How to setup a hibernate hbm xml identity generator for DB2 "GENERATED BY DEFAULT WITH SPECIFIER"

Question

I got a hbm mapping for a column, that is set to identity generator

<class name="com.dummy.TestADTO" table="table_a">
    <id name="id" type="integer" column="ID">
        <generator class="identity"/>
    </id>
...
</class>

and half of it already works together with my db2 key table column which is defined

ID NOT NULL GENERATED BY DEFAULT AS IDENTITY (START WITH 1, INCREMENT BY 1, NO CACHE)

my dto is like the following:

package com.dummy;

    class TestADTO {

        private Integer id;

        public Integer getId(){ return id; }
        public void setId(Integer id) { this.id = id; }
    }

1. 'half of it' means, that I can set a null value for the id property and the ID column is set by the generator when hibernate session.merge(testadto) is called:

insert into test_a(id) values (default);

2. The other half which does not work is, that I can set a value for TestADTO.id=155, but the value is not used when hibernate inserts to db with session.merge(testadto). Instead the next value from the identity generator is used:

what I wanted was:

insert into test_a(id) values (?); (where ? will be replaced by 155)

what I get:

insert into test_a(id) values (default);

How can I get that to work?