最近开发中遇到需要下载文件的问题，对于一般的下载来说不用考虑断点续传，不用考虑多个线程，比如下载一个apk之类的，这篇文章讨论的就是这种情形。

这里主要讨论三种方式：AsyncTask、Service和使用DownloadManager。

一、使用AsyncTask并在进度对话框中显示下载进度

这种方式的优势是你可以在后台执行下载任务的同时，也可以更新UI(这里我们用progress bar来更新下载进度)

下面的代码是使用的例子// declare the dialog as a member field of your activity

ProgressDialog mProgressDialog;

// instantiate it within the onCreate method

mProgressDialog = new ProgressDialog(YourActivity.this);

mProgressDialog.setMessage("A message");

mProgressDialog.setIndeterminate(true);

mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);

mProgressDialog.setCancelable(true);

// execute this when the downloader must be fired

final DownloadTask downloadTask = new DownloadTask(YourActivity.this);

downloadTask.execute("the url to the file you want to download");

mProgressDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {

@Override

public void onCancel(DialogInterface dialog) {

downloadTask.cancel(true);

}

});

DownloadTask继承自AsyncTask，按照如下框架定义，你需要将代码中的某些参数替换成你自己的。// usually, subclasses of AsyncTask are declared inside the activity class.

// that way, you can easily modify the UI thread from here

private class DownloadTask extends AsyncTask {

private Context context;

private PowerManager.WakeLock mWakeLock;

public DownloadTask(Context context) {

this.context = context;

}

@Override

protected String doInBackground(String... sUrl) {

InputStream input = null;

OutputStream output = null;

HttpURLConnection connection = null;

try {

URL url = new URL(sUrl[0]);

connection = (HttpURLConnection) url.openConnection();

connection.connect();

// expect HTTP 200 OK, so we don't mistakenly save error report

// instead of the file

if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {

return "Server returned HTTP " + connection.getResponseCode()

+ " " + connection.getResponseMessage();

}

// this will be useful to display download percentage

// might be -1: server did not report the length

int fileLength = connection.getContentLength();

// download the file

input = connection.getInputStream();

output = new FileOutputStream("/sdcard/file_name.extension");

byte data[] = new byte[4096];

long total = 0;

int count;

while ((count = input.read(data)) != -1) {

// allow canceling with back button

if (isCancelled()) {

input.close();

return null;

}

total += count;

// publishing the progress....

if (fileLength > 0) // only if total length is known

publishProgress((int) (total * 100 / fileLength));

output.write(data, 0, count);

}

} catch (Exception e) {

return e.toString();

} finally {

try {

if (output != null)

output.close();

if (input != null)

input.close();

} catch (IOException ignored) {

}

if (connection != null)

connection.disconnect();

}

return null;

}

上面的代码只包含了doInBackground，这是执行后台任务的代码块，不能在这里做任何的UI操作，但是onProgressUpdate和onPreExecute是运行在UI线程中的，所以我们应该在这两个方法中更新progress bar。

接上面的代码：

@Override

protected void onPreExecute() {

super.onPreExecute();

// take CPU lock to prevent CPU from going off if the user

// presses the power button during download

PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);

mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,

getClass().getName());

mWakeLock.acquire();

mProgressDialog.show();

}

@Override

protected void onProgressUpdate(Integer... progress) {

super.onProgressUpdate(progress);

// if we get here, length is known, now set indeterminate to false

mProgressDialog.setIndeterminate(false);

mProgressDialog.setMax(100);

mProgressDialog.setProgress(progress[0]);

}

@Override

protected void onPostExecute(String result) {

mWakeLock.release();

mProgressDialog.dismiss();

if (result != null)

Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();

else

Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();

}

注意需要添加如下权限：

二、在service中执行下载

在service中执行下载任务的麻烦之处在于如何通知activity更新UI。下面的代码中我们将用ResultReceiver和IntentService来实现下载。ResultReceiver允许我们接收来自service中发出的广播，IntentService继承自service，这IntentService中我们开启一个线程开执行下载任务(service和你的app其实是在一个线程中，因此不想阻塞主线程的话必须开启新的线程)。public class DownloadService extends IntentService {

public static final int UPDATE_PROGRESS = 8344;

public DownloadService() {

super("DownloadService");

}

@Override

protected void onHandleIntent(Intent intent) {

String urlToDownload = intent.getStringExtra("url");

ResultReceiver receiver = (ResultReceiver) intent.getParcelableExtra("receiver");

try {

URL url = new URL(urlToDownload);

URLConnection connection = url.openConnection();

connection.connect();

// this will be useful so that you can show a typical 0-100% progress bar

int fileLength = connection.getContentLength();

// download the file

InputStream input = new BufferedInputStream(connection.getInputStream());

OutputStream output = new FileOutputStream("/sdcard/BarcodeScanner-debug.apk");

byte data[] = new byte[1024];

long total = 0;

int count;

while ((count = input.read(data)) != -1) {

total += count;

// publishing the progress....

Bundle resultData = new Bundle();

resultData.putInt("progress" ,(int) (total * 100 / fileLength));

receiver.send(UPDATE_PROGRESS, resultData);

output.write(data, 0, count);

}

output.flush();

output.close();

input.close();

} catch (IOException e) {

e.printStackTrace();

}

Bundle resultData = new Bundle();

resultData.putInt("progress" ,100);

receiver.send(UPDATE_PROGRESS, resultData);

}

}

注册DownloadService：

activity中这样调用DownloadService// initialize the progress dialog like in the first example

// this is how you fire the downloader

mProgressDialog.show();

Intent intent = new Intent(this, DownloadService.class);

intent.putExtra("url", "url of the file to download");

intent.putExtra("receiver", new DownloadReceiver(new Handler()));

startService(intent);

使用ResultReceiver接收来自DownloadService的下载进度通知private class DownloadReceiver extends ResultReceiver{

public DownloadReceiver(Handler handler) {

super(handler);

}

@Override

protected void onReceiveResult(int resultCode, Bundle resultData) {

super.onReceiveResult(resultCode, resultData);

if (resultCode == DownloadService.UPDATE_PROGRESS) {

int progress = resultData.getInt("progress");

mProgressDialog.setProgress(progress);

if (progress == 100) {

mProgressDialog.dismiss();

}

}

}

}

2.1使用 Groundy library

Groundy 可以帮助你在后台service中运行一些代码，其实也是基于刚刚用到的 ResultReceiver，下面是使用Groundy的大致代码：public class MainActivity extends Activity {

private ProgressDialog mProgressDialog;

@Override

public void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

findViewById(R.id.btn_download).setOnClickListener(new View.OnClickListener() {

public void onClick(View view) {

String url = ((EditText) findViewById(R.id.edit_url)).getText().toString().trim();

Bundle extras = new Bundler().add(DownloadTask.PARAM_URL, url).build();

Groundy.create(DownloadExample.this, DownloadTask.class)

.receiver(mReceiver)

.params(extras)

.queue();

mProgressDialog = new ProgressDialog(MainActivity.this);

mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);

mProgressDialog.setCancelable(false);

mProgressDialog.show();

}

});

}

private ResultReceiver mReceiver = new ResultReceiver(new Handler()) {

@Override

protected void onReceiveResult(int resultCode, Bundle resultData) {

super.onReceiveResult(resultCode, resultData);

switch (resultCode) {

case Groundy.STATUS_PROGRESS:

mProgressDialog.setProgress(resultData.getInt(Groundy.KEY_PROGRESS));

break;

case Groundy.STATUS_FINISHED:

Toast.makeText(DownloadExample.this, R.string.file_downloaded, Toast.LENGTH_LONG);

mProgressDialog.dismiss();

break;

case Groundy.STATUS_ERROR:

Toast.makeText(DownloadExample.this, resultData.getString(Groundy.KEY_ERROR), Toast.LENGTH_LONG).show();

mProgressDialog.dismiss();

break;

}

}

};

}

其中GroundyTask的定义如下：public class DownloadTask extends GroundyTask {

public static final String PARAM_URL = "com.groundy.sample.param.url";

@Override

protected boolean doInBackground() {

try {

String url = getParameters().getString(PARAM_URL);

File dest = new File(getContext().getFilesDir(), new File(url).getName());

DownloadUtils.downloadFile(getContext(), url, dest, DownloadUtils.getDownloadListenerForTask(this));

return true;

} catch (Exception pokemon) {

return false;

}

}

}

但是请记住要在activity中注册了相关service才行：

三、使用DownloadManager

其实这才是解决下载问题的终极方法，因为他使用起来实在是太简单了。可惜只有在GingerBread 之后才能使用。

先判断能不能使用DownloadManager：/**

* @param context used to check the device version and DownloadManager information

* @return true if the download manager is available

*/

public static boolean isDownloadManagerAvailable(Context context) {

try {

if (Build.VERSION.SDK_INT < Build.VERSION_CODES.GINGERBREAD) {

return false;

}

Intent intent = new Intent(Intent.ACTION_MAIN);

intent.addCategory(Intent.CATEGORY_LAUNCHER);

intent.setClassName("com.android.providers.downloads.ui", "com.android.providers.downloads.ui.DownloadList");

List list = context.getPackageManager().queryIntentActivities(intent,

PackageManager.MATCH_DEFAULT_ONLY);

return list.size() > 0;

} catch (Exception e) {

return false;

}

}

如果能，那么只需要这样就可以开始下载一个文件了：String url = "url you want to download";

DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));

request.setDescription("Some descrition");

request.setTitle("Some title");

// in order for this if to run, you must use the android 3.2 to compile your app

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {

request.allowScanningByMediaScanner();

request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);

}

request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "name-of-the-file.ext");

// get download service and enqueue file

DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);

manager.enqueue(request);

下载的进度会在消息通知中显示。

总结

前两种方法需要你考虑的东西很多，除非是你想完全控制下载的整个过程，否则用最后一种比较省事。