handler.sendMessage(Message msg);handler.sendMessageAtTime(Message msg, int time);handler.sendMessageDelayed(Message msg, int time);

handler.post(Runnable r);handler.postDelayed(Runnable r, int time);

<uses-permission android:name="android.permission.INTERNET" /><uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /><uses-permission android:name="android.permission.READ_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:orientation="vertical" android:padding="10dp" tools:context="com.studying.network.DownloadActivity"> <ProgressBar android:id="@+id/progress_bar" style="?android:progressBarStyleHorizontal" android:layout_width="match_parent" android:layout_height="wrap_content" android:max="100" /> <Button android:id="@+id/download" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_marginTop="10dp" android:text="@string/download" /></LinearLayout>

public class DownloadActivity extends Activity { private static final int DOWNLOAD_MESSAGE_CODE = 100001; private static final int DOWNLOAD_MESSAGE_FAIL_CODE = 100002; private static final String APP_URL = "http://clfile.imooc.com/class/assist/119/1328281/Android%20Studio%20教輔%20.pdf"; private MyHandler mHandler; private ProgressBar mProgressBar; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_download); findViewById(R.id.download).setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //開啟子線程 new Thread(new Runnable() {  @Override  public void run() {  download(APP_URL);  } }).start(); } }); mProgressBar = (ProgressBar) findViewById(R.id.progress_bar); mHandler = new MyHandler(this); } private void download(String appUrl) { try { URL url = new URL(appUrl); URLConnection conn = url.openConnection(); InputStream in = conn.getInputStream(); int contentLength = conn.getContentLength();//獲取文件總大小 String downloadPath = Environment.getExternalStorageDirectory() + File.separator + "imooc" + File.separator; File file = new File(downloadPath); if (!file.exists()) { file.mkdir(); } String fileName = downloadPath + "test.pdf"; File apkFile = new File(fileName); if (apkFile.exists()) { apkFile.delete(); } int downloadSize = 0;//記錄已經下載的大小 byte[] bytes = new byte[1024]; int length = 0; OutputStream out = new FileOutputStream(fileName); while ((length = in.read(bytes)) != -1) { out.write(bytes, 0, length); downloadSize += length; Message msg = Message.obtain(); msg.obj = downloadSize / contentLength * 100;//progress的值為0到100，因此得到的百分數要乘以100 msg.what = DOWNLOAD_MESSAGE_CODE; mHandler.sendMessage(msg); } in.close(); out.close(); } catch (IOException e) { notifyDownloadFailed(); e.printStackTrace(); } } private void notifyDownloadFailed() { Message msg = Message.obtain(); msg.what = DOWNLOAD_MESSAGE_FAIL_CODE; mHandler.sendMessage(msg); } private static class MyHandler extends Handler{ private WeakReference<DownloadActivity> weakReference; MyHandler(DownloadActivity activity) { this.weakReference = new WeakReference<>(activity);//以弱引用的形式傳遞Activity，避免內存泄漏 } @Override public void handleMessage(Message msg) { super.handleMessage(msg); DownloadActivity activity = weakReference.get(); //消息處理 switch (msg.what) { case DOWNLOAD_MESSAGE_CODE:  activity.mProgressBar.setProgress((Integer) msg.obj);  break; case DOWNLOAD_MESSAGE_FAIL_CODE:  Toast.makeText(activity, "下載失?。?quot;, Toast.LENGTH_SHORT).show();  break; } } }}