有许多下载文件的方法。接下来，我将发布最常见的方式；由您决定哪种方法更适合您的应用。
1.使用AsyncTask并在对话框中显示下载进度
通过此方法，您可以执行一些后台进程并同时更新UI（在这种情况下，我们将更新进度条）。
进口：
import android.os.PowerManager;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.FileOutputStream;
import java.net.HttpURLConnection;
这是一个示例代码：
// 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); //cancel the task
}
});
该AsyncTask会是这样的：
// 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<String, Integer, String> {
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;
}
上面的方法（doInBackground）始终在后台线程上运行。您不应该在那里执行任何UI任务。另一方面，onProgressUpdateand onPreExecute在UI线程上运行，因此您可以在其中更改进度栏：
@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();
}
要运行此程序，您需要WAKE_LOCK权限。
<uses-permission android:name="android.permission.WAKE_LOCK" />
2.从服务下载
这里的最大问题是：如何从服务更新我的活动？。在下一个示例中，我们将使用您可能不知道的两个类：ResultReceiver和IntentService。ResultReceiver是一个允许我们从服务更新线程的线程；IntentService是的子类，Service它从那里产生一个线程来执行后台工作（您应该知道一个Service线程实际上在应用程序的同一线程中运行；扩展时Service，您必须手动产生新线程来运行CPU阻塞操作）。
下载服务如下所示：
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 {
//create url and connect
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());
String path = "/sdcard/BarcodeScanner-debug.apk" ;
OutputStream output = new FileOutputStream(path);
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);
}
// close streams
output.flush();
output.close();
input.close();
} catch (IOException e) {
e.printStackTrace();
}
Bundle resultData = new Bundle();
resultData.putInt("progress" ,100);
receiver.send(UPDATE_PROGRESS, resultData);
}
}
将服务添加到清单中：
<service android:name=".DownloadService"/>
活动将如下所示：
// 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);
这里是ResultReceiver来玩：
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"); //get the progress
dialog.setProgress(progress);
if (progress == 100) {
dialog.dismiss();
}
}
}
}
2.1使用Groundy库
Groundy是一个基于ResultReceiver上面显示的概念的库，基本上可以帮助您在后台服务中运行代码。目前不推荐使用该库。这是怎么整的代码将如下所示：
显示对话框的活动...
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;
}
}
};
}
一个GroundyTask使用实施Groundy下载文件，并显示进度：
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;
}
}
}
并将其添加到清单中：
<service android:name="com.codeslap.groundy.GroundyService"/>
我认为这再简单不过了。只需从Github获取最新的Jar ，就可以开始了。请记住，Groundy的主要目的是在后台服务中调用外部REST api，并将结果轻松发布到UI。如果您在应用中执行类似的操作，则可能会非常有用。
3.使用DownloadManager课程（GingerBread仅适用于较新的课程）
GingerBread带来了一项新功能，DownloadManager它使您可以轻松下载文件，并将处理线程，流等的辛苦工作委托给系统。
首先，让我们看一个实用程序方法：
/**
* @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) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD) {
return true;
}
return false;
}
方法的名称说明了一切。确定DownloadManager有空后，可以执行以下操作：
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);
下载进度将显示在通知栏中。
最后的想法
第一种和第二种方法只是冰山一角。如果要使应用程序更强大，则必须记住很多事情。这是一个简短的清单：
- 您必须检查用户是否可以连接互联网
- 确保您具有正确的权限（
INTERNET和WRITE_EXTERNAL_STORAGE）；另外，ACCESS_NETWORK_STATE如果您想检查互联网的可用性。
- 确保要下载文件的目录存在并且具有写权限。
- 如果下载太大，则您可能希望采用一种方法来恢复以前的尝试失败的下载。
- 如果您允许用户中断下载，他们将不胜感激。
除非您需要详细控制下载过程，否则请考虑使用DownloadManager（3），因为它已经可以处理上面列出的大多数项目。
但也请考虑您的需求可能会改变。例如，DownloadManager 不响应缓存。它将多次盲目下载相同的大文件。事后没有简单的方法来修复它。如果您从基本HttpURLConnection（1，2）开始，那么您只需添加一个HttpResponseCache。因此，学习基本的标准工具的初期工作可能是一项不错的投资。
此类已在API级别26中弃用。ProgressDialog是一个模式对话框，可防止用户与应用程序进行交互。而不是使用此类，您应该使用进度指示器（例如ProgressBar），该指示器可以嵌入应用程序的UI中。或者，您可以使用通知来通知用户任务的进度。有关更多详细信息，请链接