Why you are not trying to use HSQLDB?
You can use it in embedded mode which needs no installation  or
configuration.

There is a doc folder in the Spring-x-y-z-with-dependancies.zip, in the doc folder, you will find a MVC_step_by_step, the test  application that you need is already in there. You can set it up as explained in the folder with a lightweight database  like HSQL in 2 hours or so...
I guess starting JackRabbit plus the Appfuse or tools like that will need more time to setup....

I think HSQLDB which is an Embedded Java RDBMS is the tool you need, you can simply integrate it with Spring and deploy it anywhere you want. You can use standard SQL to store ,query and retrieve data  .

On the other hand Jackrabbit is a content repository not a database  which has its own methods to store, query and retrieve “content”, I don’t thing a content repository can replace a DBMS unless your problem at hand demands a CMS.

Jackrabbit is a good implementation of JSR-170 for Content management systems,
it is very good to store tree like structures.

if you need a database  which doesn't need installation  there are plenty of them available,
you can start with HSQL which is bundled with lots of components.

from JDK 6, Java DB is also bundled in JDK so you can use Java DB ( formerly derby and cloudscape)

you can still use database  in your project  without need any instalation in server !
many java database (also free) exist that you can connect to it with its jar file  and all of need
is put that jar file on your project too handling any thing as fast as any other db that need instalation !

one of most popular of this is derby that provided by apache  and you can now download and
learn it from this url :
http://db.apache.org/derby/

i use it in one of my project that need run  on autoplay cd (something like multimedia cd) .

Try embedded databases like Hypersonic (hSql). These databases
are placed in memory and are as fast as you need to demo your
application. Also you do not need to change a lot of things in your
app, just a little change in data  source.

You need some knowledege about JCR to using Jackrabbit functionality. a JCR implementation isn't rigth solution, when you are going to choose a minimal system. I suggest using HSQL using spring  that is working without any extra installation  and configuration.

You can use Hypersonic database  (RDBMS) or DB4O (OODBMS) easily without any special installation.
Most of Java EE open source projects use Hypersonic.
And you will loose Transactions capabilies if you use text  files same as xml  files to store data  by xml or io libraries.