I need to have standalone ID generator which is not part of any table mapping. I need to write function which takes a sequence name as argument and returns the next val. what is the best way of doing this ?

Writing your own ID generator is pretty simple. You need to create a class that implements IdentifierGenerator. Inside this class you can do whatever you want to return the id.

Then in your mapping files, just specify your class as the <generator>



You can look at the code for the standard implementations for examples.