Let say we have a server that manage a library in which you can add a book, find a book and get all the books. The server code will probably look like this:
with the class Book being something like that.
To ignore the metaClass property a custom type mapping must be defined (for details refer to Aegis Binding).
However, if you compile custom data types from Java the bytecode won't contain a metaClass property and, hence, there is no need to define a custom mapping.