java.lang.IllegalArgumentException : Iteration variable cannot be null

Question

enter image description here > 被这个问题困扰了好几天了。我相信这个问题必须做

with the class itself. Or the relationship between the class in question and the class its column is joined on.

Here are my classes: Mod

package com.matt.Keyword.models;
    import javax.persistence.*;
    import javax.validation.constraints.NotNull;
    import javax.validation.constraints.Size;


    @Entity
    @Table(name = "Mods")
    public class Mod {

        @Id
        @GeneratedValue
        private int id;

        @NotNull
        private Integer role;

        @NotNull
        @Size(min = 1, message = "An entry is required")
        private String entry;

        @Column(name = "user_id")
        private Integer userid;

        public Mod(Integer role, String entry) {
            this.role = role;
            this.entry = entry;
        }

        public Mod() {
        }

        public int getId() {

            return id;
        }

        public int getRole() {
            return role;
        }

        public void setRole(Integer role) {
            this.role = role;
        }

        public String getEntry() {
            return entry;
        }

        public void setEntry(String entry) {
            this.entry = entry;
        }

        public Integer getUserid() {
            return userid;
        }

        public void setUserid(Integer userid) {
            this.userid= userid;
        }

    }

帐户

package com.matt.Keyword.models;

    import javax.persistence.*;
    import javax.validation.constraints.NotNull;
    import javax.validation.constraints.Size;

    @Entity
    public class Account {

        @Id
        @GeneratedValue
        private int id;

        @NotNull
        @Size(min = 3, message = "Try again")
        private String name;

        @NotNull
        @Size(min = 1, message = "A password is required")
        private String password;

        @Column(name = "user_id")
        private Integer userid;

        public Account(String name, String password) {
            this.name = name;
            this.password = password;
        }

        public Account() {
        }

        public int getId() {

            return id;
        }

        public String getName() {

            return name;
        }

        public void setName(String name) {

            this.name = name;
        }

        public String getPassword() {

            return password;
        }

        public void setPassword(String password) {

            this.password = password;
        }

        public Integer getUserid() {
            return userid;
        }

        public void setUserid(Integer userid) {
            this.userid = userid;
        }

    }

Account and Mod classes are very similar and share a relationship with the user_id column in the user class.

用户

package com.matt.Keyword.models;

import javax.persistence.*;
import javax.validation.constraints.Size;
import java.util.List;

@Entity
public class User {

    @Id
    @GeneratedValue
    @Column(name = "user_id")
    private int id;

    @Size(min=3, message = "Try again")
    private String email;

    @Size(min=1, message = "A password is required")
    private String password;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "user_id", referencedColumnName = "user_id")
    private List<Account> accounts;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "user_id", referencedColumnName = "user_id")
    private List<Mod> mods;

    public User(String email, String password) {
        this.email = email;
        this.password = password;
    }

    public User() {}

    public int getId() {

        return id;
    }

    public void setId(Integer id){
        this.id = id;
    }
    public String getEmail() {

        return email;
    }

    public void setEmail(String email) {

        this.email = email;
    }

    public String getPassword() {

        return password;
    }

    public void setPassword(String password) {

        this.password = password;
    }

    public List<Account> getAccounts() {
        return accounts;
    }

    public void setAccounts(List<Account> accounts) {
        this.accounts = accounts;
    }


    public List<Mod> getMods() {
        return mods;
    }

    public void setMods(List<Mod> mods) {
        this.mods = mods;
    }
    @Override
    public String toString()
    {
        return email + " " + password;
    }
}

The exception gets thrown when I load the index page from my mods controller. I am using thymeleaf to render views. Here is my mods controller. The Mods database has objects saved to it and in this controller during debugging the attribute being passed to the view contains a populated list of Mod objects. ModController

package com.matt.Keyword.controllers;


    import com.matt.Keyword.models.Mod;
    import com.matt.Keyword.models.User;
    import com.matt.Keyword.models.data.ModDao;
    import com.matt.Keyword.models.data.UserDao;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Controller;
    import org.springframework.ui.Model;
    import org.springframework.validation.Errors;
    import org.springframework.web.bind.annotation.ModelAttribute;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RequestMethod;

    import javax.servlet.http.HttpSession;
    import javax.validation.Valid;

    //test commit note
    //second try
    //third try
    /**
     * Created by LaunchCode
     */
    @Controller
    @RequestMapping("mod")

    public class ModController {

        @Autowired
        private UserDao userDao;

        @Autowired
        private ModDao modDao;

        @RequestMapping(value = "")
        public String index(Model model, HttpSession session) {

            if (session.getAttribute("currentUser") == null) {

                return "redirect:/keyword/login";
            }

            session.getAttribute("currentUser");
            model.addAttribute("title", "Mods");
            model.addAttribute("mods", modDao.findAll());
            return "mods/index";

        }

        @RequestMapping(value = "add", method = RequestMethod.GET)
        public String displayModForm(Model model, HttpSession session) {

            if (session.getAttribute("currentUser") == null) {
                return "redirect:/keyword/login";
            }

            model.addAttribute("title", "Add a new mod");
            model.addAttribute(new Mod());
            return "mods/add";
        }

        @RequestMapping(value = "add", method = RequestMethod.POST)
        public String processAddModForm(@ModelAttribute @Valid Mod newMod,
                                            Errors errors, Model model, HttpSession session) {

            if (errors.hasErrors()) {
                model.addAttribute("title", "List of Mods");
                return "mods/add";
            }
            User currentUser = (User) session.getAttribute("currentUser");

            session.getId();

            currentUser.setId(currentUser.getId());

            newMod.setUserid(currentUser.getId());

            modDao.save(newMod);

            return "redirect:";

        }

    }

Here is the mod/index page using thymeleaf.

模组/指数

<!DOCTYPE html>
    <html lang="en" xmlns:th="http://www.thymeleaf.org/">
    <head th:replace="fragments :: head"></head>

    <body class="container">

    <h1 th:text="${title}">Default Title</h1>

    <p th:unless="${mods} and ${mods.size()}">No current mods</p>

    <nav th:replace="fragments :: navigation"></nav>

    <p th:text="${session.currentUser.email}"></p>

    <table class="table">
        <tr>
            <th>Type</th>
            <th>Entry</th>
        </tr>
            <div th:if="${mods} != null">
                <tr th:each="mod : ${mods}">
                    <div th:if="${mod.userid} == ${session.currentUser.id}">
                        <td th:text="${mod.role}"></td>
                        <td th:text="${mod.entry}"></td>
                    </div>
                </tr>
            </div>

    </table>
    </body>
    </html>

The exception seems to be associated with the for th:each statement. Particularly with the "mod" iterator variable. Here is the stack trace for the full error: Trace

There was an unexpected error (type=Internal Server Error, status=500).
An error happened during template parsing (template: "class path resource [templates/mods/index.html]")
org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/mods/index.html]")
    at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241)
    at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100)
    at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:666)
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1098)
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1072)
    at org.thymeleaf.spring5.view.ThymeleafView.renderFragment(ThymeleafView.java:362)
    at org.thymeleaf.spring5.view.ThymeleafView.render(ThymeleafView.java:189)
    at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1370)
    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1116)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1055)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:897)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:834)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1417)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)
Caused by: org.attoparser.ParseException: Error during execution of processor 'org.thymeleaf.standard.processor.StandardEachTagProcessor' (template: "mods/index" - line 21, col 17)
    at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:393)
    at org.attoparser.MarkupParser.parse(MarkupParser.java:257)
    at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230)
    ... 52 more
Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Error during execution of processor 'org.thymeleaf.standard.processor.StandardEachTagProcessor' (template: "mods/index" - line 21, col 17)
    at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:117)
    at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95)
    at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1314)
    at org.thymeleaf.engine.TemplateHandlerAdapterMarkupHandler.handleOpenElementEnd(TemplateHandlerAdapterMarkupHandler.java:304)
    at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler$InlineMarkupAdapterPreProcessorHandler.handleOpenElementEnd(InlinedOutputExpressionMarkupHandler.java:278)
    at org.thymeleaf.standard.inline.OutputExpressionInlinePreProcessorHandler.handleOpenElementEnd(OutputExpressionInlinePreProcessorHandler.java:186)
    at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler.handleOpenElementEnd(InlinedOutputExpressionMarkupHandler.java:124)
    at org.attoparser.HtmlElement.handleOpenElementEnd(HtmlElement.java:109)
    at org.attoparser.HtmlMarkupHandler.handleOpenElementEnd(HtmlMarkupHandler.java:297)
    at org.attoparser.MarkupEventProcessorHandler.handleOpenElementEnd(MarkupEventProcessorHandler.java:402)
    at org.attoparser.ParsingElementMarkupUtil.parseOpenElement(ParsingElementMarkupUtil.java:159)
    at org.attoparser.MarkupParser.parseBuffer(MarkupParser.java:710)
    at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:301)
    ... 54 more
Caused by: java.lang.IllegalArgumentException: Iteration variable cannot be null
    at org.thymeleaf.util.Validate.notNull(Validate.java:37)
    at org.thymeleaf.standard.expression.Each.<init>(Each.java:49)
    at org.thymeleaf.standard.expression.EachUtils.composeEach(EachUtils.java:169)
    at org.thymeleaf.standard.expression.EachUtils.internalParseEach(EachUtils.java:94)
    at org.thymeleaf.standard.expression.EachUtils.parseEach(EachUtils.java:65)
    at org.thymeleaf.standard.processor.StandardEachTagProcessor.doProcess(StandardEachTagProcessor.java:59)
    at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74)
    ... 67 more

感谢任何帮助。预先感谢您看看是否你有时间。

Debugger in mod controller Here is a to link to my repo if you want to download it and give it a run.

Hibernate exception in session

Edit to mod/index controller

Answer 1

事实证明，这个问题是一个相当棘手的问题。其决议的故事如下...

通过堆栈跟踪判断:

Caused by: java.lang.IllegalArgumentException: Iteration variable cannot be null

thymeleaf 模板中有一个迭代，其中有 null正在迭代的变量。唯一使用th:each我能找到的是:

<div th:if="${mods} != null">
    <tr th:each="mod : ${mods}">
         ...
    </tr>
</div>

看来mods是null因此th:each="mod : ${mods}"这就是问题所在。

但是，对此进行了检查 <div th:if="${mods} != null"> 。或者有吗？以下内容会更有用吗:

<div th:if="${mods != null}">

即。移动}这样null检查是否在表达式内。

这没有帮助，所以结果是 modDao.findAll()经过检查，发现了两个元素。所以它一定是别的东西。

查看 basics of iteration in Thymeleaf建议也许 findAll() 返回的集合不是 th:each 支持的类型之一可以迭代。

因此这个集合被复制到 Iterable 中并给出不同的名称，问题就解决了。

有趣的是，如果迭代变量的名称为 mod问题仍然出现。结果mod是 Thymeleaf 中的关键字，用于模运算 ( % )。

最终，看似正确的代码存在两个问题:

1. DAO 返回了一个集合，但不是一个可以通过 th:each 迭代的集合。 ;和
2. mod是 Thymeleaf 中的关键字，不应用作变量名。