private void trigger(){
		
		Set<String> fileSignatureSet = filePathModifiedTimeMap.keySet();
		
		for ( String fileSignature : fileSignatureSet ){
			File file = new File(fileSignature);
			long currentLastModifiedTime = file.lastModified();
			Long lastestLastModifiedTime = filePathModifiedTimeMap.get(fileSignature);
			// if different, invoke onChanged method
			if( lastestLastModifiedTime!=null && 
					currentLastModifiedTime != lastestLastModifiedTime ){
				ResourceUpdaterListener listener = listenerMap.get(fileSignature);
				listener.onChanged();
				logger.info("Reloading the resource. signature = "+listener.signature());
				filePathModifiedTimeMap.put(fileSignature, currentLastModifiedTime);
				logger.info("Reload the resource successfully. signature = "+listener.signature());
			}
		}
		
	}