Question

I am having trouble loading a bitmap from a url on Android based on this answer: https://stackoverflow.com/a/8993175/1062794

I’ve simplified the case to the absolute minimum:

public void loadBitmap(View view) {
    Bitmap b = getBitmapFromURL("http://upload.wikimedia.org/wikipedia/en/7/70/Example.png");
}

public Bitmap getBitmapFromURL(String src) {
    try {
        URL url = new URL(src);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setDoInput(true);
        connection.connect();
        InputStream input = connection.getInputStream();
        Bitmap myBitmap = BitmapFactory.decodeStream(input);
        return myBitmap;
    } catch (IOException e) {
        e.printStackTrace();
        return null;
    }
}

I have enabled internet access in manifest (I believe):

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

When I run the app it crashes with null details when it tries to run connection.connect(). Stepping through I see it tries to throw this error from StrictMode.class:

if ((mPolicyMask & PENALTY_DEATH_ON_NETWORK) != 0) {
    throw new NetworkOnMainThreadException();
}

This is my first day trying to make an Android app so I could be making an obvious mistake. I am using the emulator and Win7.

Answer 1

Starting with Android 3.0, synchronous operations can no longer be run directly from a UI thread. If you try to call the loadBitmap(View view) method directly in your onCreate() method, your application will crash when it is run on a device running Android 3.0 and later. Because loadBitmap() method is synchronous – that is, it will not return control until the image is downloaded – calling it directly will freeze the UI of your activity. This is not allowed in Android 3.0 and later; all synchronous code must be wrapped using an AsyncTask class. Using AsyncTask enables you to perform background tasks in a separate thread and then return the result in a UI thread. That way, you can perform background operations without needing to handle complex threading issues. To call the loadBitamp() method asynchronously, you need to wrap the code in a subclass of the AsyncTask class, as shown here:

private class DownloadImage extends AsyncTask<String, Void, Bitmap> {
    protected Bitmap  doInBackground(String... urls) {
    return getBitmapFromUrl(urls[0]);
    }
    protected void onPostExecute(Bitamp result) {
    ImageView img = (ImageView) findViewById(R.id.img);
    img.setImageBitmap(result);
    }
}

Now in the onCreate() method create a new instance of AsyncTask class and execute it:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    new DownloadImage().execute("http://upload.wikimedia.org/wikipedia/en/7/70/Example.png");
}