :

log4j.properties I used once for a project :

# ALL < DEBUG < INFO < WARN < ERROR < FATAL < OFF

# No appenders for rootLogger 
log4j.rootLogger=OFF

folder=..
prefix=
fileExtension=.log
htmlExtension=${fileExtension}.html
datestamp=yyyy-MM-dd/HH:mm:ss.SSS/zzz
layout=%d{${datestamp}} ms=%-4r [%t] %-5p %l %n%m %n%n

# myLogger logger
log4j.logger.myLogger=ALL, stdout, infoFile, infoHtml, errorFile

# stdout 
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=${layout}
# infoFile 
log4j.appender.infoFile=org.apache.log4j.FileAppender
log4j.appender.infoFile.File=${folder}/${prefix}_info${fileExtension}
log4j.appender.infoFile.layout=org.apache.log4j.PatternLayout
log4j.appender.infoFile.layout.ConversionPattern=${layout}
# infoHtml 
log4j.appender.infoHtml=org.apache.log4j.FileAppender
log4j.appender.infoHtml.File=${folder}/${prefix}_info${htmlExtension}
log4j.appender.infoHtml.layout=org.apache.log4j.HTMLLayout
log4j.appender.infoHtml.layout.Title=Logs
log4j.appender.infoHtml.layout.LocationInfo=true
# errorFile 
log4j.appender.errorFile=org.apache.log4j.FileAppender
log4j.appender.errorFile.File=${folder}/${prefix}_error${fileExtension}
log4j.appender.errorFile.layout=org.apache.log4j.PatternLayout
log4j.appender.errorFile.layout.ConversionPattern=${layout}

# APPENDERS SETTINGS
log4j.appender.stdout.Threshold = ALL
log4j.appender.infoFile.Threshold = INFO
log4j.appender.infoHtml.Threshold = INFO
log4j.appender.errorFile.Threshold = WARN.

To change the variables in your java code you can do :

Loading Configuration

Log4j will automatically load the configuration if it is stored in a file called "log4j.properties" and is present on the classpath under "" (e.g. WEB-INF/classes/log4j.properties).

I don't like that approach and prefer to load the configuration explicitly by calling:

PropertyConfigurator.configure( Config.ETC + "/log4j.properties" ); This way I can reload the configuration at any time as long as my application is still running. I like to add a button to an administrative jsp, "Reload Log4J".

Dynamic Log File Location

Many people complain that Log4j forces you to hard-code the location where your logs will be kept. Actually, it is possible to dynamically choose the log-file location, especially if you use the ${log.dir} property substitution technique above. Here's how:

String dynamicLog = // log directory somehow chosen... 
Properties p = new Properties( Config.ETC + "/log4j.properties" ); 
p.put( "log.dir", dynamicLog ); // overwrite "log.dir" 
PropertyConfigurator.configure( p );