question: private boolean isCacheValid ( ) { long now = CurrentTime . currentTime ( ) ; if ( ( now - _lastTime < 100 ) && ! CurrentTime . isTest ( ) ) return true ; long oldLastModified = _lastModified ; long oldLength = _length ; long newLastModified = _backing . getLastModified ( ) ; long newLength = _backing . length ( ) ; _lastTime = now ; if ( newLastModified == oldLastModified && newLength == oldLength ) { _lastTime = now ; return true ; } else { _changeSequence . incrementAndGet ( ) ; clearCache ( ) ; _depend = null ; _isSigned = null ; _zipEntryCache . clear ( ) ; _lastModified = newLastModified ; _length = newLength ; _lastTime = now ; return false ; } }
target:  Returns the last modified time for the path.