http://youtu.be/x65ua5x4sMc
Root Xperia U
For Gingerbread:
First read everything before you start, might need to update your phone first.
The video can be found via this
http://youtu.be/x65ua5x4sMclink.
http://forum.xda-developers.com/showthread.php?t=1526866
For a complete Rooting Wiki by DoomLord on XDA, read the first 8 posts on the page to really understand everything you are doing to your phone!
I take no responsibility for any damage done by following this guide!
Unlock Bootloader
visit
http://unlockbootloader.sonymobile.com/Bootloader Unlock Sony and follow guide
1.Dial *#*#7378423#*#* this will bring you to the service menu
2. *Service > Service info > Configuration >
Rooting status: unlock allowed must be YES
if it says NO you cannot root your phone (no solution yet at this time)
*Service > Service info > Configuration >
write down your IMEI number to get your Unlock Code
3.Go to [unlockbootloader.sonymobile.com Bootloader Unlock Sony]
read it and click through all pages to create your unlock request.
If your IMEI doesn't fit, remove the last digit.
4.download & install the Android SDK
http://developer.android.com/sdk
5.download Fastboot driver
http://developer.sonymobile.com/downloads/drivers/fastboot-driver, but do not yet install it
6.take out usb cable & turn off your Xperia U
7.Enter FastBoot mode > hold the Volume Up button while putting back the USB cable. The Left Blue Led will turn on.
8.Install the fastboot driver while the LED is on
9.if you have problems to install the driver and the blue light turns off multiple times before you can update the driver: Open your device manager and go back to step 6
At the moment you connect the USB cable, you’ll see an unknown device being highlighted. Right click on it to manually update the driver via Properties > Driver > Update Driver > Browse my computer and select the fastboot driver.
10.Windows start menu > typ CMD in search box
11.type CD C:\Program Files (x86)\android-sdk\platform-tools
12.run fastboot.exe -i 0x0fce getvar version
verify response ( version: 0.x finished)
if it says < waiting for device > go to back to step 6
If you continue you will not be able to auto-update your software again.
New updates need to be installed manually and can be found at
XDA
Your warranty will most likely be voided. For more information read here
http://talk.sonymobile.com/message/220509#220509
Last Step:
run fastboot.exe -i 0x0fce oem unlock 0x22222222
replace 22222222 with your unlock code.
done! reinstall your phone
ROOT your Xperia U
1.dail *#*#7378423#*#* this will bring you back to the service menu
2.Service > Service info > Software info >
3.Check your software version, most likely it will be one of the following: 6.0.B.1.564 or 6.0.B.3.184 (if it is another version, update your phone first)
4.get the matching kernel from the /kernel/ directory in this zip.
CREDITS
The 1.564 kernel is made by DoomLord
The 3.184 kernel is made by KeiranFTW
5.Copy the matching kernel number from the zip to
C:\Program Files (x86)\android-sdk\platform-tools
If you install the wrong version your phone will break
6.Go into FastBoot Mode again (blue light thingy)
if you forgot / run into problems > step 6-12 of Unlock Bootloader guide
7.open up the CMD again
to double check the connection type fastboot.exe -i 0x0fce getvar version
8.type or paste cd C:\Program Files (x86)\android-sdk\platform-tools
9.type fastboot flash boot 222_kernel.elf
replace 22222 with your specific kernel
USSD-Codes
IMEI: *#06#
Extended Info menu: *#*#4636#*#*
Service menu: *#*#7378423#*#*
ehema ICS eka U ekata daapu aya innwa nam ekath kynna hondada kyla 
