/**
     * 判断用户是否存在
     * @param username
     * @return
     */
public boolean exsit(String username){
        long count = (Long)em.createQuery("select count(o) from Buyer o where o.username=?1")
        .setParameter(1, username).getSingleResult();
        return count>0;
    }
/**
     * 判断用户名及密码是否正确
     * @param username
     * @param password
     * @return
     */
public boolean checkUser(String username, String password){
        long count = (Long)em.createQuery("select count(o) from Buyer o where o.username=?1 and o.password=?2")
        .setParameter(1, username).setParameter(2, MD5.MD5Encode(password)).getSingleResult();
        return count>0;
    }