Default How to relock boot loader for xperia u

PLZ HELP ME RELOCK THE BOOT LOADER FOR XPERIA U
AND HOW I CHECK IS MY BOOT LOADER IS LOCKED OR UNLOCKED
WHEN I DIAL THE *#*#7378423#*#* to access the service menu.
IT SAYS BOOTLOADER UNLOCK ALLOWED:- YES
WHAT IS MEAN THAT PLZ TELL ME
AND ALSO AFTER RELOCKING BOOT LOADER IS MY XPERIA U WARRANTY BACK OR NOT
SO PLZ HELP ME