如何从下拉列表传递值

Question

我正在使用Spring MVC Thymeleaf项目，我在将字段值传递给对象时遇到了问题。有malt和Country实体。在malt表单中，有一个下拉列表，由DB填充-仅限国家名称-没什么大不了的。我可以填充列表，但当我单击“提交”按钮时，有一些错误。下面的代码（仅限相关部分）：

麦芽实体：

@Setter
@Getter
@NoArgsConstructor
@Entity
@ToString
@Table(name="malt")
public class Malt extends BaseEntity {

    @Column(name="malt_name")
    private String maltName;

    @ManyToOne(fetch=FetchType.EAGER,
            cascade= {CascadeType.PERSIST, CascadeType.MERGE,
                    CascadeType.DETACH, CascadeType.REFRESH})
    @JoinColumn(name="producer_id")
    private Producer producer;

    @Column(name="malt_filling")
    private int maltFilling;

    @Column(name="malt_ebc")
    private int maltEbc;

    @Column(name="malt_usage")
    private String maltUsage;

    @ManyToOne(fetch=FetchType.EAGER,
            cascade= {CascadeType.PERSIST, CascadeType.MERGE,
                    CascadeType.DETACH, CascadeType.REFRESH})
    @JoinColumn(name="country_id")
    private Country country;

    @ManyToMany(mappedBy="malts")
    private Set<Batch> batches;

麦芽控制器：

@Controller
@RequestMapping("/malt")
public class MaltController {

@ModelAttribute("countries")
public Collection<Country> populateCountries() {
    return countryService.findAll();
}

@RequestMapping("{id}/update")
public String updateMalt(@PathVariable String id, Model model) {
    model.addAttribute("malt", maltService.findById(Long.valueOf(id)));
    return "malt-form";
}

@PostMapping
public String saveOrUpdate(@ModelAttribute Malt malt) {
    Malt savedMalt = maltService.save(malt);
    return "redirect:/malt/" + savedMalt.getId() + "/malt-show";
}

麦芽形态：

<div class="form-field-input">
    <select class="form-control" th:field="*{id}">
        <option value="0">Select country</option>
        <option
            th:each="country : ${countries}"
            th:value="${country.id}"
            th:text="${country?.countryName}">
        </option>
    </select>
</div>
<div class="form-field-submit">
    <button class="submit-button" type="submit">Submit</button>
</div>

Malt展示模板：

<div class="wrapper">
            <div class="main">
                <div class="page-title">
                    <p th:text="${malt.maltName}">Malt name</p>
                </div>
                <div class="show">
                    <div class="form-row">
                        <div class="form-field-name">
                            <label>Producer:</label>
                        </div>
                        <div class="form-field-input">
                            <p th:text="${malt.producer.producerName}">Producer name</p>
                        </div>
                    </div>
                    <div class="form-row">
                        <div class="form-field-name">
                            <label>Country:</label>
                        </div>
                        <div class="form-field-input">
                            <p th:text="${malt.country.countryName}">Country</p>
                        </div>
                    </div>
                    <div class="form-row">
                        <div class="form-field-name">
                            <label>Malt filling:</label>
                        </div>
                        <div class="form-field-input">
                            <p th:text="${malt.maltFilling}">Malt filling</p>                       
                        </div>
                    </div>
                    <div class="form-row">
                        <div class="form-field-name">
                            <label>Malt usage:</label>
                        </div>
                        <div class="form-field-input">
                            <p th:text="${malt.maltUsage}">Malt usage</p>
                        </div>
                    </div>
                    <div class="form-row">
                        <div class="form-field-name">
                            <label>Malt EBC:</label>
                        </div>
                        <div class="form-field-input">
                            <p th:text="${malt.maltEbc}">Malt EBC</p>
                        </div>
                    </div>

                </div>
            </div>
        </div>

我得到的结束错误：

An error happened during template parsing (template: "class path resource [templates/malt-show.html]")
org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/malt-show.html]")
.
.
.
Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "malt.country.countryName" (template: "malt-show" - line 44, col 11)
    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: Exception evaluating SpringEL expression: "malt.country.countryName" (template: "malt-show" - line 44, col 11)
    at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:290)
.
.
.
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'countryName' cannot be found on null    

链接到回购：https://github.com/fangirsan/maruszka-new/tree/malt-form-problem

我尝试了很多不同的方法，但没有结果。

Answer 1

正如例外情况所述，问题是您的malt show表单中的${malt.country.countryName}。在异常stacktrace的最后一行中，我看到在null上找不到属性或字段“countryName”。这意味着您正试图获取一个空的相关模型属性。您的malt表中的country\u id列可能为空。换言之，国家/地区id未与其他字段一起保存。根据这些假设，您将在保存麦芽糖的表格中发现问题。我检查了此表单，可能问题是<代码>

重要提示：

一些相关模型可以为空，例如，假设您的malt模型中的国家/地区可以为空（取决于应用程序业务逻辑）。如果关系为空，访问模式中的模型关系字段可能会产生上述错误。因此，在这些情况下，您应该在thymeleaf模板中使用null检查。