Example of how to parallelize file tree walk and parse.

AsynchronousParse.java

import java.io.FileInputStream;
import java.io.IOException;
import java.nio.file.FileVisitResult;
import java.nio.file.FileVisitor;
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;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;

import com.github.javaparser.JavaParser;

@SuppressWarnings("serial")
public class AsynchronousParse extends RecursiveAction {

    private static final String SRC_DIR_PATH = "";
    private final Path dir;
    private static int numFilesParsed;

    public AsynchronousParse(Path dir) {
        this.dir = dir;
    }

    @Override
    protected void compute() {
        final List<AsynchronousParse> walks = new ArrayList<>();
        try {
            Files.walkFileTree(dir, new SimpleFileVisitor<Path>() {
                @Override
                public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) throws IOException {
                    if (!dir.equals(AsynchronousParse.this.dir)) {
                        AsynchronousParse w = new AsynchronousParse(dir);
                        w.fork();
                        walks.add(w);
                        return FileVisitResult.SKIP_SUBTREE;
                    } else {
                        return FileVisitResult.CONTINUE;
                    }
                }

                @Override
                public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
                    if (file.toString().endsWith(".java")) {
                        numFilesParsed++;
                        JavaParser.parse(new FileInputStream(file.toFile()));
                    }
                    return FileVisitResult.CONTINUE;
                }
            });
        } catch (IOException e) {
            e.printStackTrace();
        }

        for (AsynchronousParse w : walks) {
            w.join();
        }
    }

    public static void main(String[] args) throws IOException {
        AsynchronousParse w = new AsynchronousParse(Paths.get(SRC_DIR_PATH).toRealPath());
        ForkJoinPool p = new ForkJoinPool();
        long startTime = System.nanoTime();
        p.invoke(w);
        double elapsedTime = (System.nanoTime() - startTime) / 1000000;
        System.out.println("Compilation of " + numFilesParsed + " files took " + elapsedTime + " milliseconds.");
    }
}

It's from? https://github.com/javaparser/javaparser/blob/cfc1bcdf1fbd596ac11cbf14be565ffdee8903a5/javaparser-core/src/main/java/com/github/javaparser/utils/SourceRoot.java#L568