监听文件变化的实现有很多种，之前使用的是commons-VFS的api，使用起来有一些不是很方便，后来摸索了下，发现commons-io-2.0 也可以实现文件的监听。

今天就来介绍一下它的用法。

首先，继承FileAlterationListenerAdaptor 实现它的功能：

import java.io.File;

import org.apache.commons.io.monitor.FileAlterationListenerAdaptor;
import org.apache.commons.io.monitor.FileAlterationObserver;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class FileListerAdapter extends FileAlterationListenerAdaptor {
	private static final Logger logger = LoggerFactory.getLogger(FileListerAdapter.class);	
	
	@Override
	public void onFileChange(File file) {
		if (!file.exists() || !file.canRead()) {
			logger.info("The file {} is not exists or is not readable!", file);
			return;
		}
		logger.info("----The file {} is change.", file);
		//TODO 读取操作
		super.onFileChange(file);
	}

	@Override
	public void onFileCreate(File file) {
		if (!file.exists()) {
			logger.info("The file {} is not exists!", file);
			return;
		}
		if (!file.canRead()) {
			logger.info("The file {} is not readable!", file);
			return;
		}
		logger.info("----The file {} is created.", file);
		//TODO 读取操作
		super.onFileCreate(file);
	}

	@Override
	public void onFileDelete(File file) {
		logger.info("----The file {} is deleted.", file);
		super.onFileDelete(file);
	}

	@Override
	public void onDirectoryChange(File directory) {
		if (!directory.exists()) {
			logger.info("The directory {} is not exists!", directory);
			return;
		}
		logger.info("----The directory {} has changed.", directory);
		super.onDirectoryChange(directory);
	}

	@Override
	public void onDirectoryCreate(File directory) {
		if (!directory.exists()) {
			logger.info("The directory {} is not exists!", directory);
			return;
		}
		logger.info("----The directory {} is created.", directory);
		super.onDirectoryCreate(directory);
	}

	@Override
	public void onDirectoryDelete(File directory) {
		logger.info("----The directory {} is deleted.", directory);		
		super.onDirectoryDelete(directory);
	}
}

然后在你的程序中调用它：

FileAlterationMonitor monitor = new FileAlterationMonitor(1000L);// 每隔1000毫秒扫描一次
for (File listenFile:files) {
    FileAlterationObserver observer = new FileAlterationObserver(listenFile, new FileFilterImpl());
    FileListerAdapter listener = new FileListerAdapter();
    observer.addListener(listener);
    monitor.addObserver(observer);
}
monitor.start();

new FileAlterationObserver 的时候还需要使用到一个FileFilter 的接口实现，实现它的accept 方法即可。

这样就可以轻松实现对多个目录的文件监听，需要监听的文件可以使用正则表达式去过滤。