Question

使用JPA，MVC-JSP + Servlets，JSTL

实体和重要财产：

@Entity
public class Post {
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "post_id", unique = true, nullable = false)
    private Integer id;
    @OneToMany(cascade = { ALL }, fetch = EAGER, mappedBy = "post")
    private Set<Comment> comments = new HashSet<Comment>();
    //...
}

@Entity
public class Comment {
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "comment_id", unique = true, nullable = false)
    private Integer id;

    @Column(name = "comment_date", unique = false, nullable = false)
    private Date date;

    @ManyToOne
    @JoinColumn (name = "post_id", referencedColumnName="post_id", nullable = false)
    private Post post;
    //...
}

现在，在servlet的doGet方法中，我有以下代码摘录：

Post post = null;
int postId = Integer.parseInt(request.getParameter("postId"));
post = postDao.findById(postId);

request.setAttribute("post", post);
RequestDispatcher dispatcher = request.getRequestDispatcher("post.jsp");
dispatcher.forward(request, response);

在JSP页面中：

<c:if test="${fn:length(post.comments)>0}">
        <section id="postComments">
            <strong>Comments:</strong>

            <c:forEach items="${post.comments}" var="comment">
                <div class="postComment">
                    <div class="commentInfo"><strong>${comment.user.name}</strong> says:</div>
                    <br>
                    <div class="commentTitle">${comment.title}</div>
                    <hr>
                    <div>${comment.content}</div>
                </div>
            </c:forEach>    
        </section>
    </c:if>

这样我就收到了帖子的所有评论，但我希望按照date属性对它们进行排序。我知道Set不是有序集合，那么如何阅读这些注释以使它们按date属性排序？

Answer 1

您可以使用@OrderBy注释JPA关系，LinkedHashSet将使用提供的字段以指定的顺序从数据库中检索实体。同时将set实现更改为@OneToMany(cascade = { ALL }, fetch = EAGER, mappedBy = "post") @OrderBy("date DESC") private Set<Comment> comments = new LinkedHashSet<Comment>();以保留数据库中的顺序。

@OrderBy

有关详细信息，您可以观看我创建的video tutorial，其中说明了如何使用{{1}}。

使用JSTL，如何读取set的元素，以便按“date”（java.util.Date）属性进行排序？

1 个答案: