【问题标题】：Can't get the size of a model attribute in JSP无法在 JSP 中获取模型属性的大小
【发布时间】：2018-06-07 12:11:22
【问题描述】：

这是我第一个使用 jsp 的项目，我正在尝试构建一个表格，其中每一行看起来有点像这样

在最后一列（喜欢）中，目前只有一个按钮，但我想在它旁边显示喜欢的数量。

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 类

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 文件中显示信息的表格。

<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。

我查看了其他类似的问题，但似乎没有一个可以解决我的问题。

顺便说一句，我确实在文件开头有<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

编辑

这是likeEntity类

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);
    }
}

【问题讨论】：

如果你尝试只输出它会发生什么：${mes.likes} 那你看到了什么？
我得到另一个错误：org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: isep.project.web.entity.MessageEntity.likes, could not initialize proxy - no Session
你能发布你的LikeEntity类吗？
我在帖子中添加了 LikeEntity 类
好吧，我明白为什么您可能会收到错误消息。我对您在那里使用的任何技术都不是很熟悉。为什么不以创建消息实体的方式创建此点赞实体？

标签： jsp jstl jsp-tags

【解决方案1】：

免责声明：这是一种解决方法，而不是真正的解决方案

我仍然不知道为什么喜欢无法延迟加载，但是将 fetchtype 从延迟更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，这种解决方法是合适的，因为它只是一个学校项目，所以我永远不会在一条消息上获得成千上万的赞。

但是，如果您处理大量数据，我认为此解决方案不合适。

【讨论】：