使用AsyncTask下载远端资源到SD卡

下载远端资源，需要INTERNET权限；
将文件写入到SD，需要WRITE_EXTERNAL_STORAGE权限；

在AndroidManifest.xml中进行如下配置：

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

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

Android AsyncTask提供了简单易用的方式，执行后台操作并更新UI。

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

示例代码：
功能：下载网络资源到本地

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package lizhen.download;

 

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 extendsAsyncTask<String, Integer, Boolean> {

 

    protectedint size;

    protectedString errorMessage;

 

    @Override

    protectedBoolean 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

                publicvoid onProgressUpdate(intprogress) {

                    publishProgress(progress);

                }

            });

 

        }catch (ClientProtocolException e) {

            errorMessage = e.getMessage();

            cancel(true);

            returnfalse;

        }catch (IOException e) {

            errorMessage = e.getMessage();

            cancel(true);

            returnfalse;

        }

        returntrue;

    }

 

    /**

     * 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

     * */

    privatevoid fileWrite(InputStream in, String path,

            OnProgressUpdateListener listener)throws IOException {

        File file = createFile(path);

        FileOutputStream fileOutputStream =new FileOutputStream(file);

        bytebuffer[] = new byte[1024];

        intprogress = 0;

        intreadBytes = 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

     * */

    privateinterface OnProgressUpdateListener {

        /**

         * 下载进度更新

         *

         * @param progress

         *            进度

         * */

        publicvoid onProgressUpdate(intprogress);

    }

 

    /**

     * 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

     * */

    privateInputStream requestInputStream(String url)

            throwsClientProtocolException, IOException {

        InputStream result =null;

        HttpGet httpGet =new HttpGet(url);

        HttpClient httpClient =new DefaultHttpClient();

        HttpResponse httpResponse = httpClient.execute(httpGet);

        inthttpStatusCode = httpResponse.getStatusLine().getStatusCode();

        if(httpStatusCode == HttpStatus.SC_OK) {

            HttpEntity httpEntity = httpResponse.getEntity();

            size = (int) httpEntity.getContentLength();

            result = httpEntity.getContent();

        }

        returnresult;

    }

 

    /**

     * 根据文件路径创建文件

     *

     * @param path

     *            文件路径

     * @return 文件File实例

     * @return IOException

     * */

    privateFile createFile(String path) throwsIOException {

        File file =new File(path);

        file.createNewFile();

        returnfile;

    }

 

    /**

     * 返回错误信息

     *

     * @return 错误信息

     * */

    publicString getErrorString() {

        returnthis.errorMessage;

    }

 

}

功能：用户界面，更新进度条和下载进度，显示下载图片

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package lizhen.download;

 

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 extendsActivity {

 

    privateButton startButton;

    privateProgressBar downloadProgressBar;

    privateTextView progressTextView;

    privateImageView downloadImageView;

 

    privatefinal String source = "http://www.android-study.com/resource/atm.gif"; // 源文件地址

    privatefinal String path = Environment.getExternalStorageDirectory()

            .toString() +"/atm.gif"; // 目标文件地址

 

    @Override

    publicvoid onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.download);

 

        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(newView.OnClickListener() {

 

            @Override

            publicvoid onClick(View v) {

                newDownloadTask().execute(source, path);

            }

        });

    }

 

    privateclass DownloadTask extends DownloadAsyncTask {

 

        @Override

        protectedvoid onProgressUpdate(Integer... values) {

            super.onProgressUpdate(values);

            intprogress = values[0];

            /*

             * 更新进度条和下载百分率

             */

            downloadProgressBar.setMax(size);

            downloadProgressBar.setProgress(progress);

            intpercentage = progress * 100/ size;

            progressTextView.setText("已完成"+ percentage + "%");

        }

 

        @Override

        protectedvoid onPostExecute(Boolean result) {

            super.onPostExecute(result);

            if(result) {

                Bitmap bitmap = BitmapFactory.decodeFile(path);

                downloadImageView.setImageBitmap(bitmap);

            }else {

                Toast.makeText(Download.this,"Error: " + errorMessage,1000)

                        .show();

            }

        }

    }

}

运行结果：