java classes are,

class Account
{
private long acctId;
}
class Position
{
private long positionId;
}

class UserHolding
{
Account acct;
Position pos;
}

i'm newbie to Hibernate. In UserHolding table acctId and positionId as Composite key. i don't know how to map more than foreign keys as composite key. can anyone send me Hibernate mapping file.