Autostart an app at bootup

Generally speaking it's not possible in Android to make your app to start automatically at bootup. But you can use a trick to achieve the same result.

Basically you have to register a BroadcastReceiver in your AndroidManifest.xml file to listen to the Intent.ACTION_BOOT_COMPLETED event. In your BroadcastReceiver class you can now start your app or Service.
This method is often used to start a Service to perform certain operations in the background (for example, to register an alarm clock).

Let's look at the code of the AndroidManifest.xml file:

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
...
<receiver android:enabled="true" android:name=".BootUpReceiver"
        android:permission="android.permission.RECEIVE_BOOT_COMPLETED">

        <intent-filter>
                <action android:name="android.intent.action.BOOT_COMPLETED" />
                <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
</receiver>

Now, let's suppose that you want to start a Service at bootup. The code of our BroadcastReceiver is the following:

public class BootUpReceiver extends BroadcastReceiver{

        @Override
        public void onReceive(Context context, Intent intent) {
                Intent i = new Intent(INTENT_ACTION);  
                i.putExtra...
                [...]
                context.startService(i);  
        }

}

Of course you have to use this code only when really needed: an app starting app every time the device is booting could possibly waste system's resources.