Android AsyncTask异步实现大文件下载

Android AsyncTask异步实现大文件下载

![异步下载显示图片](http://img.blog.csdn.net/20151114001557431)
public class Third extends Activity {
private Button btn;
private ImageView iv;
private TextView textview;
private Handler handlers;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.setContentView(R.layout.download);
btn = (Button) findViewById(R.id.btn);
iv = (ImageView) findViewById(R.id.iv);
textview = (TextView) findViewById(R.id.textview);

btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DownLoadImageTask imt = new DownLoadImageTask();
imt.execute("http://img2.niutuku.com/desk/1208/0850/bizhi-0850-17120.jpg");//传一个网址
}
});
handlers = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
if(msg.what==0x001){
Log.e("---------------------", "" + msg.obj);
textview.setText(msg.obj.toString());

}else if(msg.what==0x002){
Bitmap bitmap = (Bitmap) msg.obj;
iv.setImageBitmap(bitmap);

}
}
};

}

/**
* 完成图片下载的异步任务
* @author Administrator
*
*/
class DownLoadImageTask
extends AsyncTask<String, Integer, byte[]> {//参数含义：传进去的值；后台百分比；返回的结果
private ProgressDialog progressDialog;//进度条
private byte[] current = new byte[2 * 1024];//每次读到的字节数组
private byte[] total;//下载图片后汇总的字节数组
private boolean flag; //是否被取消

//任务执行之前回调
@Override
protected void onPreExecute() {
progressDialog = new ProgressDialog(Third.this);
progressDialog.setMessage("正在操作中，请稍候……");
progressDialog.setMax(100);//进度条最大值
progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);//水平样式
progressDialog.setIndeterminate(false);//进度条的动画效果（有动画则无进度值）

flag = true;
//退出对话框事件
progressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
@Override
public void onCancel(DialogInterface dialog) {
flag = false;
}
});
progressDialog.show();
}

@Override
protected  byte[] doInBackground(String... params) {//字符串数组
try {
//创建URL对象，用于访问网络资源
URL url = new URL(params[0]);
//获得HttpUrlConnection对象
HttpURLConnection http = (HttpURLConnection) url.openConnection();
//设置超时时间
http.setConnectTimeout(5000);
//获得总长度
int length = http.getContentLength();
Log.d("=======", length + "");
//为total分配空间
total = new byte[length];

//开始读取数据
int pointer = 0;//已用掉的索引
InputStream is = http.getInputStream();
int real = is.read(current); //读取当前批次的字节并保存到数组中
while(flag && real > 0){
//如果读取到了字节，则保存到total中
for(int i = 0; i < real; i ++){
total[pointer + i] = current[i];
}
//指针向后移
pointer += real;
//计算进度
int progress = (int)((double)pointer / length * 100);//先计算出百分比在转换成整型
//更新进度
publishProgress(progress, pointer, length);
//继续读取下一批
real = is.read(current);
}
//关闭流对象
is.close();
//将获得的所有字节全部返回
return total;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}

//更新进度条回调
@Override
protected void onProgressUpdate(Integer... values) {
progressDialog.setMessage(
String.format("已读%d M, 共%d M",//字节为单位
values[1] / 1024 / 1024, values[2] / 1024 / 1024));//将values[1]赋给第一个%d,第二个同理

progressDialog.setProgress(values[0]);//进度动态提示
Message message001 = handlers.obtainMessage(0x001);
message001.obj=values[0];
handlers.sendMessage(message001);
}

//任务被取消时回调
@Override
protected void onCancelled() {
super.onCancelled();
Toast.makeText(getApplication(),"任务已取消",Toast.LENGTH_SHORT).show();
}

//任务结束后回调
@Override
protected void onPostExecute(byte[] result) {
progressDialog.dismiss();//关闭对话框

Bitmap bitmap = null;
if(flag){
//将获得的字节数组转换成Bitmap图片对象（在安卓中用位图显示图片）
bitmap = BitmapFactory.decodeByteArray(
result, 0, result.length);
//iv.setImageBitmap(bitmap);
Message message002 = handlers.obtainMessage(0x002);
message002.obj=bitmap;
handlers.sendMessage(message002);
}
flag = false;

//回收内存
if(!flag && !bitmap.isRecycled()){
//bitmap.recycle();
}
}
}

}

download.xml文件：-----------------------------------

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<ImageView
android:id="@+id/iv"
android:layout_width="match_parent"
android:layout_height="match_parent" />

<ProgressBar
style="?android:attr/progressBarStyleHorizontal"
android:layout_gravity="center_vertical|center_horizontal"
android:id="@+id/progressbar"
android:layout_width="200dp"
android:layout_height="20dp" />

<TextView
android:textColor="#FF0000"
android:id="@+id/textview"
android:layout_below="@id/progressbar"
android:gravity="center_horizontal"
android:textSize="14dp"
android:layout_width="match_parent"
android:layout_height="wrap_content" />

<Button
android:id="@+id/btn"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:background="@android:color/holo_blue_dark"
android:text="下载图片"
android:textColor="#FFFFFF" />

</RelativeLayout>