public List getPersonList(int max, int whichpage) {
        try {
            int index = (whichpage - 1) * max;
            Query query = em.createQuery("from Person p order by personid asc");
            List list = query.setMaxResults(max).setFirstResult(index).getResultList();
            em.clear();// 分离内存中受EntityManager管理的实体bean，让VM进行垃圾回收
            return list;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

    public String getLimitString(String querySelect, int offset, int limit) {
        if ( offset > 0 ) {
            throw new UnsupportedOperationException( "query result offset is not supported" );
        }
        return new StringBuffer( querySelect.length() + 8 )
                .append( querySelect )
                .insert( getAfterSelectInsertPoint( querySelect ), " top " + limit )
                .toString();
    }