設置了 scanPeriod 之后，過了好長時間，都不生效，后來 debug 代碼。發現了下面這段。

  private volatile long mask = 0xF;
@Override
public FilterReply decide(Marker marker, Logger logger, Level level,
String format, Object[] params, Throwable t) {
if (!isStarted()) {
return FilterReply.NEUTRAL;
}
// for performance reasons, skip change detection (MASK-1) times out of MASK.
// Only once every MASK calls is change detection code executed
// Note that MASK is a variable itself.
if (((invocationCounter++) & mask) != mask) {
return FilterReply.NEUTRAL;
}
long now = System.currentTimeMillis();
synchronized (configurationWatchList) {
updateMaskIfNecessary(now);
if (changeDetected(now)) {
// Even though reconfiguration involves resetting the loggerContext,
// which clears the list of turbo filters including this instance, it is
// still possible for this instance to be subsequently invoked by another
// thread if it was already executing when the context was reset.
disableSubsequentReconfiguration();
detachReconfigurationToNewThread();
}
}
return FilterReply.NEUTRAL;
}

這行 if (((invocationCounter++) & mask) != mask) { mask = 0xf，其實要每循環 0xf 次，也就是 15 次，才會去 check 一次是否要更新，也就是說，不管過了多久，如果沒到這 15 次，也不會去檢查是否更新配置。
也就是說，我多打幾次 log，配置文件就生效了。

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