@Skyhan – an xml example is provided in http://blog.xebia.com/2007/02/let-hibernate-connect-your-world/ that is linked from this article, you have to look to the source code of the page as the xml is not shown in the browser

@Skyhan – as seen in https://forum.hibernate.org/viewtopic.php?f=6&t=978321 (and later described in http://stackoverflow.com/questions/7313463/multiple-relationships-with-single-mapping-table-without-generating-foreign-keys/7319977#7319977) you can use following annotation:
@ForeignKey( name = “none” )

An annotation would be even better, but cannot seem to locate a complete example anywhere.

Depends on which database you are using and what you are doing.

If you never actually join across the tables having a foreign key, the key only gives you referential integrity assurance. Referential integrity can be assured in other ways.

Some databases do not cope with foreign keys and composite primary keys nicely. I have seen examples of tables where the index and key spaces were an order of magnitude larger than the data set. This is a major issue if you have nontrivial amounts of data.