public DailyRollingFileAppender() 
{

}

The default constructor does nothing.

 public DailyRollingFileAppender(Layout layout,
    String filename,
    String datePattern) throws IOException 
{
    super(layout, filename, true);
    this.datePattern = datePattern;
    activateOptions();
}

Instantiate a DailyRollingFileAppender and open the file designated by filename. The opened filename will become the ouput destination for this appender.

 public  void activateOptions() 
{
    super.activateOptions();
    if(datePattern != null && fileName != null) {
      now.setTime(System.currentTimeMillis());
      sdf = new SimpleDateFormat(datePattern);
      int type = computeCheckPeriod();
      printPeriodicity(type);
      rc.setType(type);
      File file = new File(fileName);
      scheduledFilename = fileName+sdf.format(new Date(file.lastModified()));
    } else {
      LogLog.error("Either File or DatePattern options are not set for appender ["
		   +name+"].");
    }
}

 int computeCheckPeriod() 
{
    RollingCalendar rollingCalendar = new RollingCalendar(gmtTimeZone, Locale.ENGLISH);
    // set sate to 1970-01-01 00:00:00 GMT
    Date epoch = new Date(0);
    if(datePattern != null) {
      for(int i = TOP_OF_MINUTE; i < = TOP_OF_MONTH; i++) {
	SimpleDateFormat simpleDateFormat = new SimpleDateFormat(datePattern);
	simpleDateFormat.setTimeZone(gmtTimeZone); // do all date formatting in GMT
	String r0 = simpleDateFormat.format(epoch);
	rollingCalendar.setType(i);
	Date next = new Date(rollingCalendar.getNextCheckMillis(epoch));
	String r1 =  simpleDateFormat.format(next);
	//System.out.println("Type = "+i+", r0 = "+r0+", r1 = "+r1);
	if(r0 != null && r1 != null && !r0.equals(r1)) {
	  return i;
	}
      }
    }
    return TOP_OF_TROUBLE; // Deliberately head for trouble...
}

 public String getDatePattern() 
{
    return datePattern;
}

Returns the value of the DatePattern option.

  void printPeriodicity(int type) 
{
    switch(type) {
    case TOP_OF_MINUTE:
      LogLog.debug("Appender ["+name+"] to be rolled every minute.");
      break;
    case TOP_OF_HOUR:
      LogLog.debug("Appender ["+name
		   +"] to be rolled on top of every hour.");
      break;
    case HALF_DAY:
      LogLog.debug("Appender ["+name
		   +"] to be rolled at midday and midnight.");
      break;
    case TOP_OF_DAY:
      LogLog.debug("Appender ["+name
		   +"] to be rolled at midnight.");
      break;
    case TOP_OF_WEEK:
      LogLog.debug("Appender ["+name
		   +"] to be rolled at start of week.");
      break;
    case TOP_OF_MONTH:
      LogLog.debug("Appender ["+name
		   +"] to be rolled at start of every month.");
      break;
    default:
      LogLog.warn("Unknown periodicity for appender ["+name+"].");
    }
}

  void rollOver() throws IOException 
{
    /* Compute filename, but only if datePattern is specified */
    if (datePattern == null) {
      errorHandler.error("Missing DatePattern option in rollOver().");
      return;
    }
    String datedFilename = fileName+sdf.format(now);
    // It is too early to roll over because we are still within the
    // bounds of the current interval. Rollover will occur once the
    // next interval is reached.
    if (scheduledFilename.equals(datedFilename)) {
      return;
    }
    // close current file, and rename it to datedFilename
    this.closeFile();
    File target  = new File(scheduledFilename);
    if (target.exists()) {
      target.delete();
    }
    File file = new File(fileName);
    boolean result = file.renameTo(target);
    if(result) {
      LogLog.debug(fileName +" - > "+ scheduledFilename);
    } else {
      LogLog.error("Failed to rename ["+fileName+"] to ["+scheduledFilename+"].");
    }
    try {
      // This will also close the file. This is OK since multiple
      // close operations are safe.
      this.setFile(fileName, false, this.bufferedIO, this.bufferSize);
    }
    catch(IOException e) {
      errorHandler.error("setFile("+fileName+", false) call failed.");
    }
    scheduledFilename = datedFilename;
}

Rollover the current file to a new file.

 public  void setDatePattern(String pattern) 
{
    datePattern = pattern;
}

The DatePattern takes a string in the same format as expected by SimpleDateFormat . This options determines the rollover schedule.

 protected  void subAppend(LoggingEvent event) 
{
    long n = System.currentTimeMillis();
    if (n  >= nextCheck) {
      now.setTime(n);
      nextCheck = rc.getNextCheckMillis(now);
      try {
	rollOver();
      }
      catch(IOException ioe) {
	LogLog.error("rollOver() failed.", ioe);
      }
    }
    super.subAppend(event);
}

This method differentiates DailyRollingFileAppender from its super class.

Before actually logging, this method will check whether it is time to do a rollover. If it is, it will schedule the next rollover time and then rollover.