为什么java DirectoryStream的执行速度如此之慢？

时间：2015-06-18 16:06:39

标签： java performance stream nio

我已经使用nio-package的DirectoryStreams特别对Streams进行了一些测试。我只是尝试获取目录中按上次修改日期和大小排序的所有文件的列表。

旧File.listFiles（）的JavaDoc向文件 s 中的方法声明了Note：

  

请注意，Files类将newDirectoryStream方法定义为   打开一个目录并遍历该文件的名称   目录。使用非常大的资源时，这可能会占用更少的资源   目录。

我在下面运行代码很多次（前三次）：

首轮：

Run time of Arrays.sort: 1516
Run time of Stream.sorted as Array: 2912
Run time of Stream.sorted as List: 2875

第二运行：

Run time of Arrays.sort: 1557
Run time of Stream.sorted as Array: 2978
Run time of Stream.sorted as List: 2937

第三运行：

Run time of Arrays.sort: 1563
Run time of Stream.sorted as Array: 2919
Run time of Stream.sorted as List: 2896

我的问题是：为什么流表现得如此糟糕？

import java.io.File;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.FileTime;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FileSorter {

  // This sorts from old to young and from big to small
  Comparator<Path> timeSizeComparator = (Path o1, Path o2) -> {
    int sorter = 0;
    try {
      FileTime lm1 = Files.getLastModifiedTime(o1);
      FileTime lm2 = Files.getLastModifiedTime(o2);
      if (lm2.compareTo(lm1) == 0) {
        Long s1 = Files.size(o1);
        Long s2 = Files.size(o2);
        sorter = s2.compareTo(s1);
      } else {
        sorter = lm1.compareTo(lm2);
      }
    } catch (IOException ex) {
      throw new UncheckedIOException(ex);
    }
    return sorter;
  };

  public String[] getSortedFileListAsArray(Path dir) throws IOException {
    Stream<Path> stream = Files.list(dir);
    return stream.sorted(timeSizeComparator).
            map(Path::getFileName).map(Path::toString).toArray(String[]::new);
  }

  public List<String> getSortedFileListAsList(Path dir) throws IOException {
    Stream<Path> stream = Files.list(dir);
    return stream.sorted(timeSizeComparator).
            map(Path::getFileName).map(Path::toString).collect(Collectors.
            toList());
  }

  public String[] sortByDateAndSize(File[] fileList) {
    Arrays.sort(fileList, (File o1, File o2) -> {
      int r = Long.compare(o1.lastModified(), o2.lastModified());
      if (r != 0) {
        return r;
      }
      return Long.compare(o1.length(), o2.length());
    });
    String[] fileNames = new String[fileList.length];
    for (int i = 0; i < fileNames.length; i++) {
      fileNames[i] = fileList[i].getName();
    }
    return fileNames;
  }

  public static void main(String[] args) throws IOException {
    // File (io package)
    File f = new File("C:\\Windows\\system32");
    // Path (nio package)
    Path dir = Paths.get("C:\\Windows\\system32");

    FileSorter fs = new FileSorter();

    long before = System.currentTimeMillis();
    String[] names = fs.sortByDateAndSize(f.listFiles());
    long after = System.currentTimeMillis();
    System.out.println("Run time of Arrays.sort: " + ((after - before)));

    long before2 = System.currentTimeMillis();
    String[] names2 = fs.getSortedFileListAsArray(dir);
    long after2 = System.currentTimeMillis();
    System.out.
            println("Run time of Stream.sorted as Array: " + ((after2 - before2)));

    long before3 = System.currentTimeMillis();
    List<String> names3 = fs.getSortedFileListAsList(dir);
    long after3 = System.currentTimeMillis();
    System.out.
            println("Run time of Stream.sorted as List: " + ((after3 - before3)));
  }
}

更新

应用Peter的代码后，我得到了这样的结果：

Run time of Arrays.sort: 1615
Run time of Stream.sorted as Array: 3116
Run time of Stream.sorted as List: 3059
Run time of Stream.sorted as List with caching: 378

更新2

在对Peter的解决方案做了一些研究之后，我可以说，阅读文件属性为ex。 Files.getLastModified必须是一个沉重的紧缩。仅将比较器中的该部分更改为：

  Comparator<Path> timeSizeComparator = (Path o1, Path o2) -> {
    File f1 = o1.toFile();
    File f2 = o2.toFile();
    long lm1 = f1.lastModified();
    long lm2 = f2.lastModified();
    int cmp = Long.compare(lm2, lm1);
    if (cmp == 0) {
      cmp = Long.compare(f2.length(), f1.length());
    }
    return cmp;
  };

在我的电脑上获得更好的结果：

Run time of Arrays.sort: 1968
Run time of Stream.sorted as Array: 1999
Run time of Stream.sorted as List: 1975
Run time of Stream.sorted as List with caching: 488

但正如您所看到的，缓存对象是最好的方法。正如jtahlborn所说，它是一种稳定的类型。

更新3（我找到的最佳解决方案）

经过一番研究后，我发现，Files.lastModified和Files.size这两个方法在同一个方面做了大量工作：属性。所以我做了三个版本的PathInfo类来测试：

  1. Peters版本如下所述
  2. 旧样式文件版本，我在构造函数中执行一次Path.toFile（）并使用f.lastModified和f.length获取该文件中的所有值
  3. Peters解决方案的一个版本，但现在我用Files.readAttributes（path，BasicFileAttributes.class）读取了一个Attribute对象，并在此处做了一些事情。

    4. 将它全部放在一个循环中，每次100次，我想出了这些结果：

    After doing all hundred times
Mean performance of Peters solution: 432.26
Mean performance of old File solution: 343.11
Mean performance of read attribute object once solution: 255.66

    最佳解决方案的PathInfo构造函数中的代码：

    public PathInfo(Path path) {
  try {
    // read the whole attributes once
    BasicFileAttributes bfa = Files.readAttributes(path, BasicFileAttributes.class);
    fileName = path.getFileName().toString();
    modified = bfa.lastModifiedTime().toMillis();
    size = bfa.size();
  } catch (IOException ex) {
    throw new UncheckedIOException(ex);
  }
}

    我的结果：永远不要读取属性两次，而在对象中缓存会破坏性能。

1 个答案:

答案 0 :(得分：4)

Files.list（）是O（N）操作，而排序是O（N log N）。排序中的操作更有可能是重要的。鉴于比较不做同样的事情，这是最可能的解释。在C：/ Windows / System32下有很多具有相同修改日期的文件，这意味着将经常检查大小。

为了显示大多数时间没有花费在FIles.list（dir）Stream中，我已经优化了比较，因此每个文件只获取一次文件的数据。

import java.io.File;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.FileTime;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FileSorter {

    // This sorts from old to young and from big to small
    Comparator<Path> timeSizeComparator = (Path o1, Path o2) -> {
        int sorter = 0;
        try {
            FileTime lm1 = Files.getLastModifiedTime(o1);
            FileTime lm2 = Files.getLastModifiedTime(o2);
            if (lm2.compareTo(lm1) == 0) {
                Long s1 = Files.size(o1);
                Long s2 = Files.size(o2);
                sorter = s2.compareTo(s1);
            } else {
                sorter = lm1.compareTo(lm2);
            }
        } catch (IOException ex) {
            throw new UncheckedIOException(ex);
        }
        return sorter;
    };

    public String[] getSortedFileListAsArray(Path dir) throws IOException {
        Stream<Path> stream = Files.list(dir);
        return stream.sorted(timeSizeComparator).
                map(Path::getFileName).map(Path::toString).toArray(String[]::new);
    }

    public List<String> getSortedFileListAsList(Path dir) throws IOException {
        Stream<Path> stream = Files.list(dir);
        return stream.sorted(timeSizeComparator).
                map(Path::getFileName).map(Path::toString).collect(Collectors.
                toList());
    }

    public String[] sortByDateAndSize(File[] fileList) {
        Arrays.sort(fileList, (File o1, File o2) -> {
            int r = Long.compare(o1.lastModified(), o2.lastModified());
            if (r != 0) {
                return r;
            }
            return Long.compare(o1.length(), o2.length());
        });
        String[] fileNames = new String[fileList.length];
        for (int i = 0; i < fileNames.length; i++) {
            fileNames[i] = fileList[i].getName();
        }
        return fileNames;
    }

    public List<String> getSortedFile(Path dir) throws IOException {
        return Files.list(dir).map(PathInfo::new).sorted().map(p -> p.getFileName()).collect(Collectors.toList());
    }

    static class PathInfo implements Comparable<PathInfo> {
        private final String fileName;
        private final long modified;
        private final long size;

        public PathInfo(Path path) {
            try {
                fileName = path.getFileName().toString();
                modified = Files.getLastModifiedTime(path).toMillis();
                size = Files.size(path);
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
        }

        @Override
        public int compareTo(PathInfo o) {
            int cmp = Long.compare(modified, o.modified);
            if (cmp == 0)
                cmp = Long.compare(size, o.size);
            return cmp;
        }

        public String getFileName() {
            return fileName;
        }
    }

    public static void main(String[] args) throws IOException {
        // File (io package)
        File f = new File("C:\\Windows\\system32");
        // Path (nio package)
        Path dir = Paths.get("C:\\Windows\\system32");

        FileSorter fs = new FileSorter();

        long before = System.currentTimeMillis();
        String[] names = fs.sortByDateAndSize(f.listFiles());
        long after = System.currentTimeMillis();
        System.out.println("Run time of Arrays.sort: " + ((after - before)));

        long before2 = System.currentTimeMillis();
        String[] names2 = fs.getSortedFileListAsArray(dir);
        long after2 = System.currentTimeMillis();
        System.out.println("Run time of Stream.sorted as Array: " + ((after2 - before2)));

        long before3 = System.currentTimeMillis();
        List<String> names3 = fs.getSortedFileListAsList(dir);
        long after3 = System.currentTimeMillis();
        System.out.println("Run time of Stream.sorted as List: " + ((after3 - before3)));
        long before4 = System.currentTimeMillis();
        List<String> names4 = fs.getSortedFile(dir);
        long after4 = System.currentTimeMillis();
        System.out.println("Run time of Stream.sorted as List with caching: " + ((after4 - before4)));
    }
}

这在我的笔记本电脑上打印。

Run time of Arrays.sort: 1980
Run time of Stream.sorted as Array: 1295
Run time of Stream.sorted as List: 1228
Run time of Stream.sorted as List with caching: 185

如您所见，大约85％的时间用于重复获取文件的修改日期和大小。

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