Quiero leer un documento XML de unaURL:

public void DownloadXmlFile() throws IOException{
        //TODO
        String url = "http://api.m1858.com/coursebook.xml";
        URL u = new URL(url);
        HttpURLConnection conn = (HttpURLConnection) u.openConnection();
        conn.setReadTimeout(10000);
        conn.setConnectTimeout(15000);
        conn.setRequestMethod("GET");
        conn.setDoInput(true);
        conn.connect();
    }

Me sale una excepción de error

android.os.NetworkOnMainThreadException

Agregué usos-permiso enmanifiesto expediente:

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