多线程遍历怎么实现？

多线程遍历可以将文件夹分成多个小段，每个线程遍历其中一段，最后合并结果。以下是一个示例代码：

import java.io.File;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

public class MultiThreadTraverseFolder {
    public static void main(String[] args) {
        String folderPath = "path/to/folder";
        int threadCount = Runtime.getRuntime().availableProcessors();
        ExecutorService executor = Executors.newFixedThreadPool(threadCount);
        List<Future<List<String>>> futures = new ArrayList<>();
        File folder = new File(folderPath);
        if (folder.isDirectory()) {
            File[] files = folder.listFiles();
            if (files != null) {
                int step = files.length / threadCount;
                for (int i = 0; i < threadCount; i++) {
                    int start = i * step;
                    int end = (i == threadCount - 1) ? files.length : (i + 1) * step;
                    Callable<List<String>> task = new TraverseTask(files, start, end);
                    Future<List<String>> future = executor.submit(task);
                    futures.add(future);
                }
            }
        }
        List<String> fileList = new ArrayList<>();
        for (Future<List<String>> future : futures) {
            try {
                fileList.addAll(future.get());
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        }
        executor.shutdown();
        for (String file : fileList) {
            System.out.println(file);
        }
    }

    static class TraverseTask implements Callable<List<String>> {
        private final File[] files;
        private final int start;
        private final int end;

        public TraverseTask(File[] files, int start, int end) {
            this.files = files;
            this.start = start;
            this.end = end;
        }

        @Override
        public List<String> call() throws Exception {
            List<String> fileList = new ArrayList<>();
            for (int i = start; i < end; i++) {
                File file = files[i];
                if (file.isDirectory()) {
                    fileList.addAll(traverseFolder(file));
                } else {
                    fileList.add(file.getAbsolutePath());
                }
            }
            return fileList;
        }

        private List<String> traverseFolder(File folder) {
            List<String> fileList = new ArrayList<>();
            File[] files = folder.listFiles();
            if (files != null) {
                for (File file : files) {
                    if (file.isDirectory()) {
                        fileList.addAll(traverseFolder(file));
                    } else {
                        fileList.add(file.getAbsolutePath());
                    }
                }
            }
            return fileList;
        }
    }
}

在代码中，我们首先获取 CPU 核心数，然后创建一个线程池来执行遍历任务。将文件夹分成多个小段，每个线程遍历其中一段，最后将每个线程的结果合并到一个列表中。最后输出所有文件名。在遍历过程中，如果遇到一个子文件夹，就递归调用 `traverseFolder` 方法来遍历这个子文件夹。如果遇到一个文件，就输出文件名。

需要注意的是，在多线程遍历中，要处理好线程间的同步和互斥问题，避免出现资源竞争和数据不一致的问题。