<?xml version="." encoding="utf-"?>
 <LinearLayout
   xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:id="@+id/activity_main"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:orientation="vertical"
   tools:context="com.example.administrator.asynctask.MainActivity">
   <TextView
     android:id="@+id/tv"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:text="panhouye!"
     android:textSize="sp"/>
   <ProgressBar
     android:id="@+id/progress"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     style="@style/Base.Widget.AppCompat.ProgressBar.Horizontal"
     android:visibility="visible"/>
   <Button
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:onClick="image"
     android:text="下载图片"/>
 </LinearLayout>

 import android.os.AsyncTask;
 import android.os.Environment;
 import android.support.v.app.AppCompatActivity;
 import android.os.Bundle;
 import android.view.View;
 import android.widget.ProgressBar;
 import android.widget.TextView;
 import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
 import java.io.File;
 import java.io.FileOutputStream;
 import java.net.HttpURLConnection;
 import java.net.URL;
 /**
 * Created by panchengjia on //.
 */
 public class MainActivity extends AppCompatActivity {
 //声明publishProgress的更新标记
   private static final int PROGRESS_MAX = X;
   private static final int UPDATE = X;
   private TextView tv;
   ProgressBar progress;
   int contentLen;//声明要下载的文件总长
   @Override
   protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);
     tv = (TextView) findViewById(R.id.tv);
     progress = (ProgressBar) findViewById(R.id.progress);
   }
   public void image(View view){
   //启用AsyncTask，传入需要执行的内容（图片地址）
     new DownLoad().execute("http://cdnq.duitang.com/uploads/item///_jWNmx.thumb._.jpeg");
   }
   class DownLoad extends AsyncTask<String,Integer,String>{
   //在执行实际的后台操作前被UI Thread调用
     @Override
     protected void onPreExecute() {
       super.onPreExecute();
       //准备下载前的初始进度
       progress.setProgress();
     }
   //在onPreExecute执行，该方法运行在后台线程中
     @Override
     protected String doInBackground(String... params) {
       try {
         URL url = new URL(params[]);
         //获取连接
         HttpURLConnection connection = (HttpURLConnection) url.openConnection();
         //获取下载文件的大小
         contentLen = connection.getContentLength();
         //根据下载文件大小设置进度条最大值（使用标记区别实时进度更新）
         publishProgress(PROGRESS_MAX,contentLen);
         //循环下载（边读取边存入）
         BufferedInputStream bis = new BufferedInputStream(connection.getInputStream());
         BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(new
             File(Environment.getExternalStorageDirectory()+"/Download/ss.jpg")));
         int len =-;
         byte[] bytes = new byte[];
         while((len=bis.read(bytes))!=-){
           bos.write(bytes,,len);
           bos.flush();
           //实时更新下载进度（使用标记区别最大值）
           publishProgress(UPDATE,len);
           //演示下载的图片太小，网速太快，休眠毫秒，方便大家观察
           Thread.sleep();
         }
         bos.close();
         bis.close();
       } catch (Exception e) {
         e.printStackTrace();
       }
       return "下载完成";
     }
   //在publishProgress被调用后，UI thread会调用这个方法
     @Override
     protected void onProgressUpdate(Integer... values) {
       super.onProgressUpdate(values);
       switch (values[]){
         case PROGRESS_MAX:
           progress.setMax(values[]);
           break;
         case UPDATE:
           progress.incrementProgressBy(values[]);
           //获取下载进度百分比并更新textview
           int i=(progress.getProgress()*)/contentLen;
           tv.setText("下载进度为："+i+"%");
           break;
       }
     }
   //doInBackground方法执行完后被UI thread执行
     @Override
     protected void onPostExecute(String s) {
       super.onPostExecute(s);
       progress.setVisibility(View.GONE);
       tv.setText(s);
     }
   }
 }