内部有“临界区”控制：
void
Appender::doAppend(const log4cplus::spi::InternalLoggingEvent& event)
{
LOG4CPLUS_BEGIN_SYNCHRONIZE_ON_MUTEX( access_mutex )
if(closed) {
getLogLog().error( LOG4CPLUS_TEXT("Attempted to append to closed appender named [")
+ name
+ LOG4CPLUS_TEXT("]."));
return;
}

if(!isAsSevereAsThreshold(event.getLogLevel())) {
return;
}

if(checkFilter(filter.get(), event) == DENY) {
return;
}

append(event);
LOG4CPLUS_END_SYNCHRONIZE_ON_MUTEX
}