I recently completed this tutorial,
http://www.objectdb.com/tutorial/jpa/eclipse/spring/run
It says the .odb file is created under the Tomcat directory, but I can't find the Tomcat directory. I did a search on my Mac OSX machine and could not find any .odb files. I searched my project and could not find it. I downloaded Tomcat a while ago and searched the folder which eclipse is pointing to. It has to exist somewhere because when I stop the tomcat server in eclipse and restart it the data comes back and persists.
You may specify a full path to your preferred ObjectDB database file path in step 2.
Edit the property javax.persistence.jdbc.url in the persistence.xml file.
Any path on your machine with appropriate permissions is a valid value.