Start AlarmManager if device is rebooted

Create Boot Receiver using following code :

public class BootBroadcastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context pContext, Intent intent) {
        if(intent.getAction().equals(Intent.ACTION_BOOT_COMPLETED)){
            // Do your work related to alarm manager
        }
    }
}

In your Manifest, register this Broadcast receiver :

<receiver
android:name="com.yourapp.BootBroadcastReceiver"
android:enabled="true" >
<intent-filter>
    <action android:name="android.intent.action.BOOT_COMPLETED" />
</intent-filter>
</receiver>

And don’t forget to add permission in AndroidManifest.xml :

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