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：
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android: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
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 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;
}
}
功能：用户界面，更新进度条和下载进度，显示下载图片
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 extends Activity {
private Button startButton;
private ProgressBar downloadProgressBar;
private TextView progressTextView;
private ImageView downloadImageView;
private final String source = "http://www.android100.org/uploadfile/2012/0311/20120311040516591.gif"; // 源文件地址
private final String path = Environment.getExternalStorageDirectory()
.toString() + "/atm.gif"; // 目标文件地址
@Override
public void 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(new View.OnClickListener() {
@Override
public void onClick(View v) {
new DownloadTask().execute(source, path);
}
});
}
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(path);
downloadImageView.setImageBitmap(bitmap);
} else {
Toast.makeText(Download.this, "Error: " + errorMessage, 1000)
.show();
}
}
}
}
运行结果：
