如果你要从互联网下载东西，不要忘记在你的清单文件中添加权限！

有很多方法可以下载文件。接下来我会发布最常见的方式;选择哪种方法更适合您的应用程序取决于您。

1. 使用AsyncTask并在对话框中显示下载进度
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这种方法将允许您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）。

导入:

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
    }
});

AsyncTask将如下所示:

// 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;
    }

上面的方法(doInBackground)总是在后台线程上运行。您不应在那里执行任何UI任务。另一方面，onProgressUpdate和onPreExecute在UI线程上运行，所以您可以在那里更改进度条:

    @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();
    }
}

为了运行此代码，您需要WAKE_LOCK权限。

2. 从服务下载
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这里的大问题是：如何从服务更新我的活动？在下一个示例中，我们将使用两个您可能不知道的类：ResultReceiver和IntentService。ResultReceiver 将允许我们从服务更新我们的线程；IntentService是 Service的子类，它产生一个线程来做后台工作（您应该知道 Service 实际上运行在您的应用程序的同一线程上;当您扩展 Service 时，您必须手动生成新的线程来运行CPU阻塞操作)。

下载服务可以像这样：

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：

然后活动将看起来像这样：

// 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);

这就是ResultReceiver的用法：

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是一个库，它基本上帮助您在后台服务中运行代码片段，并且它基于上面展示的ResultReceiver概念。该库目前已过时。这就是整个代码的实现：

您正在显示对话框的活动...

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;
            }
        }
    };
}

Groundy用于下载文件并显示进度的GroundyTask实现：

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;
        }
    }
}

只需将其添加到清单中：

我认为这是再简单不过的了。只需从Github获取最新的jar包，你就可以开始了。请记住，Groundy的主要目的是在后台服务中调用外部REST API并轻松地将结果发布到UI中。如果您的应用程序正在执行此类操作，这可能非常有用。

2.2 使用https://github.com/koush/ion

3. 使用DownloadManager类（仅适用于及更高版本）

GingerBread带来了一个新功能，DownloadManager，它允许您轻松下载文件，并将处理线程，流等艰苦工作委托给系统。

首先，让我们看一个实用方法：

/**
 * @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;
}

方法的名称就解释了一切。一旦你确定DownloadManager可用，你可以这样做：

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);

下载进度会显示在通知栏中。

最后的想法

第一和第二个方法只是冰山一角。如果你想要你的应用程序更加强大，有很多要牢记的事情。以下是一个简要列表：

• 你必须检查用户是否有可用的网络连接
• 确保你有正确的权限（INTERNET和WRITE_EXTERNAL_STORAGE）；如果你想检查互联网的可用性，还要有ACCESS_NETWORK_STATE。
• 确保你要下载文件的目录存在并具有写入权限。
• 如果下载太大，你可能需要实现一种方法，以便在之前的尝试失败时恢复下载。
• 如果允许用户打断下载，用户会很感激。

除非你需要详细控制下载过程，否则考虑使用DownloadManager（3），因为它已经处理了上面列出的大多数项。

但也要考虑到你的需求可能会改变。例如，DownloadManager 没有响应缓存。它会盲目地下载同一个大文件多次。之后很难修复。如果你从基本的HttpURLConnection（1、2）开始，你只需要添加一个HttpResponseCache。所以学习基本的、标准的工具的初始投入可能是一个不错的投资。

这个类在API级别26中被弃用。ProgressDialog是一个模态对话框，防止用户与应用程序进行交互。你应该使用像ProgressBar这样的进度指示器来替代使用这个类，它可以嵌入到你的应用程序的UI中。另外，你也可以使用通知来通知用户任务的进展。更多细节请参考链接