files - java list subdirectories recursively




Recursively list files in Java (14)

// Ready to run

import java.io.File;

public class Filewalker {

    public void walk( String path ) {

        File root = new File( path );
        File[] list = root.listFiles();

        if (list == null) return;

        for ( File f : list ) {
            if ( f.isDirectory() ) {
                walk( f.getAbsolutePath() );
                System.out.println( "Dir:" + f.getAbsoluteFile() );
            }
            else {
                System.out.println( "File:" + f.getAbsoluteFile() );
            }
        }
    }

    public static void main(String[] args) {
        Filewalker fw = new Filewalker();
        fw.walk("c:\\" );
    }

}

How do I recursively list all files under a directory in Java? Does the framework provide any utility?

I saw a lot of hacky implementations. But none from the framework or nio



Apart from the recursive traversal one can use a Visitor based approach as well.

Below code is uses Visitor based approach for the traversal.It is expected that the input to the program is the root directory to traverse.

public interface Visitor {
    void visit(DirElement d);
    void visit(FileElement f);
}

public abstract class Element {
    protected File rootPath;
    abstract void accept(Visitor v);

    @Override
    public String toString() {
        return rootPath.getAbsolutePath();
    }
}

public class FileElement extends Element {
    FileElement(final String path) {
        rootPath = new File(path);
    }

    @Override
    void accept(final Visitor v) {
        v.visit(this);
    }
}

public class DirElement extends Element implements Iterable<Element> {
    private final List<Element> elemList;
    DirElement(final String path) {
        elemList = new ArrayList<Element>();
        rootPath = new File(path);
        for (File f : rootPath.listFiles()) {
            if (f.isDirectory()) {
                elemList.add(new DirElement(f.getAbsolutePath()));
            } else if (f.isFile()) {
                elemList.add(new FileElement(f.getAbsolutePath()));
            }
        }
    }

    @Override
    void accept(final Visitor v) {
        v.visit(this);
    }

    public Iterator<Element> iterator() {
        return elemList.iterator();
    }
}

public class ElementWalker {
    private final String rootDir;
    ElementWalker(final String dir) {
        rootDir = dir;
    }

    private void traverse() {
        Element d = new DirElement(rootDir);
        d.accept(new Walker());
    }

    public static void main(final String[] args) {
        ElementWalker t = new ElementWalker("C:\\temp");
        t.traverse();
    }

    private class Walker implements Visitor {
        public void visit(final DirElement d) {
            System.out.println(d);
            for(Element e:d) {
                e.accept(this);
            }
        }

        public void visit(final FileElement f) {
            System.out.println(f);
        }
    }
}

Based on stacker answer. Here is a solution working in JSP without any external libraries so you can put it almost anywhere on your server:

<!DOCTYPE html>
<%@ page session="false" %>
<%@ page import="java.util.*" %>
<%@ page import="java.io.*" %>
<%@ page contentType="text/html; charset=UTF-8" %>

<%!
    public List<String> files = new ArrayList<String>();
    /**
        Fills files array with all sub-files.
    */
    public void walk( File root ) {
        File[] list = root.listFiles();

        if (list == null) return;

        for ( File f : list ) {
            if ( f.isDirectory() ) {
                walk( f );
            }
            else {
                files.add(f.getAbsolutePath());
            }
        }
    }
%>
<%
    files.clear();
    File jsp = new File(request.getRealPath(request.getServletPath()));
    File dir = jsp.getParentFile();
    walk(dir);
    String prefixPath = dir.getAbsolutePath() + "/";
%>

Then you just do something like:

    <ul>
        <% for (String file : files) { %>
            <% if (file.matches(".+\\.(apk|ipa|mobileprovision)")) { %>
                <li><%=file.replace(prefixPath, "")%></li>
            <% } %>
        <% } %>
    </ul>

Here a simple but perfectly working solution using recursion:

public static List<Path> listFiles(String rootDirectory)
{
    List<Path> files = new ArrayList<>();
    listFiles(rootDirectory, files);

    return files;
}

private static void listFiles(String path, List<Path> collectedFiles)
{
    File root = new File(path);
    File[] files = root.listFiles();

    if (files == null)
    {
        return;
    }

    for (File file : files)
    {
        if (file.isDirectory())
        {
            listFiles(file.getAbsolutePath(), collectedFiles);
        } else
        {
            collectedFiles.add(file.toPath());
        }
    }
}

I prefer using a queue over recursion for this kind of simple traversion:

List<File> allFiles = new ArrayList<File>();
Queue<File> dirs = new LinkedList<File>();
dirs.add(new File("/start/dir/"));
while (!dirs.isEmpty()) {
  for (File f : dirs.poll().listFiles()) {
    if (f.isDirectory()) {
      dirs.add(f);
    } else if (f.isFile()) {
      allFiles.add(f);
    }
  }
}

I would go with something like:

public void list(File file) {
    System.out.println(file.getName());
    File[] children = file.listFiles();
    for (File child : children) {
        list(child);
    }
}

The System.out.println is just there to indicate to do something with the file. there is no need to differentiate between files and directories, since a normal file will simply have zero children.


In Java 8, we can now use the Files utility to walk a file tree. Very simple.

Files.walk(root.toPath())
      .filter(path -> !Files.isDirectory(path))
      .forEach(path -> System.out.println(path));

Java 8 provides a nice stream to process all files in a tree.

Files.walk(Paths.get(path))
        .filter(Files::isRegularFile)
        .forEach(System.out::println);

This provides a natural way to traverse files. Since it's a stream you can do all nice stream operations on the result such as limit, grouping, mapping, exit early etc.

UPDATE: I might point out there is also Files.find which takes a BiPredicate that could can be more efficient if you need to check file attributes.

Files.find(Paths.get(path),
           Integer.MAX_VALUE,
           (filePath, fileAttr) -> fileAttr.isRegularFile())
        .forEach(System.out::println);

Note that while the JavaDoc eludes that this method could be more efficient than Files.walk it is effectively identical, the difference in performance can be observed if you are also retrieving file attributes within your filter. In the end, if you need to filter on attributes use Files.find, otherwise use Files.walk, mostly because there's overloads and it's more convenient.

TESTS: As requested I've provided a performance comparison of many of the answers. Check out the Github project which contains results and a test case.


My version (of course I could have used the built in walk in Java 8 ;-) ):

public static List<File> findFilesIn(File rootDir, Predicate<File> predicate) {
        ArrayList<File> collected = new ArrayList<>();
        walk(rootDir, predicate, collected);
        return collected;
    }

    private static void walk(File dir, Predicate<File> filterFunction, List<File> collected) {
        Stream.of(listOnlyWhenDirectory(dir))
                .forEach(file -> walk(file, filterFunction, addAndReturn(collected, file, filterFunction)));
    }

    private static File[] listOnlyWhenDirectory(File dir) {
        return dir.isDirectory() ? dir.listFiles() : new File[]{};
    }

    private static List<File> addAndReturn(List<File> files, File toAdd, Predicate<File> filterFunction) {
        if (filterFunction.test(toAdd)) {
            files.add(toAdd);
        }
        return files;
    }

Non-recursive BFS with a single list (particular example is searching for *.eml files):

    final FileFilter filter = new FileFilter() {
        @Override
        public boolean accept(File file) {
            return file.isDirectory() || file.getName().endsWith(".eml");
        }
    };

    // BFS recursive search
    List<File> queue = new LinkedList<File>();
    queue.addAll(Arrays.asList(dir.listFiles(filter)));

    for (ListIterator<File> itr = queue.listIterator(); itr.hasNext();) {
        File file = itr.next();
        if (file.isDirectory()) {
            itr.remove();
            for (File f: file.listFiles(filter)) itr.add(f);
        }
    }

With Java 7 you can use the following class:

import java.io.IOException;
import java.nio.file.FileVisitResult;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.SimpleFileVisitor;
import java.nio.file.attribute.BasicFileAttributes;

public class MyFileIterator extends SimpleFileVisitor<Path>
{
    public MyFileIterator(String path) throws Exception
    {
        Files.walkFileTree(Paths.get(path), this);
    }

    @Override
    public FileVisitResult visitFile(Path file,
            BasicFileAttributes attributes) throws IOException
    {
        System.out.println("File: " + file);
        return FileVisitResult.CONTINUE;
    }

    @Override
    public FileVisitResult preVisitDirectory(Path dir,
            BasicFileAttributes attributes) throws IOException
    {
        System.out.println("Dir: " + dir);
        return FileVisitResult.CONTINUE;
    }
}

just write it yourself using simple recursion:

public List<File> addFiles(List<File> files, File dir)
{
    if (files == null)
        files = new LinkedList<File>();

    if (!dir.isDirectory())
    {
        files.add(dir);
        return files;
    }

    for (File file : dir.listFiles())
        addFiles(files, file);
    return files;
}

    private void fillFilesRecursively(File file, List<File> resultFiles) {
        if (file.isFile()) {
            resultFiles.add(file);
        } else {
            for (File child : file.listFiles()) {
                fillFilesRecursively(child, resultFiles);
            }
        }
    }




nio