Spying an AsyncTask doesn't work

Issue

I am testing an AsyncTask. I want to stub an HttpURLConnection to return my mocked object. This is how I do it (PackageDownloader represents an AsyncTask):

 ...
 PackageDownloader packageDownloader = new PackageDownloader();
 packageDownloader.setParameters(URL, downloadFolder, downloadProgressCallback);
 PackageDownloader mPackageDownloader = spy(packageDownloader);
 HttpURLConnection connectionMock = Mockito.mock(HttpURLConnection.class);
 doReturn(0).when(connectionMock).getContentLength();
 doReturn(connectionMock).when(mPackageDownloader).createConnection(Mockito.any(URL.class));
 mPackageDownloader.execute();
 mPackageDownloader.get();

This is PackageDownloader:

public HttpURLConnection createConnection(URL url) throws IOException {
    HttpURLConnection connection;
    connection = (HttpURLConnection) url.openConnection();
    return connection;
}

@Override
protected DownloadResult doInBackground(Void... params) {
    HttpURLConnection connection;
    URL downloadUrl = new URL(downloadUrlString);
    connection = createConnection(downloadUrl);
    long totalBytes = connection.getContentLength();
    ...

Here, createConnection returns real, not mocked object, and I can't figure out why.

Solution

Well I have found a solution, though haven't found an explanation why it works so. The reason nothing worked was that doInBackground method is async, I assume, so I had to call it directly via reflection, like so:

 Method method = mPackageDownloader.getClass().getMethod("doInBackground", Void[].class);
 method.invoke(mPackageDownloader, new Void[] {null});

Answered By - annaoomph