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>