java - 在 JPA 中修改查询结果

Question

EntityManager em = EMF.get().createEntityManager();
EntityTransaction tx = null;

List<Profile> list = null;
Query q = null;

try{
    tx = em.getTransaction();
    tx.begin();

    q = em.createNamedQuery("Profile.getRandomProfile");
    q.setParameter("random", Math.random());
    q.setMaxResults(8);
    list = (List<Profile>) q.getResultList();

    if (list != null){
        Collections.shuffle(list);
    }

    tx.commit();

} catch(NoResultException ex){
    System.out.println("ERROR CATCHED: " +ex.getMessage());
    if(tx != null && tx.isActive())
        tx.rollback();
} catch(Exception e){
    e.printStackTrace();
}
finally{
    em.close();
}

打乱列表有错误:

java.lang.UnsupportedOperationException: Query result sets are not modifiable

如何克服这个问题？

Answer 1

将结果复制到二级列表并对其进行洗牌，而不是查询结果列表。

ArrayList copyList = new ArrayList();
Collections.copy(copyList,list);
Collections.shuffle(copyList);