Files.walkFileTree使用自定义FileVisitor泄漏目录描述符

Question

我在我的应用程序中遇到了一个奇怪的错误.我已经通过一种解决方法解决了这个问题,但我仍然很好奇为什么会发生这种错误.

下面给出了一个自定义FileVisitor的示例,该文件正在删除它遍历的空目录.如果目录不为空,并且仍然遍历这些目录,则会泄漏目录描述符.如果我使用lsof应用程序的PID,它将显示一堆指向相同的几个目录的描述符,它们遍历的目录.

private String getOldestFile() {
    fileVisitor.clearOldestFile();

    try {
        // FIXME: this was throwing FileSystemException: Too many open files after some time running. Leaking file descriptors!!
        Files.walkFileTree(Paths.get(csvPath), fileVisitor);
    } catch (IOException e) {
        e.printStackTrace();
    }

    return fileVisitor.getOldestFile().toString();
}

class CustomFileVisitor extends SimpleFileVisitor<Path> {
    private Path oldestFile = null;

    Path getOldestFile() {
        return oldestFile;
    }

    void clearOldestFile() {
        oldestFile = null;
    }

    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
        if (attrs.isDirectory())
            return FileVisitResult.CONTINUE;

        if (oldestFile == null)
            oldestFile = file;

        if (oldestFile.compareTo(file) > 0)
            oldestFile = file;

        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult postVisitDirectory(Path dir, IOException exc) throws IOException {
        if (dir.equals(Paths.get(csvPath)))
            return FileVisitResult.CONTINUE;

        if (Files.list(dir).collect(Collectors.toList()).size() == 0)
            Files.delete(dir); // throws an exception if folder is not empty -> mustn't delete folder with files

        return FileVisitResult.CONTINUE;
    }
}

CustomFileVisitor只在外部类中创建一次,并且定期调用该函数 filename = getOldestFile();

编辑:发布lsof -p {PID}输出.一开始,我发现PID就像这样.

这就是lsof -p {PID}输出的样子,只有成千上万的这些行."/ home/leon/Development/data /"是Files.walkFileTree的输入.

java    14965 leon  285r      DIR                8,2     4096  1970798 /home/leon/Development/data/2017
java    14965 leon  286r      DIR                8,2     4096  1970799 /home/leon/Development/data/2017/10
java    14965 leon  287r      DIR                8,2     4096  1970799 /home/leon/Development/data/2017/10
java    14965 leon  288r      DIR                8,2    36864  1970800 /home/leon/Development/data/2017/10/17
java    14965 leon  289r      DIR                8,2    36864  1970800 /home/leon/Development/data/2017/10/17
java    14965 leon  290r      DIR                8,2     4096  1970798 /home/leon/Development/data/2017
java    14965 leon  291r      DIR                8,2     4096  1970798 /home/leon/Development/data/2017
java    14965 leon  292r      DIR                8,2     4096  1970799 /home/leon/Development/data/2017/10
java    14965 leon  293r      DIR                8,2     4096  1970799 /home/leon/Development/data/2017/10
java    14965 leon  294r      DIR                8,2    36864  1970800 /home/leon/Development/data/2017/10/17
java    14965 leon  295r      DIR                8,2    36864  1970800 /home/leon/Development/data/2017/10/17

编辑2:我设法将问题隔离到这一行:Files.list(dir).collect(Collectors.toList()).size() == 0.这不应该是垃圾收集？

Answer 1

从Files#list()文档:

返回的流封装了一个DirectoryStream.如果需要及时处理文件系统资源,则应使用try-with-resources构造来确保在流操作完成后调用流的close方法.

最终,流将被垃圾收集,但不会立即收集.所以在这种情况下你必须自己管理它.