Limiting the Size of a Log by Using a Rotating Sequence of Files
The example Limiting the Size of a Log File shows how to
limit the log file size by automatically emptying it when it reaches
the limit. However, this approach has the disadvantage of discarding
useful information even though the file size might be well within the
limit. The FileHandler allows a more effective approach by allowing
you to use a sequence of files to hold the log information. When a
file fills up, the oldest file is emptied and logging resumes in that
file.
More specifically, if there are N log files in the
sequence, records are always dumped into logfile0. When
logfile0 is filled, logfileN-2 is renamed to logfileN,
logfileN-3 is renamed to logfileN-2, etc. Finally,
logfile0 is renamed logfile1. A new logfile0 is created
and logging resumes in the new logfile0. To read the log records
in chronological order, you need to process the files from
logfileN-1 to logfile0.
The logfile number is called the generation number and ranges
from 0 to the number of logfiles – 1. When specifying the filename
pattern to use for the logfiles, you need to include the location of
the generation number using the %g placeholder. For example,
using a filename pattern of my%g.log with three log files will result
in the files my0.log, my1.log, and my2.log.
try {
// Create a file handler that uses 3 logfiles, each with a limit of 1Mbyte
String pattern = "my%g.log";
int limit = 1000000; // 1 Mb
int numLogFiles = 3;
FileHandler fh = new FileHandler(pattern, limit, numLogFiles);
// Add to logger
Logger logger = Logger.getLogger("com.mycompany");
logger.addHandler(fh);
} catch (IOException e) {
}
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