can I unlock then lock the bootloader?

I am considering flashing stock factory image to my Nexus 7 2013.

in the guide below, it says I have to unlock the bootloader.

Manual update Nexus 7

My questions are

1. after the factory image is loaded, is the bootloader locked again?
2. if not, how do i lock it?

Thanks

I would take a look at Wugfresh Nexus Root Toolkit you can download it here click here. With this toolkit you can make backup, lock and unlock boot loader, root and unroot and so much more, all with one or two clicks.