无法在 JSP 中获取模型属性的大小

Question

这是我使用 jsp 的第一个项目，我正在尝试构建一个 table，其中每一行看起来有点像这样

Here is a line of the table

最后一栏（点赞）目前只有一个按钮，但我想在旁边显示点赞数。

这是我的问题。当我尝试使用 jsp 函数长度 ${fn:length(mes.likes)}

我得到以下异常

org.apache.jasper.JasperException: javax.el.ELException: Problems calling function [fn:length]
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:606)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:482)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:170)
org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:314)
org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1325)
org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1069)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1008)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:870)
javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:855)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:320)
org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:127)
org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:91)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:119)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:137)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:111)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:170)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.authentication.www.BasicAuthenticationFilter.doFilterInternal(BasicAuthenticationFilter.java:158)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:200)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:116)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.csrf.CsrfFilter.doFilterInternal(CsrfFilter.java:100)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:66)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:105)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:56)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334)
org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:215)
org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:178)
org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:357)
org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:270)

我从 HomeController

加载模型中的所有消息

@Controller
public class HomeController {
    @Autowired
    IMessageService messageService;
    @GetMapping("/homepage")
    public String index(Model model) {
        List<MessageEntity> messages = messageService.getAll();
        model.addAttribute("allMessages", messages);
        return "index";
    }
}

这里是 MessageEntity class

public class MessageEntity{
    private int Id;
    private String title;
    private String content;
    private Date createdAt;
    private User createdBy;
    private List<LikeEntity> likes;
}

最后，这是我在 index.jsp 文件中显示信息的 table。

<table id="panneau">
    <thead class="bg-dark text-light">
        <tr>
            <th>Message</th>
            <th>Emetteur</th>
            <th>Date/heure</th>
            <th>Likes</th>
        </tr>
    </thead>
    <tbody class="text-black">
        <c:forEach var="mes" items="${allMessages}">
            <tr id="message${mes.id}">
                <td>
                    <div><b>${mes.title}</b></div>
                    <div>${mes.content}</div>
                </td>
                <td>
                    <c:choose>
                        <c:when test="${mes.anonymous}">
                            anonyme
                        </c:when>
                        <c:otherwise>
                            ${mes.createdBy.firstName} ${mes.createdBy.lastName}
                        </c:otherwise>
                    </c:choose>
                </td>
                <td>
                    ${mes.createdAt}
                </td>
                <td>
                    <div>${fn:length(mes.likes)}</div>
                    <div><button class="btn btn-dark" id="like${mes.id}"><i class="far fa-thumbs-up fa-2x"></i></button></div>
                </td>
            </tr>
        </c:forEach>
    </tbody>
</table>

当我删除 ${fn:length(mes.likes)} 时，一切正常，但我需要一种方法来获取此信息。

我看过其他类似的问题，但 none 似乎解决了我的问题。

顺便说一下，我在文件的开头确实有 <%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

编辑

这里是likeEntityclass

import javax.persistence.*;
import java.util.Objects;
/**
 * Created by owner on 18-05-07.
 */
@Entity
@Table(name = "likes", schema = "heroku_9efd0238a94d992")
public class LikeEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "like_id", nullable = false)
    private int id;
    @Basic
    @Column(name = "is_dislike", nullable = true)
    private Boolean isDislike;
    @ManyToOne(cascade = {
        CascadeType.MERGE,
        CascadeType.PERSIST,
        CascadeType.DETACH,
        CascadeType.REFRESH,
    })
    @JoinColumn(name = "liked_by")
    private UserEntity likedBy;
    @ManyToOne(cascade = {
        CascadeType.MERGE,
        CascadeType.PERSIST,
        CascadeType.DETACH,
        CascadeType.REFRESH,
    })
    @JoinColumn(name = "liked_message") 
    private MessageEntity likedMessage;
    public LikeEntity() {
        this.setDislike(false);
    }
}

Answer 1

Disclamer：这是一个解决方法，不是真正的解决方案

我仍然不知道为什么 likes 不能延迟加载，但是将 fetchtype 从延迟加载改为急切似乎已经成功了......

我知道这个解决方案不是最优的。就我而言，此解决方法是合适的，因为它只是一个学校项目，所以我永远不会在一条消息中获得数千个赞。

但是，如果您处理大量数据，我认为此解决方案不合适。

无法在 JSP 中获取模型属性的大小

Can't get the size of a model attribute in JSP

jsp

jstl

jsp-tags

编辑