Android API 26（Oreo）从Internet下载数据文件

Question

I have now resolved this problem, which turned out to be something completely different. Since no-one has responded, the question can be deleted.

To support API 26 onward, using Android Studio I have already updated my app to explicitly ask for the dangerous permissions I need (WRITE_EXTERNAL_STORAGE, ACCESS_FINE_LOCATION). It also uses normal permission INTERNET, which is granted without needing to ask the user.

The app when first run attempts to download to local storage some data files (all text) from a specific web site that hosts these files. Although it works fine in the API 27 emulator, it fails when tested on my phone running Android 8.1.0 (Huawei P20).

Two files are successfully downloaded, but on the third file it throws an IO exception "premature end of file" when getInputStream is called on the connection, there is a 0 byte transfer, and the app exits after displaying a failure message. However, if I then Clear Data in the app's settings (on the phone), and restart the app on the phone, the full download of all files is successful.

This is the function that is called to download each file (the else clause is what's relevant):

// Download file from the internet
public static boolean downloadFile(String urlString, String fileName)
{
    mDownloadStatus = false;

    // In local file mode, download.tmp must have been pre-loaded
    if (LOCAL_FILE_MODE && fileName.equals(LOCAL_FILE_NAME))
    {
        File f = new File(dataDir, LOCAL_FILE_NAME);
        if (f.exists())
            mDownloadStatus = true;
        else
            mErrorString = "Pre-loaded " + LOCAL_FILE_NAME + " not found";
    }
    else
    {
        CountDownLatch latch = new CountDownLatch(1);
        Runnable r = new DownloadFile(urlString, fileName, latch);
        new Thread(r).start();

        try
        {
            // Wait for completion
            latch.await();
        }
        catch (InterruptedException e) {}
    }
    return mDownloadStatus;
}

And this is the DownloadFile class:

public class DownloadFile implements Runnable
{
    private String          mUrlString;
    private String          mFileName;
    private CountDownLatch  mLatch;

    public DownloadFile(String urlString, String fileName, CountDownLatch latch)
    {
        mUrlString  = urlString;
        mFileName   = fileName;
        mLatch      = latch;
    }

    public void run()
    {
        HttpURLConnection urlConnection = null;

        Spine.mDownloadStatus = false;

        try
        {
            // set the download URL, a url that points to a file on the internet
            // this is the file to be downloaded
            URL url = new URL(mUrlString);

            // create the new connection
            urlConnection = (HttpURLConnection) url.openConnection();

            // set up some things on the connection
            urlConnection.setRequestMethod("GET");
            urlConnection.setDoOutput(true);
            urlConnection.setUseCaches(false);

            // and connect!
            urlConnection.connect();

            // create a new file, specifying the path, and the filename
            // which we want to save the file as.
            File file = new File(Spine.dataDir, mFileName);

            // this will be used to write the downloaded data into the file we
            // created
            FileOutputStream fileOutput = new FileOutputStream(file);

            // this will be used in reading the data from the internet
            InputStream inputStream = urlConnection.getInputStream();

            // create a buffer...
            byte[] buffer = new byte[1024];
            int bufferLength = 0; // used to store a temporary size of the buffer

            // now, read through the input buffer and write the contents to the
            // file
            while ((bufferLength = inputStream.read(buffer)) > 0)
            {
                // add the data in the buffer to the file in the file output
                // stream (the file on the sd card
                fileOutput.write(buffer, 0, bufferLength);
            }

            // close the output stream when done
            fileOutput.close();

            Spine.mDownloadStatus = true;
        }

        // catch some possible errors...
        catch (IOException e)
        {
            Spine.mErrorString = e.getMessage();
        }

        if (urlConnection != null)
            urlConnection.disconnect();

        // Signal completion
        mLatch.countDown();
    }
}

What do I need to do to fix this? I am at my wit's end!

Answer 1

After changing the code to more properly deal with asking for permissions, the original problem described no longer existed. However I had a totally different problem which I solved. Outline of problem: Worked with API27 emulator in both debug and release builds. On Huawei P20 (API27), worked on debug, failed on release. Once I discovered I could enable debugging for the release build I was able to find the problem and fix it.

SharedPreferences for the application seem to persist on the phone somehow even when the app has been uninstalled. The problem was that SharedPreferences said that certain files had already been downloaded when of course they no longer existed on the phone. My fix was to check in this case if the file existed, and if not I reset the preferences so that the download took place.

(I have a suspicion that it was picking up previous SharedPreferences from an earlier version of the app, installed from the Google Play Store and uninstalled so I could test the new version.)