1.静态权限 manifest（xml文件未写）

  <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_CONTACTS"/>
android:requestLegacyExternalStorage="true"

2.动态权限

 boolean isAllGranted = checkPerssionAllGranted(
                        new String[]{
                                Manifest.permission.READ_CONTACTS,
                                Manifest.permission.READ_EXTERNAL_STORAGE,
                                Manifest.permission.WRITE_EXTERNAL_STORAGE
                        }

                );
//查看是否有权限，若有进行下载
                if (isAllGranted) {
                    new Download().execute(IMG_URL);
                    return;
                }
//若没有，请求授权
                ActivityCompat.requestPermissions(
                        DownloadActivity.this,
                        new String[]{
                                Manifest.permission.READ_CONTACTS,
                                Manifest.permission.READ_EXTERNAL_STORAGE,
                                Manifest.permission.WRITE_EXTERNAL_STORAGE
                        },
                        MY_PERMISSION_CODE
                );


private boolean checkPerssionAllGranted(String[] permissions) {
        for (String permission : permissions) {
            if (ContextCompat.checkSelfPermission(this, permission) != PackageManager.PERMISSION_GRANTED) {
                return false;
            }
        }
        return true;
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == MY_PERMISSION_CODE) {
            boolean isAllGranted = true;
            for (int grant : grantResults) {
                if (grant != PackageManager.PERMISSION_GRANTED) {
                    isAllGranted = false;
                    break;
                }
            }
            if (isAllGranted) {
                new Download().execute();
            } else {
                return;
            }
        }
    }

3.自定义内部类继承AsyncTask

public class Download extends AsyncTask<String, Integer, Boolean> {
//开始前 UI线程
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            downloadBtn.setText("开始下载");
            txt.setText("");
            seekBar.setProgress(0);
        }
//下载过程中 子线程
        @Override
        protected Boolean doInBackground(String... strings) {
            if (strings != null && strings.length > 0) {
                try {
                    URL url = new URL(strings[0]);
                    URLConnection conn = url.openConnection();
                    InputStream in = conn.getInputStream();
                    int fileLen = conn.getContentLength();
                    path = Environment.getExternalStorageDirectory()
                            + File.separator + FILE_NAME;
                    File file = new File(path);
                    if (file.exists()) {
                        boolean result = file.delete();
                        if (!result) {
                            return false;
                        }
                    }
                    OutputStream os = new FileOutputStream(file);
                    int len = 0;
                    byte[] b = new byte[1024];
                    int okLen = 0;
                    while ((len = in.read(b)) > -1) {
                        os.write(b, 0, len);
                        okLen += len;
                        publishProgress(okLen * 100 / fileLen);
                    }
                    in.close();
                    os.close();
                    return true;
                } catch (MalformedURLException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } else {
                return false;
            }
        }

//下载结束 UI线程
        @Override
        protected void onPostExecute(Boolean aBoolean) {
            super.onPostExecute(aBoolean);
            downloadBtn.setText((aBoolean ? "下载完成" : "下载失败"));
            txt.setText( "下载完成"  + path );
        }
//过程中更新UI 线程
        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);
            seekBar.setProgress(values[0]);
        }
    }