[NIO.2] 第四十篇 监控目录树

现在，我们编写一个应用来监控 C:\rafaelnadal 目录。此外，如果在这个目录中发生了 CREATE 事件创建了一个新目录，那么这个目录将会被立即注册，和 C:\rafaelnadal 中一开始就存在的目录一样。

首先，创建监控服务：

private WatchService watchService = FileSystems.getDefault().newWatchService();

然后，需要注册目录树的创建、删除和修改事件。稍微麻烦的一点是我们需要注册  C:\rafaelnadal 的所有子目录，而不仅仅只是一个目录。因此需要递归遍历所有子目录，并将其独立注册到监控服务中。要完成这个任务可以使用 SimpleFileVisitor 类，并覆盖 preVisitDirectory 方法，如果你需要处理一些未知的遍历异常，还可以覆盖 visitFileFailed() 方法。接下来，我们按照前面这些描述编写一个方法，名为 registerTree()：

private void registerTree(Path start) throws IOException { 
 
  Files.walkFileTree(start, new SimpleFileVisitor<Path>() { 
 
     @Override 
     public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) 
                                                                          throws IOException { 
        System.out.println("Registering:" + dir); 
        registerPath(dir); 
        return FileVisitResult.CONTINUE; 
        } 
   }); 
}

如你所见，上面的方法将注册的代码单独放到了 registerPath() 方法中：

private void registerPath(Path path) throws IOException { 
 
  //register the received path 
  WatchKey key = path.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,  
                  StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE);
}

现在，所有  C:\rafaelnadal 树下的子目录都注册了新增，修改和删除事件。

接下来，我们需要使用无限循环捕获这些事件。当事件发生后，我们更关注是否是新增事件，如果是新增子目录，那么新增的子目录也需要立即调用  registerTree() 注册到监控服务中。现在有个问题是我们不知道 WatchKey 对应的 Path，因此没办法调用 registerTree() 进行注册。解决办法是将 WatchKey 和对应的 Path 存放到 HashMap 中，在 registerPath() 方法中更新 HashMap 的内容。这样，如果事件发生后，我们就可以直接从 HashMap 中获取 Path 对象：

private final Map<WatchKey, Path> directories = new HashMap<>(); 
… 
private void registerPath(Path path) throws IOException { 
  //register the received path 
  WatchKey key = path.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,   
                  StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE); 
 
  //store the key and path 
  directories.put(key, path); 
}

现在，在无限循环中可以从 HashMap 获取 Path：

… 
while (true) { 
  … 
  if (kind == StandardWatchEventKinds.ENTRY_CREATE) { 
      final Path directory_path = directories.get(key); 
      final Path child = directory_path.resolve(filename); 
 
      if (Files.isDirectory(child, LinkOption.NOFOLLOW_LINKS)) { 
        registerTree(child); 
      } 
  } 
… 
} 
…

HashMap 还可以作为结束无线循环的条件，当 WatchKey 无效时，从 HashMap 中移除，并在 HashMap 为空时跳出循环：

… 
while (true) { 
  … 
  //reset the key 
  boolean valid = key.reset(); 
 
  //remove the key if it is not valid 
  if (!valid) { 
      directories.remove(key); 
 
      if (directories.isEmpty()) { 
         break; 
      } 
  } 
} 
…

综上所述，编写一个完整的应用：

import java.io.IOException; 
import java.nio.file.FileSystems; 
import java.nio.file.FileVisitResult; 
import java.nio.file.Files; 
import java.nio.file.LinkOption; 
import java.nio.file.Path; 
import java.nio.file.Paths; 
import java.nio.file.SimpleFileVisitor; 
import java.nio.file.StandardWatchEventKinds; 
import java.nio.file.WatchEvent; 
import java.nio.file.WatchEvent.Kind; 
import java.nio.file.WatchKey; 
import java.nio.file.WatchService; 
import java.nio.file.attribute.BasicFileAttributes; 
import java.util.HashMap; 
import java.util.Map; 
 
class WatchRecursiveRafaelNadal { 
 
    private WatchService watchService; 
    private final Map<WatchKey, Path> directories = new HashMap<>(); 
 
    private void registerPath(Path path) throws IOException { 
        //register the received path 
        WatchKey key = path.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,  
                  StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE); 
 
        //store the key and path 
        directories.put(key, path); 
    } 
    private void registerTree(Path start) throws IOException { 
 
        Files.walkFileTree(start, new SimpleFileVisitor<Path>() { 
 
            @Override 
            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) 
                    throws IOException { 
                System.out.println("Registering:" + dir); 
                registerPath(dir); 
                return FileVisitResult.CONTINUE; 
            } 
        }); 
 
    } 
 
    public void watchRNDir(Path start) throws IOException, InterruptedException { 
 
        watchService = FileSystems.getDefault().newWatchService(); 
 
        registerTree(start); 
 
        //start an infinite loop 
        while (true) { 
 
            //retrieve and remove the next watch key 
            final WatchKey key = watchService.take(); 
 
            //get list of events for the watch key 
            for (WatchEvent<?> watchEvent : key.pollEvents()) { 
 
                //get the kind of event (create, modify, delete) 
                final Kind<?> kind = watchEvent.kind(); 
 
                //get the filename for the event 
                final WatchEvent<Path> watchEventPath = (WatchEvent<Path>) watchEvent; 
                final Path filename = watchEventPath.context(); 
 
                //handle OVERFLOW event 
                if (kind == StandardWatchEventKinds.OVERFLOW) { 
                    continue; 
                } 
 
                //handle CREATE event 
                if (kind == StandardWatchEventKinds.ENTRY_CREATE) { 
                    final Path directory_path = directories.get(key); 
                    final Path child = directory_path.resolve(filename); 
 
                    if (Files.isDirectory(child, LinkOption.NOFOLLOW_LINKS)) { 
                        registerTree(child); 
                    } 
                } 
                //print it out 
                System.out.println(kind + " -> " + filename); 
            } 
 
            //reset the key 
            boolean valid = key.reset(); 
 
            //remove the key if it is not valid 
            if (!valid) { 
                directories.remove(key); 
 
                //there are no more keys registered 
                if (directories.isEmpty()) { 
                    break; 
                } 
            } 
        } 
        watchService.close(); 
    } 
} 
 
public class Main { 
 
    public static void main(String[] args) { 
 
        final Path path = Paths.get("C:/rafaelnadal"); 
        WatchRecursiveRafaelNadal watch = new WatchRecursiveRafaelNadal(); 
 
        try { 
            watch.watchRNDir(path); 
        } catch (IOException | InterruptedException ex) { 
            System.err.println(ex); 
        } 
 
    } 
} 

测试的时候，可以在子目录中创建目录或文件，删除移动目录或文件，观察输出结果。

文章来源：http://www.aptusource.org/2014/04/nio-2-watching-a-directory-tree/