下载文件的方法有很多。下面我将列出最常见的方法;由你来决定哪种方法更适合你的应用程序。

使用AsyncTask并在对话框中显示下载进度 =============================================================

此方法将允许您执行一些后台进程并同时更新UI(在本例中，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
    }
});

AsyncTask看起来是这样的:

// 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)总是在后台线程上运行。你不应该在那里做任何UI任务。另一方面，onProgressUpdate和onPreExecute运行在UI线程上，所以你可以在那里改变进度条:

    @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();
    }
}

要运行此操作，您需要WAKE_LOCK权限。

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

从服务下载 ========================

The big question here is: how do I update my activity from a service?. In the next example we are going to use two classes you may not be aware of: ResultReceiver and IntentService. ResultReceiver is the one that will allow us to update our thread from a service; IntentService is a subclass of Service which spawns a thread to do background work from there (you should know that a Service runs actually in the same thread of your app; when you extends Service, you must manually spawn new threads to run CPU blocking operations).

下载服务可以是这样的:

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);
    }
}

将服务添加到您的清单:

<service android:name=".DownloadService"/>

这个活动看起来是这样的:

// 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);

这是ResultReceiver开始发挥作用的情况:

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

grounddy是一个库，主要帮助您在后台服务中运行代码段，它基于上面所示的ResultReceiver概念。这个库目前已弃用。这是整个代码的样子:

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;
            }
        }
    };
}

grounddy用来下载文件并显示进度的一个grounddytask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;
        }
    }
}

把这个加到清单上

<service android:name="com.codeslap.groundy.GroundyService"/>

我觉得这再容易不过了。只要从Github抓取最新的罐子，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。如果你在你的应用程序中做类似的事情，它可能真的很有用。

2.2使用https://github.com/koush/ion

使用DownloadManager类(仅限GingerBread和更新版本) =============================================================

GingerBread带来了一个新特性，DownloadManager，它允许你轻松地下载文件，并将处理线程、流等的困难工作委托给系统。

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;
}

方法的名称说明了这一切。一旦你确定DownloadManager是可用的，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);

下载进度将显示在通知栏中。

最终的想法

第一种和第二种方法只是冰山一角。如果你想让你的应用健壮，有很多事情你必须记住。以下是一个简短的列表:

您必须检查用户是否有可用的互联网连接 确保你有正确的权限(INTERNET和WRITE_EXTERNAL_STORAGE);还有ACCESS_NETWORK_STATE如果你想检查互联网可用性。 确保你要下载文件的目录存在并且有写权限。 如果下载太大，你可能想要实现一种方法来恢复下载，如果以前的尝试失败。 如果你允许用户中断下载，他们会很感激的。

除非您需要对下载过程进行详细的控制，否则可以考虑使用DownloadManager(3)，因为它已经处理了上面列出的大部分项目。

但也要考虑到你的需求可能会改变。例如，DownloadManager不做响应缓存。它会盲目地多次下载同一个大文件。事后补救是不容易的。如果你从一个基本的HttpURLConnection(1,2)开始，那么你所需要的就是添加一个HttpResponseCache。因此，最初努力学习基本的标准工具是一项很好的投资。

这个类在API级别26中已弃用。ProgressDialog是一个模式 对话框，这会阻止用户与应用程序进行交互 使用这个类时，您应该使用一个进度指示器 ProgressBar，它可以嵌入到你的应用的UI中。另外, 您可以使用通知通知用户任务的进度。欲知详情Link

如果你要从网上下载东西，别忘了给你的manifest文件添加权限!

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.helloandroid"
    android:versionCode="1"
    android:versionName="1.0">

        <uses-sdk android:minSdkVersion="10" />

        <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>
        <uses-permission android:name="android.permission.INTERNET"></uses-permission>
        <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>
        <uses-permission android:name="android.permission.READ_PHONE_STATE"></uses-permission>

        <application 
            android:icon="@drawable/icon" 
            android:label="@string/app_name" 
            android:debuggable="true">

        </application>

</manifest>

是的，上面的代码是可以工作的，但是如果你在Asynctask的onProgressUpdate中更新你的进度条，你按下后退按钮或完成你的活动，Asynctask将失去它与你的UI的跟踪，当你回到你的活动，即使下载正在后台运行，你也不会看到进度条上的更新。因此，OnResume()尝试运行一个线程，如runOnUIThread与一个定时器任务更新进度条的值从AsyncTask运行后台更新。

private void updateProgressBar(){
    Runnable runnable = new updateProgress();
    background = new Thread(runnable);
    background.start();
}

public class updateProgress implements Runnable {
    public void run() {
        while(Thread.currentThread()==background)
            //while (!Thread.currentThread().isInterrupted()) {
            try {
                Thread.sleep(1000); 
                Message msg = new Message();
                progress = getProgressPercentage();        
                handler.sendMessage(msg);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            } catch (Exception e) {
        }
    }
}

private Handler handler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
        progress.setProgress(msg.what);
    }
};

当你的活动不可见时，不要忘记销毁线程。

private void destroyRunningThreads() {
    if (background != null) {
        background.interrupt();
        background=null;
    }
}

我个人的建议是使用进度对话框并在执行前进行构建，或者在OnPreExecute()上初始化，如果你使用进度对话框的水平风格的进度条，则经常发布进度。剩下的部分是优化doInBackground的算法。

不要忘记用新文件("/mnt/sdcard/…")替换"/sdcard…"，否则你会得到一个FileNotFoundException

我推荐你使用我的Project Netroid，它是基于Volley的。我添加了一些功能，如多事件回调，文件下载管理。这可能会有帮助。

当我开始学习android开发时，我知道ProgressDialog是正确的方法。ProgressDialog的setProgress方法可以在下载文件时调用它来更新进度级别。

我在许多应用程序中看到的最好的情况是，它们自定义了进度对话框的属性，使进度对话框的外观和感觉比库存版本更好。很好地保持用户的一些动画，如青蛙，大象或可爱的猫/小狗。任何有进度对话框的动画都能吸引用户，他们不喜欢长时间等待。

使用Android查询库，确实很酷。你可以改变它使用ProgressDialog，就像你在其他例子中看到的那样，这个将从你的布局中显示进度视图，并在完成后隐藏它。

File target = new File(new File(Environment.getExternalStorageDirectory(), "ApplicationName"), "tmp.pdf");
new AQuery(this).progress(R.id.progress_view).download(_competition.qualificationScoreCardsPdf(), target, new AjaxCallback<File>() {
    public void callback(String url, File file, AjaxStatus status) {
        if (file != null) {
            // do something with file  
        } 
    }
});

我已经修改了AsyncTask类来处理progressDialog在同一上下文中的创建。我认为下面的代码将更可重用。 (它可以从任何活动调用，只要传递上下文，目标文件，对话框消息)

public static class DownloadTask extends AsyncTask<String, Integer, String> {
    private ProgressDialog mPDialog;
    private Context mContext;
    private PowerManager.WakeLock mWakeLock;
    private File mTargetFile;
    //Constructor parameters :
    // @context (current Activity)
    // @targetFile (File object to write,it will be overwritten if exist)
    // @dialogMessage (message of the ProgresDialog)
    public DownloadTask(Context context,File targetFile,String dialogMessage) {
        this.mContext = context;
        this.mTargetFile = targetFile;
        mPDialog = new ProgressDialog(context);

        mPDialog.setMessage(dialogMessage);
        mPDialog.setIndeterminate(true);
        mPDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        mPDialog.setCancelable(true);
        // reference to instance to use inside listener
        final DownloadTask me = this;
        mPDialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
            @Override
            public void onCancel(DialogInterface dialog) {
                me.cancel(true);
            }
        });
        Log.i("DownloadTask","Constructor done");
    }

    @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();
            }
            Log.i("DownloadTask","Response " + connection.getResponseCode());

            // 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(mTargetFile,false);

            byte data[] = new byte[4096];
            long total = 0;
            int count;
            while ((count = input.read(data)) != -1) {
                // allow canceling with back button
                if (isCancelled()) {
                    Log.i("DownloadTask","Cancelled");
                    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;
    }
    @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) mContext.getSystemService(Context.POWER_SERVICE);
        mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK,
                getClass().getName());
        mWakeLock.acquire();

        mPDialog.show();

    }

    @Override
    protected void onProgressUpdate(Integer... progress) {
        super.onProgressUpdate(progress);
        // if we get here, length is known, now set indeterminate to false
        mPDialog.setIndeterminate(false);
        mPDialog.setMax(100);
        mPDialog.setProgress(progress[0]);

    }

    @Override
    protected void onPostExecute(String result) {
        Log.i("DownloadTask", "Work Done! PostExecute");
        mWakeLock.release();
        mPDialog.dismiss();
        if (result != null)
            Toast.makeText(mContext,"Download error: "+result, Toast.LENGTH_LONG).show();
        else
            Toast.makeText(mContext,"File Downloaded", Toast.LENGTH_SHORT).show();
    }
}

权限

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

使用HttpURLConnection

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

import android.app.Activity;
import android.app.Dialog;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.view.Window;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;

public class DownloadFileUseHttpURLConnection extends Activity {

ProgressBar pb;
Dialog dialog;
int downloadedSize = 0;
int totalSize = 0;
TextView cur_val;
String dwnload_file_path =  
"http://coderzheaven.com/sample_folder/sample_file.png";
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    Button b = (Button) findViewById(R.id.b1);
    b.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
             showProgress(dwnload_file_path);

                new Thread(new Runnable() {
                    public void run() {
                         downloadFile();
                    }
                  }).start();
        }
    });
}

void downloadFile(){

    try {
        URL url = new URL(dwnload_file_path);
        HttpURLConnection urlConnection = (HttpURLConnection)   
 url.openConnection();

        urlConnection.setRequestMethod("GET");
        urlConnection.setDoOutput(true);

        //connect
        urlConnection.connect();

        //set the path where we want to save the file           
        File SDCardRoot = Environment.getExternalStorageDirectory(); 
        //create a new file, to save the downloaded file 
        File file = new File(SDCardRoot,"downloaded_file.png");

        FileOutputStream fileOutput = new FileOutputStream(file);

        //Stream used for reading the data from the internet
        InputStream inputStream = urlConnection.getInputStream();

        //this is the total size of the file which we are downloading
        totalSize = urlConnection.getContentLength();

        runOnUiThread(new Runnable() {
            public void run() {
                pb.setMax(totalSize);
            }               
        });

        //create a buffer...
        byte[] buffer = new byte[1024];
        int bufferLength = 0;

        while ( (bufferLength = inputStream.read(buffer)) > 0 ) {
            fileOutput.write(buffer, 0, bufferLength);
            downloadedSize += bufferLength;
            // update the progressbar //
            runOnUiThread(new Runnable() {
                public void run() {
                    pb.setProgress(downloadedSize);
                    float per = ((float)downloadedSize/totalSize) *     
                    100;
                    cur_val.setText("Downloaded " + downloadedSize +  

                    "KB / " + totalSize + "KB (" + (int)per + "%)" );
                }
            });
        }
        //close the output stream when complete //
        fileOutput.close();
        runOnUiThread(new Runnable() {
            public void run() {
                // pb.dismiss(); // if you want close it..
            }
        });         

    } catch (final MalformedURLException e) {
        showError("Error : MalformedURLException " + e);        
        e.printStackTrace();
    } catch (final IOException e) {
        showError("Error : IOException " + e);          
        e.printStackTrace();
    }
    catch (final Exception e) {
        showError("Error : Please check your internet connection " +  
e);
    }       
}

void showError(final String err){
    runOnUiThread(new Runnable() {
        public void run() {
            Toast.makeText(DownloadFileDemo1.this, err,  
      Toast.LENGTH_LONG).show();
        }
    });
}

void showProgress(String file_path){
    dialog = new Dialog(DownloadFileDemo1.this);
    dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
    dialog.setContentView(R.layout.myprogressdialog);
    dialog.setTitle("Download Progress");

    TextView text = (TextView) dialog.findViewById(R.id.tv1);
    text.setText("Downloading file from ... " + file_path);
    cur_val = (TextView) dialog.findViewById(R.id.cur_pg_tv);
    cur_val.setText("Starting download...");
    dialog.show();

     pb = (ProgressBar)dialog.findViewById(R.id.progress_bar);
     pb.setProgress(0);
            pb.setProgressDrawable(
      getResources().getDrawable(R.drawable.green_progress));  
  }
}

我正在为我现在使用的其他解决方案添加另一个答案，因为Android查询太大了，无法保持健康。所以我搬到了这个https://github.com/amitshekhariitbhu/Fast-Android-Networking。

    AndroidNetworking.download(url,dirPath,fileName).build()
      .setDownloadProgressListener(new DownloadProgressListener() {
        public void onProgress(long bytesDownloaded, long totalBytes) {
            bar.setMax((int) totalBytes);
            bar.setProgress((int) bytesDownloaded);
        }
    }).startDownload(new DownloadListener() {
        public void onDownloadComplete() {
            ...
        }

        public void onError(ANError error) {
            ...
        }
    });

您可以使用LiveData和协程观察下载管理器的进度，参见下面的要点

https://gist.github.com/FhdAlotaibi/678eb1f4fa94475daf74ac491874fc0e

data class DownloadItem(val bytesDownloadedSoFar: Long = -1, val totalSizeBytes: Long = -1, val status: Int)

class DownloadProgressLiveData(private val application: Application, private val requestId: Long) : LiveData<DownloadItem>(), CoroutineScope {

    private val downloadManager by lazy {
        application.getSystemService(Context.DOWNLOAD_SERVICE) as DownloadManager
    }

    private val job = Job()

    override val coroutineContext: CoroutineContext
        get() = Dispatchers.IO + job

    override fun onActive() {
        super.onActive()
        launch {
            while (isActive) {
                val query = DownloadManager.Query().setFilterById(requestId)
                val cursor = downloadManager.query(query)
                if (cursor.moveToFirst()) {
                    val status = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS))
                    Timber.d("Status $status")
                    when (status) {
                        DownloadManager.STATUS_SUCCESSFUL,
                        DownloadManager.STATUS_PENDING,
                        DownloadManager.STATUS_FAILED,
                        DownloadManager.STATUS_PAUSED -> postValue(DownloadItem(status = status))
                        else -> {
                            val bytesDownloadedSoFar = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR))
                            val totalSizeBytes = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES))
                            postValue(DownloadItem(bytesDownloadedSoFar.toLong(), totalSizeBytes.toLong(), status))
                        }
                    }
                    if (status == DownloadManager.STATUS_SUCCESSFUL || status == DownloadManager.STATUS_FAILED)
                        cancel()
                } else {
                    postValue(DownloadItem(status = DownloadManager.STATUS_FAILED))
                    cancel()
                }
                cursor.close()
                delay(300)
            }
        }
    }

    override fun onInactive() {
        super.onInactive()
        job.cancel()
    }

}

重要的

AsyncTask在Android 11中已弃用。

欲了解更多信息，请查看以下帖子

Android AsyncTask API在Android 11中已弃用。有什么替代方案? https://developer.android.com/reference/android/os/AsyncTask

可能应该移动到谷歌建议的并发框架

我们可以使用协程和工作管理器在kotlin中下载文件。

在build.gradle中添加一个依赖

    implementation "androidx.work:work-runtime-ktx:2.3.0-beta01"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.1"

WorkManager类

    import android.content.Context
    import android.os.Environment
    import androidx.work.CoroutineWorker
    import androidx.work.WorkerParameters
    import androidx.work.workDataOf
    import com.sa.chat.utils.Const.BASE_URL_IMAGE
    import com.sa.chat.utils.Constants
    import kotlinx.coroutines.delay
    import java.io.BufferedInputStream
    import java.io.File
    import java.io.FileOutputStream
    import java.net.URL

    class DownloadMediaWorkManager(appContext: Context, workerParams: WorkerParameters)
        : CoroutineWorker(appContext, workerParams) {

        companion object {
            const val WORK_TYPE = "WORK_TYPE"
            const val WORK_IN_PROGRESS = "WORK_IN_PROGRESS"
            const val WORK_PROGRESS_VALUE = "WORK_PROGRESS_VALUE"
        }

        override suspend fun doWork(): Result {

            val imageUrl = inputData.getString(Constants.WORK_DATA_MEDIA_URL)
            val imagePath = downloadMediaFromURL(imageUrl)

            return if (!imagePath.isNullOrEmpty()) {
                Result.success(workDataOf(Constants.WORK_DATA_MEDIA_URL to imagePath))
            } else {
                Result.failure()
            }
        }

        private suspend fun downloadMediaFromURL(imageUrl: String?): String? {

            val file = File(
                    getRootFile().path,
                    "IMG_${System.currentTimeMillis()}.jpeg"
            )

            val url = URL(BASE_URL_IMAGE + imageUrl)
            val connection = url.openConnection()
            connection.connect()

            val lengthOfFile = connection.contentLength
            // download the file
            val input = BufferedInputStream(url.openStream(), 8192)
            // Output stream
            val output = FileOutputStream(file)

            val data = ByteArray(1024)
            var total: Long = 0
            var last = 0

            while (true) {

                val count = input.read(data)
                if (count == -1) break
                total += count.toLong()

                val progress = (total * 100 / lengthOfFile).toInt()

                if (progress % 10 == 0) {
                    if (last != progress) {
                        setProgress(workDataOf(WORK_TYPE to WORK_IN_PROGRESS,
                                WORK_PROGRESS_VALUE to progress))
                    }
                    last = progress
                    delay(50)
                }
                output.write(data, 0, count)
            }

            output.flush()
            output.close()
            input.close()

            return file.path

        }

        private fun getRootFile(): File {

            val rootDir = File(Environment.getExternalStorageDirectory().absolutePath + "/AppName")

            if (!rootDir.exists()) {
                rootDir.mkdir()
            }

            val dir = File("$rootDir/${Constants.IMAGE_FOLDER}/")

            if (!dir.exists()) {
                dir.mkdir()
            }
            return File(dir.absolutePath)
        }
    }

开始下载通过工作管理器在活动类

 private fun downloadImage(imagePath: String?, id: String) {

            val data = workDataOf(WORK_DATA_MEDIA_URL to imagePath)
            val downloadImageWorkManager = OneTimeWorkRequestBuilder<DownloadMediaWorkManager>()
                    .setInputData(data)
                    .addTag(id)
                    .build()

            WorkManager.getInstance(this).enqueue(downloadImageWorkManager)

            WorkManager.getInstance(this).getWorkInfoByIdLiveData(downloadImageWorkManager.id)
                    .observe(this, Observer { workInfo ->

                        if (workInfo != null) {
                            when {
                                workInfo.state == WorkInfo.State.SUCCEEDED -> {
                                    progressBar?.visibility = View.GONE
                                    ivDownload?.visibility = View.GONE
                                }
                                workInfo.state == WorkInfo.State.FAILED || workInfo.state == WorkInfo.State.CANCELLED || workInfo.state == WorkInfo.State.BLOCKED -> {
                                    progressBar?.visibility = View.GONE
                                    ivDownload?.visibility = View.VISIBLE
                                }
                                else -> {
                                    if(workInfo.progress.getString(WORK_TYPE) == WORK_IN_PROGRESS){
                                        val progress = workInfo.progress.getInt(WORK_PROGRESS_VALUE, 0)
                                        progressBar?.visibility = View.VISIBLE
                                        progressBar?.progress = progress
                                        ivDownload?.visibility = View.GONE

                                    }
                                }
                            }
                        }
                    })

        }

    private class DownloadTask extends AsyncTask<String, Integer, String> {

        private PowerManager.WakeLock mWakeLock;
        String onlinePathBundle, onlinePathMusic, offlinePathBundle, offlinePathMusic;

        CircleProgressBar progressBar;
        RelativeLayout rl_progress;
        ImageView btn_download;
        TextView tv_progress;

        public DownloadTask(String onlinePathBundle, String onlinePathMusic, String offlinePathBundle, String offlinePathMusic, CircleProgressBar progressBar, RelativeLayout rl_progress, ImageView btn_download,
                            TextView tv_progress) {
            this.offlinePathBundle = offlinePathBundle;
            this.offlinePathMusic = offlinePathMusic;
            this.progressBar = progressBar;
            this.rl_progress = rl_progress;
            this.btn_download = btn_download;
            this.tv_progress = tv_progress;
            this.onlinePathBundle = onlinePathBundle;
            this.onlinePathMusic = onlinePathMusic;
        }

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressBar.setMax(100);
            rl_progress.setVisibility(View.VISIBLE);
            btn_download.setVisibility(View.GONE);

            PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
            mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, getClass().getName());
            mWakeLock.acquire();
        }

        @Override
        protected String doInBackground(String... sUrl) {
            String firstResult = downTask(onlinePathBundle, 1, offlinePathBundle);
            String seondResult = downTask(onlinePathMusic, 2, offlinePathMusic);
            if (firstResult != null) {
                Toast.makeText(SplashActivity.this, "Download error: " + firstResult, Toast.LENGTH_LONG).show();
                return firstResult;
            } else {
                return seondResult;
            }

        }

        @Override
        protected void onProgressUpdate(Integer... progress) {
            super.onProgressUpdate(progress);
            progressBar.setProgress(progress[0]);
            tv_progress.setText(progress[0] + "%");

        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            rl_progress.setVisibility(View.GONE);
            btn_download.setVisibility(View.VISIBLE);
            if (result != null) {
                Toast.makeText(SplashActivity.this, "Download error: " + result, Toast.LENGTH_LONG).show();
            } else {
                rl_progress.setVisibility(View.GONE);
                btn_download.setVisibility(View.GONE);
            }

        }

        public String downTask(String onlineNewPath, int numberOfTask, String offlineNewPath) {
            InputStream input = null;
            OutputStream output = null;
            HttpURLConnection connection = null;
            try {
                URL url = new URL(onlineNewPath);
                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(offlineNewPath);

                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
                        if (numberOfTask == 1) {
                            publishProgress((int) (total * 50 / fileLength));
                        } else if (numberOfTask == 2) {
                            publishProgress(((int) (total * 50 / fileLength)) + 50);
                        }

                    }
                    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;
        }
    }

我遇到了一个简单文件下载库 获取，重要的是，它有存储访问框架，内容提供程序和URI支持。如果有人还在寻找，可能会有帮助。

//AndroidX .tonyodev.fetch2:xfetch2:3.1.6

实现"com.tonyodev.fetch2:fetch2:3.0.12" //支持lib

需要的权限，如果你不是使用应用程序特定的目录。

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

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

网络权限

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

private Fetch fetch;
FetchConfiguration fetchConfiguration = new FetchConfiguration.Builder(this)
                .setDownloadConcurrentLimit(3)//Concurrent Download limit
                .build();

        fetch = Fetch.Impl.getInstance(fetchConfiguration);

        String url = "http:www.example.com/test.txt";//URL of file
        String file = "/downloads/test.txt";//Path of file
        
        final Request request = new Request(url, file);
        request.setPriority(Priority.HIGH);
        request.setNetworkType(NetworkType.ALL);//Preferred network type
        request.addHeader("clientKey", "SD78DF93_3947&MVNGHE1WONG");//Auth header if any
        
        fetch.enqueue(request, updatedRequest -> {
            //Request was successfully enqueued for download.
        }, error -> {
            //An error occurred enqueuing the request.
        });

    }

倾听更新和进展

FetchListener fetchListener = new FetchListener() {
    @Override
    public void onQueued(@NotNull Download download, boolean waitingOnNetwork) {
        if (request.getId() == download.getId()) {
            showDownloadInList(download);
        }
    }

    @Override
    public void onCompleted(@NotNull Download download) {

    }

    @Override
    public void onError(@NotNull Download download) {
        Error error = download.getError();
    }

    @Override
    public void onProgress(@NotNull Download download, long etaInMilliSeconds, long downloadedBytesPerSecond) {
        if (request.getId() == download.getId()) {
            updateDownload(download, etaInMilliSeconds);
        }
        int progress = download.getProgress();
    }

    @Override
    public void onPaused(@NotNull Download download) {

    }

    @Override
    public void onResumed(@NotNull Download download) {

    }

    @Override
    public void onCancelled(@NotNull Download download) {

    }

    @Override
    public void onRemoved(@NotNull Download download) {

    }

    @Override
    public void onDeleted(@NotNull Download download) {

    }
};

fetch.addListener(fetchListener);

//Remove listener when done.
fetch.removeListener(fetchListener);

此示例代码取自所有者页面，所有功劳都归于Tonyo Francis