spring boot에서
~~~xml
<springProfile name="fileLog">
    <appender name="consoleAppender" class="ch.qos.logback.core.helpers.NOPAppender" />
    <appender name="fileAppender" class="ch.qos.logback.core.rolling.RollingFileAppender">
      <file>${LOG_FILE}</file>
      <encoder>
        <pattern>${FILE_PATTERN}</pattern>
      </encoder>
      <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
        <fileNamePattern>${LOG_FILE}.%d{yyyy-MM-dd}</fileNamePattern>
        <maxHistory>5</maxHistory>
      </rollingPolicy>
    </appender>
  </springProfile>
~~~
이런식으로 rolling을 지정해줄 수 있다. 어떻게 rolling이 되는 걸까?

우선 두가지를 봐야하는데, 현재 위의 logback configuration에서는 RollingFileAppender를 사용하며, Rolling Policy로 TimeBasedRollingPolicy를 사용한다.
RollingFileAppender의
~~~java
public void rollover() {
    lock.lock();
    try {
        // Note: This method needs to be synchronized because it needs exclusive
        // access while it closes and then re-opens the target file.
        //
        // make sure to close the hereto active log file! Renaming under windows
        // does not work for open files.
        this.closeOutputStream();
        attemptRollover();
        attemptOpenFile();
    } finally {
        lock.unlock();
    }
}
~~~
를 확인해보면 ```attemptRollover()```라는 함수가 있다.
~~~java
private void attemptRollover() {
    try {
        rollingPolicy.rollover();
    } catch (RolloverFailure rf) {
        addWarn("RolloverFailure occurred. Deferring roll-over.");
        // we failed to roll-over, let us not truncate and risk data loss
        this.append = true;
    }
}
~~~
를 보면 rollingPolicy의 rollover함수를 실행시키고, TimeBasedRollingPolicy에서 이를 확인해보면
~~~java
public void rollover() throws RolloverFailure {

    // when rollover is called the elapsed period's file has
    // been already closed. This is a working assumption of this method.

    String elapsedPeriodsFileName = timeBasedFileNamingAndTriggeringPolicy.getElapsedPeriodsFileName();

    String elapsedPeriodStem = FileFilterUtil.afterLastSlash(elapsedPeriodsFileName);

    if (compressionMode == CompressionMode.NONE) {
        if (getParentsRawFileProperty() != null) {
            renameUtil.rename(getParentsRawFileProperty(), elapsedPeriodsFileName);
        } // else { nothing to do if CompressionMode == NONE and parentsRawFileProperty == null }
    } else {
        if (getParentsRawFileProperty() == null) {
            compressionFuture = compressor.asyncCompress(elapsedPeriodsFileName, elapsedPeriodsFileName, elapsedPeriodStem);
        } else {
            compressionFuture = renameRawAndAsyncCompress(elapsedPeriodsFileName, elapsedPeriodStem);
        }
    }

    if (archiveRemover != null) {
        Date now = new Date(timeBasedFileNamingAndTriggeringPolicy.getCurrentTime());
        this.cleanUpFuture = archiveRemover.cleanAsynchronously(now);
    }
}
~~~
rename을 하는 것을 확인할 수 있다. 그리고 다시 ```rollover```의 ```attemptOpenFile```를 확인해보면
~~~java
private void attemptOpenFile() {
    try {
        // update the currentlyActiveFile LOGBACK-64
        currentlyActiveFile = new File(rollingPolicy.getActiveFileName());

        // This will also close the file. This is OK since multiple close operations are safe.
        this.openFile(rollingPolicy.getActiveFileName());
    } catch (IOException e) {
        addError("setFile(" + fileName + ", false) call failed.", e);
    }
}
~~~
새롭게 activeFileName을 이용하여 file을 생성함을 알 수 있다.