I'm not sure if it especially complicated, but it will be quite a big job. As you say, you have to create the Java, the DDL, and the Hibernate mapping files (hbm.xml).
There are tools within Hibernate that generate the Java and DDL from the Hibernate mapping files. Similarly, you can use XDoclet to generate the Hibernate mapping files from the Java, and then in turn generate the DDL from the Hibernate mapping files.
The next issue you have is keeping the Java and DDL in sync in EA if you use EA to generate the Java and DDL. I've found the Model Transformation generators struggle with many-to-many relationships, particularly those from associative objects, which seems to form quite large parts of my models owing to vague requirements for understanding the relationship between classes.
I found quite a quick way was to modify the EA Java code generators to add XDoc comments into the headers, and then use XDoclet to do the hard work of creating the hbm.xml files. However, I abandoned its widescale use because it repeated the comments for both the get and set methods, which resulted in two column entries for each attribute. I now have a Perl script that generates the Java, DDL, and hbm.xml files. It is still a pain to create the Perl-based configuration files from the design, but the process is much faster than generating all by hand.
Saying that, if someone does do a proper job for creating the transforms, they'll have me as a customer! If I can help with some of the concepts, don't hesitate to contact me.
