android使用AsyncTask下载远端网络资源到SD卡

<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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".Download" 
    android:orientation="vertical">


    <Button
        android:id="@+id/download_StartButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="开始下载" />
    
    <ProgressBar
        android:id="@+id/download_ProgressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="203dp"
        android:layout_height="40dp" />
    
    <TextView  android:id="@+id/download_ProgressTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="center"/>
    
    <ImageView android:id="@+id/download_ImageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="center"/>
      

</LinearLayout>

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

package com.example.drawableimgasynctask;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
 
public class Download extends Activity {
 
    private Button startButton;
    private ProgressBar downloadProgressBar;
    private TextView progressTextView;
    private ImageView downloadImageView;
 
    private final String mSource = "http://d3gtl9l2a4fn1j.cloudfront.net/t/p/original/wYdFAUyCApeUbfWWP0YMsu6b4hs.jpg";// 源文件地址
    private final String mPath = Environment.getExternalStorageDirectory()
            .toString() + "/imga.gif"; // 目标文件地址
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
 
        startButton = (Button) findViewById(R.id.download_StartButton);
        downloadProgressBar = (ProgressBar) findViewById(R.id.download_ProgressBar);
        progressTextView = (TextView) findViewById(R.id.download_ProgressTextView);
        downloadImageView = (ImageView) findViewById(R.id.download_ImageView);
        startButton.setOnClickListener(new View.OnClickListener() {
 
            @Override
            public void onClick(View v) {
                new DownloadTask().execute(mSource, mPath);
            }
        });
    }
 
    private class DownloadTask extends DownloadAsyncTask {
 
        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);
            int progress = values[0];
            /*
             * 更新进度条和下载百分率
             */
            downloadProgressBar.setMax(size);
            downloadProgressBar.setProgress(progress);
            int percentage = progress * 100 / size;
            progressTextView.setText("已完成" + percentage + "%");
        }
 
        @Override
        protected void onPostExecute(Boolean result) {
            super.onPostExecute(result);
            if (result) {
                Bitmap bitmap = BitmapFactory.decodeFile(mPath);
                downloadImageView.setImageBitmap(bitmap);
            } else {
                Toast.makeText(Download.this, "Error: " + errorMessage, 1000)
                        .show();
            }
        }
    }
}

package com.example.drawableimgasynctask;
 
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
 
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
 
import android.os.AsyncTask;
 
public class DownloadAsyncTask extends AsyncTask<String, Integer, Boolean> {
 
    protected int size;
    protected String errorMessage;
 
    @Override
    protected Boolean doInBackground(String... params) {
        String url = params[0]; // 资源地址
        String path = params[1]; // 目标路径
        try {
            InputStream source = requestInputStream(url); // 请求源文件InputStream
            /**
             * 创建文件写入数据，并更新进度
             */
            fileWrite(source, path, new OnProgressUpdateListener() {
 
                @Override
                public void onProgressUpdate(int progress) {
                    publishProgress(progress);
                }
            });
 
        } catch (ClientProtocolException e) {
            errorMessage = e.getMessage();
            cancel(true);
            return false;
        } catch (IOException e) {
            errorMessage = e.getMessage();
            cancel(true);
            return false;
        }
        return true;
    }
 
    /**
     * 文件写入
     *
     * @param in
     *            数据源输出流
     * @param path
     *            文件路径
     * @param listener
     *            下载进度监听器
     * */
    private void fileWrite(InputStream in, String path,
            OnProgressUpdateListener listener) throws IOException {
        File file = createFile(path);
        FileOutputStream fileOutputStream = new FileOutputStream(file);
        byte buffer[] = new byte[1024];
        int progress = 0;
        int readBytes = 0;
        while ((readBytes = in.read(buffer)) != -1) {
            progress += readBytes;
            fileOutputStream.write(buffer, 0, readBytes);
            if (listener != null) {
                listener.onProgressUpdate(progress);
            }
        }
        in.close();
        fileOutputStream.close();
    }
 
    /**
     * 下载进度监听器
     * */
    private interface OnProgressUpdateListener {
        /**
         * 下载进度更新
         *
         * @param progress
         *            进度
         * */
        public void onProgressUpdate(int progress);
    }
 
    /**
     * 根据资源URL地址取得资源输入流
     *
     * @param url
     *            URL地址
     * @return 资源输入流
     * @throws ClientProtocolException
     * @throws IOException
     * */
    private InputStream requestInputStream(String url)
            throws ClientProtocolException, IOException {
        InputStream result = null;
        HttpGet httpGet = new HttpGet(url);
        HttpClient httpClient = new DefaultHttpClient();
        HttpResponse httpResponse = httpClient.execute(httpGet);
        int httpStatusCode = httpResponse.getStatusLine().getStatusCode();
        if (httpStatusCode == HttpStatus.SC_OK) {
            HttpEntity httpEntity = httpResponse.getEntity();
            size = (int) httpEntity.getContentLength();
            result = httpEntity.getContent();
        }
        return result;
    }
 
    /**
     * 根据文件路径创建文件
     *
     * @param path
     *            文件路径
     * @return 文件File实例
     * @return IOException
     * */
    private File createFile(String path) throws IOException {
        File file = new File(path);
        file.createNewFile();
        return file;
    }
 
    /**
     * 返回错误信息
     *
     * @return 错误信息
     * */
    public String getErrorString() {
        return this.errorMessage;
    }
 
}

AsyncTask的3个泛型

• Param  传入数据类型
• Progress  更新UI数据类型
• Result  处理结果类型

AsyncTask的4个步骤

1、onPreExecute  执行前的操作
2、doInBackGround  后台执行的操作
3、onProgressUpdate  更新UI操作
4、onPostExecute  执行后的操作

从网络下载资源到SD卡的步骤：

1、HTTP请求资源InputStream
2、在SD中创建一个空文件
3、创建该文件的FileOutputStream
4、使用while循环，InputStream每次循环读入数据到字节数组buffer中（buffer字节数组的大小一般为1024的整数倍），FileOutputStream将buffer中的数据写入文件，直到EOF，read()方法返回-1

下载网络资源到本地

  运行结果：