Add the followings in your Log4j configuration/properties file:
log4j.rootLogger=info, R log4j.appender.R=org.apache.log4j.DailyRollingFileAppender log4j.appender.R.File=filename log4j.appender.R.DatePattern='.'yyyy-MM-dd'.log' log4j.appender.R.layout=org.apache.log4j.PatternLayout log4j.appender.R.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS zzz} %5p %c{1}:%L - %m%n
The filename of the log file will look like this:
filename filename.2010-01-21.log filename.2010-01-22.log filename.2010-01-23.log