日誌檔案自動刪除功能必不可少，當然你可以讓運維去做這事，只是這不地道。而日誌元件是一個必備元件，讓其多做一件刪除的工作，無可厚非。本文就來探討下 log4j 的日誌檔案自動刪除實現吧。

0.自動刪除配置參考樣例: （log4j2.xml）

<?xml version="1.0" encoding="UTF-8" ?>
<Configuration status="warn" monitorInterval="30" strict="true"
        schema="Log4J-V2.2.xsd">
  <Properties>
    <Property name="log_level">info</Property>
  </Properties>
  <Appenders>
    <!-- 輸出到控制檯 -->
    <Console name="Console" target="SYSTEM_OUT">
      <ThresholdFilter level="${log_level}" onMatch="ACCEPT" onMismatch="DENY" />
      <PatternLayout pattern="%d{yyyy-MM-dd'T'HH:mm:ss.SSS} [%t] %p - %c - %m%n" />
    </Console>
    <!-- 與properties檔案中位置存在衝突，如有問題，請注意調整 -->
    <RollingFile name="logFile" fileName="logs/app/test.log"
           filePattern="logs/app/history/test-%d{MM-dd-yyyy}-%i.log.gz">
      <ThresholdFilter level="${log_level}" onMatch="ACCEPT" onMismatch="DENY" />
      <PatternLayout pattern="%d{yyyy-MM-dd'T'HH:mm:ss.SSS} [%p] [%c:%L] -- %m%n" />
      <Policies>
        <!-- 按天遞計算頻率 -->
        <TimeBasedTriggeringPolicy interval="1" />
        <SizeBasedTriggeringPolicy size="500 MB" />
        <OnStartupTriggeringPolicy />
      </Policies>
      <!-- 刪除策略配置 -->
      <DefaultRolloverStrategy max="5">
        <Delete basePath="logs/app/history" maxDepth="1">
          <IfFileName glob="*.log.gz"/>
          <IfLastModified age="7d"/>
        </Delete>
        <Delete basePath="logs/app/history" maxDepth="1">
          <IfFileName glob="*.docx"/>
        </Delete>
        <Delete basePath="logs/app/history" maxDepth="1">
          <IfFileName glob="*.vsdx"/>
        </Delete>
      </DefaultRolloverStrategy>
    </RollingFile>
    <Async name="Async" bufferSize="2000" blocking="false">
      <AppenderRef ref="logFile"/>
    </Async>
  </Appenders>

  <Loggers>
    <Root level="${log_level}">
      <AppenderRef ref="Console" />
      <AppenderRef ref="Async" />
    </Root>
    <!-- 配置個例 -->
    <Logger name="com.xx.filter" level="info" />
  </Loggers>
</Configuration>

　　如果僅想停留在使用層面，如上log4j2.xml配置檔案足矣！

　　不過，至少得注意一點，以上配置需要基於log4j2,而如果你是 log4j1.x,則需要做下無縫升級：主要就是換下jar包版本，換個橋接包之類的,比如下參考配置：

<dependency>
        <groupId>commons-logging</groupId>
        <artifactId>commons-logging</artifactId>
        <version>1.2</version>
      </dependency>
      <!-- 橋接：告訴commons logging使用Log4j2 -->
      <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>jcl-over-slf4j</artifactId>
        <version>1.7.26</version>
      </dependency>

      <!-- 此處老版本，需註釋掉 -->
      <!--<dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.17</version>
      </dependency>-->

      <dependency>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-compress</artifactId>
        <version>1.10</version>
      </dependency>
      <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-core</artifactId>
        <version>2.8.2</version>
      </dependency>
      <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-slf4j-impl</artifactId>
        <version>2.8.2</version>
      </dependency>
      <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-api</artifactId>
        <version>2.8.2</version>
      </dependency>
      <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-web</artifactId>
        <version>2.8.2</version>
      </dependency>

　如果還想多瞭解一點其執行原理，就跟隨本文的腳步吧:

1.自動清理大體執行流程

　　自動刪除工作的執行原理大體流程如下。（大抵都是如此）

　　　　1. 載入log4j2.xml配置檔案;
　　　　2. 讀取appenders，並新增到log4j上下文中;
　　　　3. 載入 policy,載入 rollover 配置;
　　　　4. 寫入日誌時判斷是否滿足rollover配置,預設是一天執行一次,可自行新增各種執行測試,比如大小、啟動時;

　　所以，刪除策略的核心是每一次新增日誌時。程式碼驗證如下：

// 在每次新增日誌時判定
  // org.apache.logging.log4j.core.appender.RollingRandomAccessFileAppender#append
  /**
   * Write the log entry rolling over the file when required.
   *
   * @param event The LogEvent.
   */
  @Override
  public void append(final LogEvent event) {
    final RollingRandomAccessFileManager manager = getManager();
    // 重點：直接檢查是否需要 rollover,如需要直接進行
    manager.checkRollover(event);

    // Leverage the nice batching behaviour of async Loggers/Appenders:
    // we can signal the file manager that it needs to flush the buffer
    // to disk at the end of a batch.
    // From a user's point of view,this means that all log events are
    // _always_ available in the log file,without incurring the overhead
    // of immediateFlush=true.
    manager.setEndOfBatch(event.isEndOfBatch()); // FIXME manager's EndOfBatch threadlocal can be deleted

    // LOG4J2-1292 utilize gc-free Layout.encode() method: taken care of in superclass
    super.append(event);
  }

  // org.apache.logging.log4j.core.appender.rolling.RollingFileManager#checkRollover
  /**
   * Determines if a rollover should occur.
   * @param event The LogEvent.
   */
  public synchronized void checkRollover(final LogEvent event) {
    // 由各觸發策略判定是否需要進行 rolling
    // 如需要,則呼叫 rollover()
    if (triggeringPolicy.isTriggeringEvent(event)) {
      rollover();
    }
  }

　　所以，何時進行刪除？答案是在適當的時機，這個時機可以是任意時候。

2. log4j 日誌滾動

　　日誌滾動，可以是重新命名，也可以是刪除檔案。但總體判斷是否可觸發滾動的前提是一致的。我們這裡主要關注檔案刪除。我們以時間作為依據看下判斷過程。

 // 1. 判斷是否是 觸發事件時機
  // org.apache.logging.log4j.core.appender.rolling.TimeBasedTriggeringPolicy#isTriggeringEvent
  /**
   * Determines whether a rollover should occur.
   * @param event  A reference to the currently event.
   * @return true if a rollover should occur.
   */
  @Override
  public boolean isTriggeringEvent(final LogEvent event) {
    if (manager.getFileSize() == 0) {
      return false;
    }
    final long nowMillis = event.getTimeMillis();
    // TimeBasedTriggeringPolicy,是基於時間判斷的,此處為每天一次
    if (nowMillis >= nextRolloverMillis) {
      nextRolloverMillis = manager.getPatternProcessor().getNextTime(nowMillis,interval,modulate);
      return true;
    }
    return false;
  }
  // org.apache.logging.log4j.core.appender.rolling.RollingFileManager#rollover()
  public synchronized void rollover() {
    if (!hasOutputStream()) {
      return;
    }
    // strategy 是xml配置的策略
    if (rollover(rolloverStrategy)) {
      try {
        size = 0;
        initialTime = System.currentTimeMillis();
        createFileAfterRollover();
      } catch (final IOException e) {
        logError("Failed to create file after rollover",e);
      }
    }
  }
  // RollingFileManager 統一管理觸發器
  // org.apache.logging.log4j.core.appender.rolling.RollingFileManager#rollover
  private boolean rollover(final RolloverStrategy strategy) {

    boolean releaseRequired = false;
    try {
      // Block until the asynchronous operation is completed.
      // 上鎖保證執行緒安全
      semaphore.acquire();
      releaseRequired = true;
    } catch (final InterruptedException e) {
      logError("Thread interrupted while attempting to check rollover",e);
      return false;
    }

    boolean success = true;

    try {
      // 由各觸發器執行 rollover 邏輯
      final RolloverDescription descriptor = strategy.rollover(this);
      if (descriptor != null) {
        writeFooter();
        closeOutputStream();
        if (descriptor.getSynchronous() != null) {
          LOGGER.debug("RollingFileManager executing synchronous {}",descriptor.getSynchronous());
          try {
            // 先使用同步方法，改名，然後再使用非同步方法操作更多
            success = descriptor.getSynchronous().execute();
          } catch (final Exception ex) {
            success = false;
            logError("Caught error in synchronous task",ex);
          }
        }
        // 如果配置了非同步器,則使用非同步進行 rollover
        if (success && descriptor.getAsynchronous() != null) {
          LOGGER.debug("RollingFileManager executing async {}",descriptor.getAsynchronous());
          // CompositeAction,使用非同步執行緒池執行使用者的 action
          asyncExecutor.execute(new AsyncAction(descriptor.getAsynchronous(),this));
          // 在非同步執行action期間，鎖是不會被釋放的，以避免執行緒安全問題
          // 直到非同步任務完成，再主動釋放鎖
          releaseRequired = false;
        }
        return true;
      }
      return false;
    } finally {
      if (releaseRequired) {
        semaphore.release();
      }
    }

  }

　　此處滾動有兩個處理點，1. 每個滾動策略可以自行處理業務； 2. RollingFileManager 統一管理觸發同步和非同步的滾動action；

3. DefaultRolloverStrategy 預設滾動策略驅動

　　DefaultRolloverStrategy 作為一個預設的滾動策略實現，可以配置多個 Action,然後處理刪除操作。

　　刪除有兩種方式: 1. 當次滾動的檔案數過多，會立即進行刪除; 2. 配置單獨的 DeleteAction,根據配置的具體策略進行刪除。（但該Action只會被返回給外部呼叫，自身則不會執行）

 // org.apache.logging.log4j.core.appender.rolling.DefaultRolloverStrategy#rollover
  /**
   * Performs the rollover.
   *
   * @param manager The RollingFileManager name for current active log file.
   * @return A RolloverDescription.
   * @throws SecurityException if an error occurs.
   */
  @Override
  public RolloverDescription rollover(final RollingFileManager manager) throws SecurityException {
    int fileIndex;
    // 預設 minIndex=1
    if (minIndex == Integer.MIN_VALUE) {
      final SortedMap<Integer,Path> eligibleFiles = getEligibleFiles(manager);
      fileIndex = eligibleFiles.size() > 0 ? eligibleFiles.lastKey() + 1 : 1;
    } else {
      if (maxIndex < 0) {
        return null;
      }
      final long startNanos = System.nanoTime();
      // 刪除case1: 獲取符合條件的檔案數，同時清理掉大於 max 配置的日誌檔案
      // 如配置 max=5,當前只有4個滿足時,不會立即清理檔案,但也不會阻塞後續流程
      // 只要沒有出現錯誤,fileIndex 不會小於0
      fileIndex = purge(minIndex,maxIndex,manager);
      if (fileIndex < 0) {
        return null;
      }
      if (LOGGER.isTraceEnabled()) {
        final double durationMillis = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startNanos);
        LOGGER.trace("DefaultRolloverStrategy.purge() took {} milliseconds",durationMillis);
      }
    }
    // 進入此區域即意味著，必然有檔案需要滾動，重新命名了
    final StringBuilder buf = new StringBuilder(255);
    manager.getPatternProcessor().formatFileName(strSubstitutor,buf,fileIndex);
    final String currentFileName = manager.getFileName();

    String renameTo = buf.toString();
    final String compressedName = renameTo;
    Action compressAction = null;

    FileExtension fileExtension = manager.getFileExtension();
    if (fileExtension != null) {
      renameTo = renameTo.substring(0,renameTo.length() - fileExtension.length());
      compressAction = fileExtension.createCompressAction(renameTo,compressedName,true,compressionLevel);
    }
    // 未發生檔案重新命名情況，即檔案未被重新命名未被滾動
    // 該種情況應該不太會發生
    if (currentFileName.equals(renameTo)) {
      LOGGER.warn("Attempt to rename file {} to itself will be ignored",currentFileName);
      return new RolloverDescriptionImpl(currentFileName,false,null,null);
    }
    // 新建一個重命令的 action,返回待用
    final FileRenameAction renameAction = new FileRenameAction(new File(currentFileName),new File(renameTo),manager.isRenameEmptyFiles());
    // 非同步處理器，會處理使用者配置的非同步action，如本文配置的 DeleteAction
    // 它將會在稍後被提交到非同步執行緒池中執行
    final Action asyncAction = merge(compressAction,customActions,stopCustomActionsOnError);
    // 封裝Rollover返回,renameAction 是同步方法,其他使用者配置的動態action 則是非同步方法
    // 刪除case2: 封裝非同步返回action
    return new RolloverDescriptionImpl(currentFileName,renameAction,asyncAction);
  }
  private int purge(final int lowIndex,final int highIndex,final RollingFileManager manager) {
    // 預設使用 accending 的方式進行清理檔案
    return useMax ? purgeAscending(lowIndex,highIndex,manager) : purgeDescending(lowIndex,manager);
  }
  // org.apache.logging.log4j.core.appender.rolling.DefaultRolloverStrategy#purgeAscending
  /**
   * Purges and renames old log files in preparation for rollover. The oldest file will have the smallest index,the
   * newest the highest.
   *
   * @param lowIndex low index. Log file associated with low index will be deleted if needed.
   * @param highIndex high index.
   * @param manager The RollingFileManager
   * @return true if purge was successful and rollover should be attempted.
   */
  private int purgeAscending(final int lowIndex,final RollingFileManager manager) {
    final SortedMap<Integer,Path> eligibleFiles = getEligibleFiles(manager);
    final int maxFiles = highIndex - lowIndex + 1;

    boolean renameFiles = false;
    // 依次迭代 eligibleFiles,刪除
    while (eligibleFiles.size() >= maxFiles) {
      try {
        LOGGER.debug("Eligible files: {}",eligibleFiles);
        Integer key = eligibleFiles.firstKey();
        LOGGER.debug("Deleting {}",eligibleFiles.get(key).toFile().getAbsolutePath());
        // 呼叫nio的介面刪除檔案
        Files.delete(eligibleFiles.get(key));
        eligibleFiles.remove(key);
        renameFiles = true;
      } catch (IOException ioe) {
        LOGGER.error("Unable to delete {},{}",eligibleFiles.firstKey(),ioe.getMessage(),ioe);
        break;
      }
    }
    final StringBuilder buf = new StringBuilder();
    if (renameFiles) {
      // 針對未完成刪除的檔案，繼續處理
      // 比如使用 匹配的方式匹配檔案,則不能被正常刪除
      // 還有些未超過maxFiles的檔案
      for (Map.Entry<Integer,Path> entry : eligibleFiles.entrySet()) {
        buf.setLength(0);
        // LOG4J2-531: directory scan & rollover must use same format
        manager.getPatternProcessor().formatFileName(strSubstitutor,entry.getKey() - 1);
        String currentName = entry.getValue().toFile().getName();
        String renameTo = buf.toString();
        int suffixLength = suffixLength(renameTo);
        if (suffixLength > 0 && suffixLength(currentName) == 0) {
          renameTo = renameTo.substring(0,renameTo.length() - suffixLength);
        }
        Action action = new FileRenameAction(entry.getValue().toFile(),true);
        try {
          LOGGER.debug("DefaultRolloverStrategy.purgeAscending executing {}",action);
          if (!action.execute()) {
            return -1;
          }
        } catch (final Exception ex) {
          LOGGER.warn("Exception during purge in RollingFileAppender",ex);
          return -1;
        }
      }
    }
    // 此處返回的 findIndex 一定是 >=0 的
    return eligibleFiles.size() > 0 ?
        (eligibleFiles.lastKey() < highIndex ? eligibleFiles.lastKey() + 1 : highIndex) : lowIndex;
  }

4. 符合過濾條件的檔案查詢

　　當配置了 max 引數，這個引數是如何匹配的呢？比如我某個資料夾下有很歷史檔案，是否都會匹配呢？

 // 檔案查詢規則 
  // org.apache.logging.log4j.core.appender.rolling.AbstractRolloverStrategy#getEligibleFiles
  protected SortedMap<Integer,Path> getEligibleFiles(final RollingFileManager manager) {
    return getEligibleFiles(manager,true);
  }
  protected SortedMap<Integer,Path> getEligibleFiles(final RollingFileManager manager,final boolean isAscending) {
    final StringBuilder buf = new StringBuilder();
    // 此處的pattern 即是在appender上配置的 filePattern,一般會受限於 MM-dd-yyyy-$i.log.gz
    String pattern = manager.getPatternProcessor().getPattern();
    // 此處會將時間替換為當前,然後按照此規則進行匹配要處理的檔案
    manager.getPatternProcessor().formatFileName(strSubstitutor,NotANumber.NAN);
    return getEligibleFiles(buf.toString(),pattern,isAscending);
  }
  // 細節匹配要處理的檔案
  protected SortedMap<Integer,Path> getEligibleFiles(String path,String logfilePattern,boolean isAscending) {
    TreeMap<Integer,Path> eligibleFiles = new TreeMap<>();
    File file = new File(path);
    File parent = file.getParentFile();
    if (parent == null) {
      parent = new File(".");
    } else {
      parent.mkdirs();
    }
    if (!logfilePattern.contains("%i")) {
      return eligibleFiles;
    }
    Path dir = parent.toPath();
    String fileName = file.getName();
    int suffixLength = suffixLength(fileName);
    if (suffixLength > 0) {
      fileName = fileName.substring(0,fileName.length() - suffixLength) + ".*";
    }
    String filePattern = fileName.replace(NotANumber.VALUE,"(\\d+)");
    Pattern pattern = Pattern.compile(filePattern);

    try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir)) {
      for (Path entry: stream) {
        // 該匹配相當精確
        // 只會刪除當天或者在時間交替的時候刪除上一天的資料咯
        // 如果在這個時候進行了重啟操作，就再也不會刪除此檔案了
        Matcher matcher = pattern.matcher(entry.toFile().getName());
        if (matcher.matches()) {
          Integer index = Integer.parseInt(matcher.group(1));
          eligibleFiles.put(index,entry);
        }
      }
    } catch (IOException ioe) {
      throw new LoggingException("Error reading folder " + dir + " " + ioe.getMessage(),ioe);
    }
    return isAscending? eligibleFiles : eligibleFiles.descendingMap();
  }
  // 此處會將 各種格式的檔名，替換為當前時間或者最後一次滾動的檔案的時間。所以匹配的時候，並不會匹配超時當前認知範圍的檔案
  /**
   * Formats file name.
   * @param subst The StrSubstitutor.
   * @param buf string buffer to which formatted file name is appended,may not be null.
   * @param obj object to be evaluated in formatting,may not be null.
   */
  public final void formatFileName(final StrSubstitutor subst,final StringBuilder buf,final boolean useCurrentTime,final Object obj) {
    // LOG4J2-628: we deliberately use System time,not the log4j.Clock time
    // for creating the file name of rolled-over files.
    final long time = useCurrentTime && currentFileTime != 0 ? currentFileTime :
        prevFileTime != 0 ? prevFileTime : System.currentTimeMillis();
    formatFileName(buf,new Date(time),obj);
    final LogEvent event = new Log4jLogEvent.Builder().setTimeMillis(time).build();
    final String fileName = subst.replace(event,buf);
    buf.setLength(0);
    buf.append(fileName);
  }

　　AsyncAction 是一個 Runnable 的實現,被直接提交到執行緒池執行. AsyncAction -> AbstractAction -> Action -> Runnable

　　它是一個統一管理非同步Action的包裝，主要是管理鎖和異常類操作。

 // org.apache.logging.log4j.core.appender.rolling.RollingFileManager.AsyncAction
  /**
   * Performs actions asynchronously.
   */
  private static class AsyncAction extends AbstractAction {

    private final Action action;
    private final RollingFileManager manager;

    /**
     * Constructor.
     * @param act The action to perform.
     * @param manager The manager.
     */
    public AsyncAction(final Action act,final RollingFileManager manager) {
      this.action = act;
      this.manager = manager;
    }

    /**
     * Executes an action.
     *
     * @return true if action was successful. A return value of false will cause
     *     the rollover to be aborted if possible.
     * @throws java.io.IOException if IO error,a thrown exception will cause the rollover
     *               to be aborted if possible.
     */
    @Override
    public boolean execute() throws IOException {
      try {
        // 門面呼叫 action.execute(),一般是呼叫 CompositeAction,裡面封裝了多個 action
        return action.execute();
      } finally {
        // 任務執行完成，才會釋放外部的鎖
        // 雖然不是很優雅，但是很準確很安全
        manager.semaphore.release();
      }
    }
    ...
  }

  // CompositeAction 封裝了多個 action 處理
  // org.apache.logging.log4j.core.appender.rolling.action.CompositeAction#run
  /**
   * Execute sequence of actions.
   *
   * @return true if all actions were successful.
   * @throws IOException on IO error.
   */
  @Override
  public boolean execute() throws IOException {
    if (stopOnError) {
      // 依次呼叫action
      for (final Action action : actions) {
        if (!action.execute()) {
          return false;
        }
      }

      return true;
    }
    boolean status = true;
    IOException exception = null;

    for (final Action action : actions) {
      try {
        status &= action.execute();
      } catch (final IOException ex) {
        status = false;

        if (exception == null) {
          exception = ex;
        }
      }
    }

    if (exception != null) {
      throw exception;
    }

    return status;
  }

　　DeleteAction是我們真正關心的動作。

 // CompositeAction 封裝了多個 action 處理
  // org.apache.logging.log4j.core.appender.rolling.action.CompositeAction#run
  /**
   * Execute sequence of actions.
   *
   * @return true if all actions were successful.
   * @throws IOException on IO error.
   */
  @Override
  public boolean execute() throws IOException {
    if (stopOnError) {
      // 依次呼叫action
      for (final Action action : actions) {
        if (!action.execute()) {
          return false;
        }
      }

      return true;
    }
    boolean status = true;
    IOException exception = null;

    for (final Action action : actions) {
      try {
        status &= action.execute();
      } catch (final IOException ex) {
        status = false;

        if (exception == null) {
          exception = ex;
        }
      }
    }

    if (exception != null) {
      throw exception;
    }

    return status;
  }
  
  // DeleteAction 做真正的刪除動作
  // org.apache.logging.log4j.core.appender.rolling.action.DeleteAction#execute()
  @Override
  public boolean execute() throws IOException {
    // 如果沒有script配置，則直接委託父類處理
    return scriptCondition != null ? executeScript() : super.execute();
  }
  org.apache.logging.log4j.core.appender.rolling.action.AbstractPathAction#execute()
  @Override
  public boolean execute() throws IOException {
    // 根據指定的basePath,和過濾條件，選擇相關檔案
    // 呼叫 DeleteAction 的 createFileVisitor(),返回 DeletingVisitor
    return execute(createFileVisitor(getBasePath(),pathConditions));
  }
  // org.apache.logging.log4j.core.appender.rolling.action.DeleteAction#execute(java.nio.file.FileVisitor<java.nio.file.Path>)
  @Override
  public boolean execute(final FileVisitor<Path> visitor) throws IOException {
    // 根據maxDepth設定，遍歷所有可能的檔案路徑
    // 使用 Files.walkFileTree() 實現,新增到 collected 中
    final List<PathWithAttributes> sortedPaths = getSortedPaths();
    trace("Sorted paths:",sortedPaths);

    for (final PathWithAttributes element : sortedPaths) {
      try {
        // 依次呼叫 visitFile,依次判斷是否需要刪除
        visitor.visitFile(element.getPath(),element.getAttributes());
      } catch (final IOException ioex) {
        LOGGER.error("Error in post-rollover Delete when visiting {}",element.getPath(),ioex);
        visitor.visitFileFailed(element.getPath(),ioex);
      }
    }
    // TODO return (visitor.success || ignoreProcessingFailure)
    return true; // do not abort rollover even if processing failed
  }

　　最終，即和想像的一樣：找到要查詢的資料夾，遍歷各檔案，用多個條件判斷是否滿足。刪除符合條件的檔案。

　　只是這其中注意的點：如何刪除檔案的執行緒安全性；如何保證刪除工作不影響業務執行緒；很常見的鎖和多執行緒的應用。

5.真正的刪除

　　真正的刪除動作就是在DeleteAction中配置的，但上面可以看它是呼叫visitor的visitFile方法，所以有必要看看是如何真正處理刪除的。（實際上前面在purge時已經做過一次刪除操作了，所以別被兩個點迷惑了，建議儘量只依賴於Delete配置，可以將外部max設定很大以避免兩處生效）

 // org.apache.logging.log4j.core.appender.rolling.action.DeletingVisitor#visitFile
  @Override
  public FileVisitResult visitFile(final Path file,final BasicFileAttributes attrs) throws IOException {
    for (final PathCondition pathFilter : pathConditions) {
      final Path relative = basePath.relativize(file);
      // 遍歷所有條件，只要有一個不符合，即不進行刪除。
      // 所以，所以條件是 AND 關係， 沒有 OR 關係
      // 如果想配置 OR 關係，只能配置多個DELETE
      if (!pathFilter.accept(basePath,relative,attrs)) {
        LOGGER.trace("Not deleting base={},relative={}",basePath,relative);
        return FileVisitResult.CONTINUE;
      }
    }
    // 直接刪除檔案
    if (isTestMode()) {
      LOGGER.info("Deleting {} (TEST MODE: file not actually deleted)",file);
    } else {
      delete(file);
    }
    return FileVisitResult.CONTINUE;
  }

　　刪除策略配置比如:

<RollingFile name="logFile" fileName="logs/app/test.log"
         filePattern="logs/app/history/test-%d{MM-dd-yyyy}-%i.log.gz">
    <ThresholdFilter level="${log_level}" onMatch="ACCEPT" onMismatch="DENY" />
    <PatternLayout pattern="%d{yyyy-MM-dd'T'HH:mm:ss.SSS} [%p] [%c:%L] -- %m%n" />
    <Policies>
      <!-- 按天遞計算頻率 -->
      <TimeBasedTriggeringPolicy interval="1" />
      <SizeBasedTriggeringPolicy size="500 MB" />
      <OnStartupTriggeringPolicy />
    </Policies>
    <!-- 刪除策略配置 -->
    <DefaultRolloverStrategy max="5000">
      <Delete basePath="logs/app/history" maxDepth="1">
        <!-- 配置且關係 -->
        <IfFileName glob="*.log.gz"/>
        <IfLastModified age="7d"/>
      </Delete>
      <!-- 配置或關係 -->
      <Delete basePath="logs/app/history" maxDepth="1">
        <IfFileName glob="*.docx"/>
      </Delete>
      <Delete basePath="logs/app/history" maxDepth="1">
        <IfFileName glob="*.vsdx"/>
      </Delete>
    </DefaultRolloverStrategy>
  </RollingFile>

　　另外說明，之所以能夠無縫替換，是因為利用了不同實現版本的 org/slf4j/impl/StaticLoggerBinder.class,而外部都使用 slf4j 介面定義實現的，比如 org.apache.logging.log4j:log4j-slf4j-impl 包的實現。

總結

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