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教辅%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) {
 //开启子线程
 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();//获取文件总大小
 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;//记录已经下载的大小
 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的值为0到100，因此得到的百分数要乘以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);//以弱引用的形式传递Activity，避免内存泄漏
 }
 @Override
 public void handleMessage(Message msg) {
 super.handleMessage(msg);
 DownloadActivity activity = weakReference.get();
 //消息处理
 switch (msg.what) {
 case DOWNLOAD_MESSAGE_CODE:
  activity.mProgressBar.setProgress((Integer) msg.obj);
  break;
 case DOWNLOAD_MESSAGE_FAIL_CODE:
  Toast.makeText(activity, "下载失败！", Toast.LENGTH_SHORT).show();
  break;
 }
 }
 }
}