<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" />

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:gravity="center_horizontal"
 android:orientation="vertical"
 tools:context=".MainActivity">

 <Button
  android:id="@+id/button"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_marginTop="10dp"
  android:text="下載" />

 <ProgressBar
  android:id="@+id/progressBar"
  style="?android:attr/progressBarStyleHorizontal"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:layout_margin="10dp"
  android:max="100"
  android:progress="0" />
</LinearLayout>

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

 public static final String APP_URL = "http://download.sj.qq.com/upload/connAssitantDownload/upload/MobileAssistant_1.apk";
 public static final int DOWNLOAD_MESSAGE_CODE = 100001;
 private static final int DOWNLOAD_MESSAGE_FAIL_CODE = 100002;
 public static final int REQUEST_CODE = 1;
 private Button button;
 private ProgressBar progressBar;
 private MyHandler mHandler;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  initView();
  askPermission();
  mHandler = new MyHandler(this);
 }

 public static class MyHandler extends Handler {
  final WeakReference<MainActivity> mWeakReference;

  public MyHandler(MainActivity activity) {
   this.mWeakReference = new WeakReference<>(activity);
  }

  @Override
  public void handleMessage(Message msg) {
   MainActivity activity = mWeakReference.get();
   super.handleMessage(msg);
   switch (msg.what) {
    case DOWNLOAD_MESSAGE_CODE:
     activity.progressBar.setProgress((Integer) msg.obj);
     if (((Integer) msg.obj) == 100) {
      Toast.makeText(activity, "文件下載已完成！", Toast.LENGTH_SHORT).show();
     }
     break;
    case DOWNLOAD_MESSAGE_FAIL_CODE:
     Toast.makeText(activity, "文件下載失敗！", Toast.LENGTH_SHORT).show();
     break;
   }
  }
 }

 /**
  * 申請權限
  */
 private void askPermission() {
  //將所需申請的權限添加到List集合中
  List<String> permissionList = new ArrayList<>();
  if (ContextCompat.checkSelfPermission(MainActivity.this,
    Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
   permissionList.add(Manifest.permission.WRITE_EXTERNAL_STORAGE);
  }
  //判斷權限列表是否為空，若不為空，則向用戶申請權限，否則則直接執(zhí)行操作
  if (!permissionList.isEmpty()) {
   String[] permissions = permissionList.toArray(new String[permissionList.size()]);
   ActivityCompat.requestPermissions(MainActivity.this, permissions, REQUEST_CODE);
  } else {
   //TODO
   button.setOnClickListener(this);
  }
 }

 /**
  * 用戶對請求權限的dialog做出響應之后,會處理請求權限的響應
  *
  * @param requestCode 請求碼
  * @param permissions 權限的集合
  * @param grantResults 權限授予的結果
  */
 @Override
 public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
  super.onRequestPermissionsResult(requestCode, permissions, grantResults);
  switch (requestCode) {
   case REQUEST_CODE:
    if (grantResults.length > 0) {
     for (int result : grantResults) {
      if (result != PackageManager.PERMISSION_GRANTED) {
       Toast.makeText(this, "必須同意所有權限才能使用本程序", Toast.LENGTH_SHORT).show();
       finish();
       return;
      }
     }
     //TODO
     button.setOnClickListener(this);
    } else {
     Toast.makeText(this, "權限申請失??！", Toast.LENGTH_SHORT).show();
     finish();
    }
    break;
   default:
    break;
  }
 }

 /**
  * 點擊事件
  *
  * @param v
  */
 @Override
 public void onClick(View v) {
  switch (v.getId()) {
   case R.id.button:
    new Thread(new Runnable() {
     @Override
     public void run() {
      download(APP_URL);
     }
    }).start();
    break;
  }
 }

 /**
  * 下載
  *
  * @param appUrl
  */
 private void download(String appUrl) {
  try {
   //實例化一個url對象
   URL url = new URL(appUrl);
   //獲取URLConnection對象
   URLConnection urlConnection = url.openConnection();
   //獲取輸入流
   InputStream inputStream = urlConnection.getInputStream();
   //獲取文件的總長度
   int contentLength = urlConnection.getContentLength();
   //設置軟件下載到手機存儲的位置文件夾名稱
   String downloadFolderName = Environment.getExternalStorageDirectory()
     + File.separator + "MyApp" + File.separator;
   File file = new File(downloadFolderName);
   //若文件夾不存在則進行創(chuàng)建
   if (!file.exists()) {
    file.mkdir();
   }
   String fileName = downloadFolderName + "chaixingsi.apk";
   File apkFile = new File(fileName);
   if (apkFile.exists()) {
    apkFile.delete();
   }
   int downloadContentLength = 0;
   int length = 0;
   byte[] bytes = new byte[1024];
   OutputStream outputStream = new FileOutputStream(fileName);
   while ((length = inputStream.read(bytes)) != -1) {
    outputStream.write(bytes, 0, length);
    downloadContentLength += length;
    /**
     * 發(fā)送消息通知主線程去更新UI
     */
    Message message = Message.obtain();
    message.obj = downloadContentLength * 100 / contentLength;
    message.what = DOWNLOAD_MESSAGE_CODE;
    mHandler.sendMessage(message);
   }
   inputStream.close();
   outputStream.close();
  } catch (MalformedURLException e) {
   //下載失敗的話也發(fā)送消息
   notifyDownloadFailed();
   e.printStackTrace();
  } catch (IOException e) {
   notifyDownloadFailed();
   e.printStackTrace();
  }
 }

 /**
  * 通知下載失敗
  */
 private void notifyDownloadFailed() {
  Message message = Message.obtain();
  message.what = DOWNLOAD_MESSAGE_FAIL_CODE;
  mHandler.sendMessage(message);
 }

 /**
  * 初始化視圖
  */
 private void initView() {
  button = findViewById(R.id.button);
  progressBar = findViewById(R.id.progressBar);
 }
}