下载pdf从url，保存到手机本地存储的android在颤动

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

public void downloadPDF(String url, String fileName) {
    try {
        URL downloadUrl = new URL(url);
        HttpURLConnection connection = (HttpURLConnection) downloadUrl.openConnection();
        connection.setRequestMethod("GET");
        connection.connect();

        // 创建文件对象
        File file = new File(Environment.getExternalStorageDirectory(), fileName);

        // 创建文件输出流
        FileOutputStream outputStream = new FileOutputStream(file);

        // 读取数据并写入文件
        InputStream inputStream = connection.getInputStream();
        byte[] buffer = new byte[1024];
        int len;
        while ((len = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, len);
        }

        // 关闭流
        outputStream.close();
        inputStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

String pdfUrl = "https://example.com/example.pdf";
String fileName = "example.pdf";
downloadPDF(pdfUrl, fileName);

private static final int REQUEST_CODE_WRITE_EXTERNAL_STORAGE = 1;

// 检查并请求文件写入权限
private void checkWritePermission() {
    if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE)
            != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this,
                new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                REQUEST_CODE_WRITE_EXTERNAL_STORAGE);
    } else {
        // 已经拥有权限，可以进行下载操作
        downloadPDF(pdfUrl, fileName);
    }
}

// 处理权限请求结果
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions,
                                       @NonNull int[] grantResults) {
    if (requestCode == REQUEST_CODE_WRITE_EXTERNAL_STORAGE) {
        if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            // 用户授予了文件写入权限，可以进行下载操作
            downloadPDF(pdfUrl, fileName);
        } else {
            // 用户拒绝了文件写入权限，可以给出相应的提示
            Toast.makeText(this, "需要文件写入权限才能下载PDF文件", Toast.LENGTH_SHORT).show();
        }
    }
}