使用 Files.walkFileTree 时如何忽略 AccessDeniedException？

Question

我想跟踪一个完整的硬盘分区（例如 D:），但出现以下异常：

AccessDeniedException  
java.nio.file.AccessDeniedException: D:System Volume Information
    at sun.nio.fs.WindowsException.translateToIOException(WindowsException.java:83)
    at sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:97)

如何忽略此异常并继续遍历文件树？

public class FileWatcher {
    private final WatchService watcher;
    private final Map<WatchKey, Path> keys;
    static Logger log = LoggerFactory.getLogger(GitCloneRepo.class);

    /**
     * Creates a WatchService and registers the given directory
     */
    FileWatcher(Path dir) throws IOException {
        this.watcher = FileSystems.getDefault().newWatchService();
        this.keys = new HashMap<WatchKey, Path>();

        walkAndRegisterDirectories(dir);
    }

    /**
     * Register the given directory with the WatchService; This function will be called by FileVisitor
     */
    private void registerDirectory(Path dir) throws IOException
    {
        WatchKey key = dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
        keys.put(key, dir);
    }

    /**
     * Register the given directory, and all its sub-directories, with the WatchService.
     */
    private void walkAndRegisterDirectories(final Path start) throws IOException {
        // register directory and sub-directories
        Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
            @Override
            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
                registerDirectory(dir);
                return FileVisitResult.CONTINUE;
            }
        });
    }

    /**
     * Process all events for keys queued to the watcher
     */
    void processEvents() {
        for (;;) {

            // wait for key to be signalled
            WatchKey key;
            try {
                key = watcher.take();
            } catch (InterruptedException x) {
                log.error("InterruptedException ",x);
                return;
            }

            Path dir = keys.get(key);
            if (dir == null) {
                log.warn("WatchKey not recognized!!");
                continue;
            }

            for (WatchEvent<?> event : key.pollEvents()) {
                @SuppressWarnings("rawtypes")
                WatchEvent.Kind kind = event.kind();

                // Context for directory entry event is the file name of entry
                @SuppressWarnings("unchecked")
                Path name = ((WatchEvent<Path>)event).context();
                Path child = dir.resolve(name);

                // print out event
                log.info("event.kind().name() {}: child {}", event.kind().name(), child);

                // if directory is created, and watching recursively, then register it and its sub-directories
                if (kind == ENTRY_CREATE) {
                    try {
                        if (Files.isDirectory(child)) {
                            walkAndRegisterDirectories(child);
                        }
                    } catch (IOException x) {
                        // do something useful
                    }
                }
            }

            // reset key and remove from set if directory no longer accessible
            boolean valid = key.reset();
            if (!valid) {
                keys.remove(key);

                // all directories are inaccessible
                if (keys.isEmpty()) {
                    break;
                }
            }
        }
    }


 //working code
    public static void main(String[] args) throws IOException {
        try{
        Path dir = Paths.get("D:");
        FileWatcher fileWatcher=new FileWatcher(dir);
        fileWatcher.processEvents();
        }

        catch (AccessDeniedException xx) {
            log.error("AccessDeniedException  ",xx);
        }
         catch (FileSystemException x) {
            log.error("exception",x);
        }
        }

编辑 这是完整的堆栈跟踪：

Apr 12, 2018 11:20:29 PM com.km.filewatcher.FileWatcher main
SEVERE: AccessDeniedException  
java.nio.file.AccessDeniedException: D:System Volume Information
    at sun.nio.fs.WindowsException.translateToIOException(WindowsException.java:83)
    at sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:97)
    at sun.nio.fs.WindowsException.rethrowAsIOException(WindowsException.java:102)
    at sun.nio.fs.WindowsDirectoryStream.<init>(WindowsDirectoryStream.java:86)
    at sun.nio.fs.WindowsFileSystemProvider.newDirectoryStream(WindowsFileSystemProvider.java:518)
    at java.nio.file.Files.newDirectoryStream(Files.java:457)
    at java.nio.file.FileTreeWalker.visit(FileTreeWalker.java:300)
    at java.nio.file.FileTreeWalker.next(FileTreeWalker.java:372)
    at java.nio.file.Files.walkFileTree(Files.java:2706)
    at java.nio.file.Files.walkFileTree(Files.java:2742)
    at com.km.filewatcher.FileWatcher.walkAndRegisterDirectories(FileWatcher.java:68)
    at com.km.filewatcher.FileWatcher.<init>(FileWatcher.java:51)
    at com.km.filewatcher.FileWatcher.main(FileWatcher.java:140)

Answer 1

你可以实现visitFileFailed方法和returnFileVisitResult.SKIP_SUBTREE来跳过运行进入AccessDeniedException时的目录。

public FileVisitResult visitFileFailed(T file, IOException exc) throws IOException

Invoked for a file that could not be visited. Unless overridden, this method re-throws the I/O exception that prevented the file from being visited.

Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
    @Override
    public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
        registerDirectory(dir);
        return FileVisitResult.CONTINUE;
    }
    
    @Override
    public FileVisitResult visitFileFailed(Path file, IOException exc) throws IOException {
        if (exc instanceof AccessDeniedException) {
            return FileVisitResult.SKIP_SUBTREE;
        }
        
        return super.visitFileFailed(file, exc);
    }
});

注意，如果它不是 AccessDeniedException 类型，这仍然会抛出异常。如果这不是您想要的行为，请直接 return SKIP_SUBTREE。