After you've modified the sources and built the library it is always a good idea to test your new version thoroughly. For this purpose, you can use db4o regression tests.
In order to run regression tests in java you will need the following projects:
bloat
db4oj
db4ojdk1.2
db4ojdk5
db4oj.tests
db4o_osgi
db4o_osgi_test
db4otaj
db4otools
db4ounit
db4ounit.extensions
The test suite is started from com.db4o.db4ounit.jre5.AllTestsDb4oUnitJdk5 class in db4ojdk5 project. You can check the output of the tests in the console ("GREEN" - for successful test completion).
For more information see Db4o Testing Framework.