Geração automática de chaves de hibernação com MySQL e Oracle

1. 
   Uma maneira seria mudar para os identificadores atribuídos. Você pode usar identificadores UUID, que funcionam tanto para MySQL quanto para Oracle, e também pode atribuir os valores manualmente.
   
2. 
   Outra maneira é usar um gerador de tabela personalizado.
   

    public interface Identifiable<T extends Serializable> {
        T getId();
    }

    public class AssignedTableGenerator extends TableGenerator {

        @Override
        public Serializable generate(SessionImplementor session, Object obj) {
            if(obj instanceof Identifiable) {
                Identifiable identifiable = (Identifiable) obj;
                Serializable id = identifiable.getId();
                if(id != null) {
                    return id;
                }
            }
            return super.generate(session, obj);
        }
    }

    doInTransaction(session -> {
        for (int i = 0; i < 5; i++) {
            session.persist(new AssignTableSequenceIdentifier());
        }
        AssignTableSequenceIdentifier tableSequenceIdentifier = new AssignTableSequenceIdentifier();
        tableSequenceIdentifier.id = -1L;
        session.merge(tableSequenceIdentifier);
        session.flush();
    });

    select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
    insert into sequence_table (sequence_name, next_val)  values (default,1)
    update sequence_table set next_val=2  where next_val=1 and sequence_name=default
    select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
    update sequence_table set next_val=3  where next_val=2 and sequence_name=default
    select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
    update sequence_table set next_val=4  where next_val=3 and sequence_name=default
    select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
    update sequence_table set next_val=5  where next_val=4 and sequence_name=default
    select tbl.next_val from sequence_table tbl where tbl.sequence_name=default for update
    update sequence_table set next_val=6  where next_val=5 and sequence_name=default
    select identityvs0_.id as id1_0_0_ from assigneTableIdentifier identityvs0_ where identityvs0_.id=-1
    insert into assigneTableIdentifier (id) values (1, 2)
    insert into assigneTableIdentifier (id) values (2, 4)
    insert into assigneTableIdentifier (id) values (5, -1)

public class AssignedSequenceStyleGenerator 
    extends SequenceStyleGenerator {
 
    @Override
    public Serializable generate(SessionImplementor session, 
        Object obj) {
        if(obj instanceof Identifiable) {
            Identifiable identifiable = (Identifiable) obj;
            Serializable id = identifiable.getId();
            if(id != null) {
                return id;
            }
        }
        return super.generate(session, obj);
    }
}

@Id
@GenericGenerator(
    name = "assigned-sequence",
    strategy = "com.vladmihalcea.book.hpjp.hibernate.identifier.AssignedSequenceStyleGenerator",
    parameters = @org.hibernate.annotations.Parameter(
        name = "sequence_name", 
        value = "post_sequence"
    )
)
@GeneratedValue(
    generator = "assigned-sequence", 
    strategy = GenerationType.SEQUENCE
)
private Long id;