As well as all the good stuff submitted by the community (above), I'd also expect to see evidence of some hard skills. Not perfection or über-geek powers, but a good working knowledge of the basics and basic concepts. You got the job already (congratulations by the way!) so you're pretty much there.
But refreshing your knowledge and/or getting up to speed with the tools, languages and technologies your company uses will mean that you'll hit the ground running and create a good first impression.
A hypothetical perfect junior developer will know the fundamentals of such things as
- the language.
- OO concepts like inheritance
- Object interfaces
- database access (esp. with regard to how they do it at your new code 'shop)
- the HTTP model
- database design
- SQL and stored procedures
- the IDE(s) you're going to be using
- relevant frameworks (you mention Spring)
and so on. He'll be able to concentrate on picking up new skills without having to stop and learn basics. He'll be able to plod on (asking as many sensible questions as he likes - most Senior devs love being asked sensible technical questions) with the routine fare of online forms, admin pages that many juniors face for the first few months. Becoming a reliable, junior dev may be your first team initiation test ;-)
When I was starting out as a junior in the web team of a large corporation, the bulk of the initial work I was assigned was somewhat uninspiring but taxed me to the point where I hit the books every night so I could understand what I was going to have to do the next day. I got through it, but that was a stressful five or six months after the relative luxury of my Masters course ;-)
If you can do all the basic stuff efficiently, you get more interesting stuff to do fairly quickly and this will of course help advance your career.
And echoing what many above have said, the world of computers and code is constantly evolving. You will need to learn new stuff every other day. Keep your mind wide open and try and keep sight of why you have embarked on this in the first place - because you <3 it. In the years to come there may be times when this will not be at the forefront of your mind. Enthusiasm will get you through over most of the bumps and hurdles.
Having said all that, my experience of the last twelve years or so suggests that most juniors don't stay in their first company forever, so don't get too hung up on being 'perfect'. Making mistakes is all part of the process that will get you from junior to wherever you want to end up. I've been doing this for a while now and almost-delete a production database pretty much at least a couple of times a year.
Good luck with your career.