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copied!<p>My rule of thumb is that an <code>AsyncTask</code> is for when I want to do something tied to single <code>Activity</code> and a <code>Service</code> is for when I want to do something that will carry on after the Activity which started it is in the background.</p> <p>So if I want to do a small bit of background processing in the <code>Activity</code> without tying up the UI I'll use an <code>AsyncTask</code>. I'll then use the default <code>Handler</code> from that <code>Activity</code> to pass messages back to ensure updates happen on the main thread. Processing the updates on the main thread has two benefits: UI updates happen correctly and you don't have to worry so much about synchronisation problems.</p> <p>If for example, I wanted to do a download which might take a while I'd use a <code>Service</code>. So if I went to another <code>Activity</code> in my application or another application entirely my <code>Service</code> could keep running and keep downloading the file so it would be ready when I returned to my application. In this case I'd probably use a <a href="http://developer.android.com/intl/fr/guide/topics/ui/notifiers/notifications.html" rel="noreferrer">Status Bar Notification</a> once the download was complete, so the user could choose to return to my application whenever was convenient for them.</p> <p>What you'll find if you use an <code>AsyncTask</code> for a long-running process it may continue after you've navigated away from the <code>Activity</code> but:</p> <ul> <li>If the <code>Activity</code> is in the background when your processing is complete you may have problems when you try to update the UI with the results etc.</li> <li>A background <code>Activity</code> is far more likely to be killed by Android when it needs memory than a <code>Service</code>.</li> </ul>

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