Started playing with this some today. Here is what I found for PostgreSQL. Given a function defined to return a refcursor, here is what seems like is needed:

// Note the need for the leading '? ='...
CallableStatement callableStatement = connection.prepareCall( "{? = call allPersons()}" );
// Have not tried with Java 8 and the explicit Types#REF_CURSOR added there...
callableStatement.registerOutParameter( 1, Types.OTHER );

boolean hasResults = callableStatement.execute();
assertFalse( hasResults );
assertEquals( -1, callableStatement.getUpdateCount() );

// Can't even use getObject( 1, ResultSet.class ) since the postgresql jdbc driver does not implement it...
ResultSet results = (ResultSet) callableStatement.getObject( 1 );

So against postgresql, when a parameter is registered with mode of REF_CURSOR we need to:

1. Know to append the `? ='
2. Validate that the REF_CURSOR is registered positionally and and that its position is 1
3. Disallow named parameters since the spec does not allow named/positional mixing and the REF_CURSOR parameter is implicitly positional
4. Register the JDBC parameter as Types#OTHER