Obtaining daily log bckups using log4j

Daily Logs Using log4j

Log4j has a file appender which helps you to obtain log file backups In daily basis. This is DailyRollingFileAppender. A sample log4j.properties file is shown below.

log4j.rootLogger= R

log4j.appender.R=org.apache.log4j.DailyRollingFileAppender

log4j.appender.R.File=logs/My_Log.log

log4j.appender.R.DatePattern=’.'yyyy-MM-dd’

log4j.appender.R.layout=org.apache.log4j.PatternLayout

log4j.appender.R.layout.ConversionPattern=%5p (%F:%L) – %m%n

Date pattern is what is used by the log4j to identify the required backing up pattern. Here basically you can use any valid format in SimpleDateFormatter. Above date pattern will backup the log file in daily basis. A date pattern like log4j.appender.R.DatePattern=’.'yyyy-MM-dd-hh’ will produce backup files in each hour.

When Log4j make backup files

In my original example I have mentioned that the log file will be backed up in daily basis. But this I not entirely true. Ideally log4j should make the backup file at the midnight of each day. But it does not happen in that way.

Before each log operation, log4j will check the last modified date of the log file with the current date. If they are different log4j will rename the original log file as My_Log.log.<Last modified date of the file in given format>. And it will also make a new file named My_Log.log file to insert new logs. So in the backup files list if you saw that log files for some days are missing, do not panic. It is because there were no logs written in those particular dates.

How do I get a lof file as FILE_NAME.log

Generally file names of the backup files are FILE_NAME.log. <Last modified date of the file in given format>. (When you use the DailyRollingFileAppender). So the backup file are not .log files. So you cannot open them directly using a normal textpad. Most of the time you have to explicitely rename those files to .log files. This is tedious and time consuming. But there is a small and simple trick ( or a hack). Log4j directly use the datePattern attribute to generate the file names of the backup files. So log4j.appender.R.DatePattern=’.'yyyy-MM-dd’.log will make the backup file names as  FILE_NAME.log. <Last modified date of the file in given format>.log. And it will solve the problem.