How to retrieve a list of directories QUICKLY in Java?

Question

Suppose a very simple program that lists out all the subdirectories of a given directory. Sound simple enough? Except the only way to list all subdirectories in Java is to u

Answer 1

I came across similar question when debugging performance in a Java application enumerating plenty of files. It is using old approach

for (File f : new File("C:\\").listFiles()) {
    if (f.isDirectory()) {
        continue;
    }        
}

And it appears that each f.isDirectory() is the call into native FileSsystem which, at least on NTFS, is very slow. Java7 NIO has additional API, but not all methods are good there. I'll just provide JMH benchmark result here

Benchmark                  Mode  Cnt  Score    Error  Units
MyBenchmark.dir_listFiles  avgt    5  0.437 ?  0.064   s/op
MyBenchmark.path_find      avgt    5  0.046 ?  0.001   s/op
MyBenchmark.path_walkTree  avgt    5  1.702 ?  0.047   s/op

Number come from execution of this code:

java -jar target/benchmarks.jar -bm avgt -f 1 -wi 5 -i 5 -t 1

static final String testDir = "C:/Sdk/Ide/NetBeans/src/dev/src/";
static final int nCycles = 50;

public static class Counter {
    int countOfFiles;
    int countOfFolders;
}

@Benchmark
public List dir_listFiles() {
    List files = new ArrayList<>(1000);

    for( int i = 0; i < nCycles; i++ ) {
        File dir = new File(testDir);

        files.clear();
        for (File f : dir.listFiles()) {
            if (f.isDirectory()) {
                continue;
            }
            files.add(f);
        }
    }
    return files;
}

@Benchmark
public List path_walkTree() throws Exception {
    final List files = new ArrayList<>(1000);

    for( int i = 0; i < nCycles; i++ ) {
        Path dir = Paths.get(testDir);

        files.clear();
        Files.walkFileTree(dir, new SimpleFileVisitor () {
            @Override
            public FileVisitResult visitFile(Path path, BasicFileAttributes arg1) throws IOException {
                files.add(path);
                return FileVisitResult.CONTINUE;
            }

            @Override
            public FileVisitResult preVisitDirectory(Path path, BasicFileAttributes arg1) 
                    throws IOException {
                return path == dir ? FileVisitResult.CONTINUE : FileVisitResult.SKIP_SUBTREE;
            }
        });
    }

    return files;
}

@Benchmark
public List path_find() throws Exception {
    final List files = new ArrayList<>(1000);

    for( int i = 0; i < nCycles; i++ ) {
        Path dir = Paths.get(testDir);

        files.clear();
        files.addAll(Files.find(dir, 1, (path, attrs) 
                -> true /*!attrs.isDirectory()*/).collect(Collectors.toList()));
    }

    return files;
}