Android中使用AsyncTask实现文件下载以及进度更新提示

 <?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);     }   } }