我已经寻找了很长一段时间的解决办法，但仍然没有找到合适的。
我的问题是。。。当下载一个android低于q的文件时，绝对没有问题，文件会出现在下载文件夹中。但是当下载android>=q的文件时，该文件不会出现在下载文件夹中。
所以我知道自从android10安装了downloadmanager之后，情况发生了变化。我读过很多次，你现在应该使用storagemanager，我当时就是这么做的。所以我改变了这个：

if (download_uri == null) {
    try {
       URL rurl = new URL(url);
       InputStream is = rurl.openStream();
       DataInputStream dis = new DataInputStream(is);
       byte[] buffer = new byte[4096];
       File file = new File(getExternalFilesDir(null), nameOfFile);
       FileOutputStream fos = new FileOutputStream(file);
       int length;
       while ((length = dis.read(buffer)) > 0) {
             fos.write(buffer, 0, length);
       }

       manager.addCompletedDownload(nameOfFile, "File", true, mimetype, Environment.DIRECTORY_DOWNLOADS + "/" + nameOfFile, contentLength, true);
}

为了这个

if (download_uri == null) {
    try {
       URL rurl = new URL(url);
       InputStream is = rurl.openStream();
       DataInputStream dis = new DataInputStream(is);
       byte[] buffer = new byte[4096];
       File file = new File(getExternalFilesDir(null), nameOfFile);
       FileOutputStream fos = new FileOutputStream(file);
       int length;
       while ((length = dis.read(buffer)) > 0) {
             fos.write(buffer, 0, length);
       }
       if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
          final String relativeLocation = Environment.DIRECTORY_DOWNLOADS + "/" + nameOfFile;

          ContentValues myContentValues = new ContentValues();
          myContentValues.put(MediaStore.MediaColumns.DISPLAY_NAME, nameOfFile);
          myContentValues.put(MediaStore.MediaColumns.RELATIVE_PATH, relativeLocation);
          myContentValues.put(MediaStore.MediaColumns.MIME_TYPE, mimetype);
          myContentValues.put(MediaStore.MediaColumns.IS_PENDING, 1);
         } else {
          manager.addCompletedDownload(nameOfFile, "FileDescription", true, mimetype, Environment.DIRECTORY_DOWNLOADS + "/" + nameOfFile, contentLength, true);
         }
    }

我看到这个代码很多次，但仍然在我的情况下，该文件将不会下载尚未。也许我忘了一些导致问题的代码？
我还看到一行代码，其中有人使用：

StorageManager sm = (StorageManager)getSystemService(Context.STORAGE_SERVICE);

在我的代码中，我仍然使用：

DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));

然后：

download_id = manager.enqueue(request);
Uri download_uri = manager.getUriForDownloadedFile(download_id);

不过，我不知道如何使用storagemanager（如果必须的话）。。。感谢您的帮助。
这是我与downloadmanager的旧代码：

webView.setDownloadListener((url, userAgent, contentDisposition, mimetype, contentLength) -> {

            if ( Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
                if (checkPermission()) {
                    Log.d(LogTag, "Download URL:" + url);
                    Log.d(LogTag, "Download user-Agent:" + userAgent);
                    Log.d(LogTag, "Download contentDisposition:" + contentDisposition);
                    Log.d(LogTag, "Download MIME-Type:" + mimetype);
                    Log.d(LogTag, "Download length:" + ((Long) contentLength).toString());

                    final DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
                    String nameOfFile = URLUtil.guessFileName(url, null, mimetype);
                    request.setMimeType(mimetype);
                    request.setAllowedNetworkTypes(DownloadManager.Request.NETWORK_WIFI | DownloadManager.Request.NETWORK_MOBILE);
                    request.setAllowedOverMetered(true);
                    request.setAllowedOverRoaming(true);

                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                        request.setRequiresCharging(false);
                        request.setRequiresDeviceIdle(false);
                    }

                    request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, nameOfFile);
                    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.Q) {
                        request.allowScanningByMediaScanner();
                    }

                    request.setDescription("FileDescription");
                    String cookies = CookieManager.getInstance().getCookie(url);
                    request.addRequestHeader("cookie", cookies);
                    request.addRequestHeader("User-Agent", userAgent);
                    request.setTitle(nameOfFile);
                    request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
                    //request.setVisibleInDownloadsUi(true);
                    final DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);

                    long download_id;
                    download_id = manager.enqueue(request);
                    }
                }
            }
        });