log4j使用DailyRollingFileAppender對日誌檔案按天進行歸檔時,可以使用datePattern指定歸檔檔名格式. 如日誌檔名為error.log,設定datePattern=yyyyMMdd,則歸檔檔名為error.logyyyyMMdd. 

這樣的歸檔檔名沒有統一的字尾名,看起來不是很美觀.於是我想,要是能將歸檔檔名設定為error_yyyyMMdd.log就好了.

分析DailyRollingFileAppender的原始碼,我們重點關注兩個函式

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+"].");
    }
  }
 

activeOptions函式,在初始化Appender時會被呼叫一次,設定scheduleFilename為當前日誌檔名+datePattern.

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函式每隔一段時間執行,首先使用日誌檔名+datePattern表示的當天時間賦值給dateFilename,再將dateFilename與scheduledFilename進行比較.如果兩者相等,則表示不需要進行檔案歸檔. 當一天結束進入下一天時,dateFilename發生變化,此時與scheduledFilename不相等,此時將當前日誌檔名重置成scheduledFilename,進行歸檔

從上面的分析可以看出,首先設定的datePattern只能新增到原檔名的後面.如果我們想實現將error.log檔案歸檔為error_yyyyMMdd.log需要對log4j做一下修改.

首先,修改datePattern,設定為

log4j.appender.error.datePattern='_'yyyyMMdd'.log'

接下來,建立我們自己的Appender,繼承DailyRollingFileAppender,過載上面兩個函式

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);
	      String tempFileName = fileName.substring(0, fileName.lastIndexOf("."));    
	      scheduledFilename = tempFileName+sdf.format(new Date(file.lastModified()));

	    } else {
	      LogLog.error("Either File or DatePattern options are not set for appender ["
			   +name+"].");
	    }
	  }

scheduledFilename會將當前檔案字尾名去掉,再拼接上datePattern,當然也可以按需要自定義.

void rollOver() throws IOException {

	    /* Compute filename, but only if datePattern is specified */
	    if (datePattern == null) {
	      errorHandler.error("Missing DatePattern option in rollOver().");
	      return;
	    }

	    String tempFileName = fileName.substring(0, fileName.lastIndexOf("."));    
	    String datedFilename = tempFileName+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函式中,同樣對datedFilename,也需要做同樣的處理,當發生歸檔時,會將歸檔檔名設定為scheduledFilename,再將datedFilename複製給scheduledFilename,準備下一次的歸檔.

通過過載,我們可以完全按照我們的自定義DailyRollingFileAppender的歸檔檔名了