org.springframework.beans.NullValueInNestedPathException：Spring MVC 3.2.8 中的自动增长嵌套属性路径

Question

我有一个基于 Spring Web 模型-视图-控制器 (MVC) 框架的项目。 Spring Web 模型-视图-控制器 (MVC) 框架的版本是 3.2.8。

这门课

public class DeviceForm {

    Device device;

    List<String> selectedItems = Collections.emptyList();

    public DeviceForm() {
        super();
    }

    public Device getDevice() {
        return device;
    }

    public void setDevice(Device device) {
        this.device = device;
    }

    public List<String> getSelectedItems() {
        return selectedItems;
    }

    public void setSelectedItems(List<String> selectedItems) {
        this.selectedItems = selectedItems;
    }


}

还有这个

public class Device implements java.io.Serializable {

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "CRITERIA")
    private BaseCriteria criteria;

    public BaseCriteria getCriteria() {
        return criteria;
    }

    public void setCriteria(BaseCriteria criteria) {
        this.criteria = criteria;
    }
}

还有这个

@Entity
@Table(name = "CRITERIA")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DISCRIMINATOR", discriminatorType = DiscriminatorType.STRING)
@SequenceGenerator(name = "seqCriteria", sequenceName = "SEQ_CRITERIA", allocationSize = 1)
public abstract class BaseCriteria {

    public BaseCriteria() {
        super();
    }   

    private Long id;
    private String code;
    private Date adoptionDate;
    private Date expirationDate;

    @Transient
    public abstract String getGroupKey();

    @Transient
    public abstract Long getGroupId();

    @Transient
    public abstract String getRefColumnName();

    @Id
    @Column(name = "ID", unique = true, nullable = true)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seqCriteria")
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Column(name = "CODE")
    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    @Column(name = "ADOPTION_DATE")
    @Temporal(TemporalType.TIMESTAMP)
    public Date getAdoptionDate() {
        return adoptionDate;
    }

    public void setAdoptionDate(Date adoptionDate) {
        this.adoptionDate = adoptionDate;
    }

    @Column(name = "EXPIRATION_DATE")
    @Temporal(TemporalType.TIMESTAMP)
    public Date getExpirationDate() {
        return expirationDate;
    }

    @Transient
    public boolean isExpired() {
        return getExpirationDate().before(new Date());
    }

    public void setExpirationDate(Date expirationDate) {
        this.expirationDate = expirationDate;
    }


    @Override
    public String toString() {
        return "BaseCriteria [id=" + id + ", code=" + code + ", adoptionDate="
                + adoptionDate + ", expirationDate=" + expirationDate + "]";
    }


}

和 JSP

<form:form  commandName="deviceForm"
             name="deviceForm"
             id="deviceFormId" 
             method="post"
             action="${contextPath}/newdesign/manage/device/${deviceForm.device.id}"
             htmlEscape="yes">

 <div class="col-sm-6 text-right">
     <button class="btn btn-primary" type="submit">Save device</button>
</div>
</div>

<c:forEach items="${deviceForm.device.productGroup.criteria}" var="criteria">                                                
     <div class="row">                                                                                                 
            <div class="col-md-3">
                <form:radiobutton path="device.criteria.id" value="${criteria.id}"/>
                <label for="basic-url">Criteria:</label>
                <input value="${criteria.code}" disabled="disabled" class="form-control"/>
            </div>
            <div class="col-md-3">                                                                                                              
                <label for="basic-url">Adoption date:</label>
                <input value="<fmt:formatDate type="date" value="${criteria.adoptionDate}" />"      disabled="disabled" class="form-control"/>
            </div>                
            <div class="col-md-3">                                                                                              
                <label for="basic-url">Expiration Date:</label>
                <input value="<fmt:formatDate type="date" value="${criteria.expirationDate}" />"    disabled="disabled" class="form-control"/>
            </div>                                                                                                                                            
    </div>
</c:forEach>
</form:form>

控制器：

/**
     * @throws Exception    
     *                                 
     */
    @RequestMapping(value = {       "/newdesign/manage/device/{appId}",
                                    "/newdesign/manage/device/{appId}/"}, method = {RequestMethod.GET})
    public String viewDevicesWithStatus(                        
                                     @ModelAttribute("deviceForm") DeviceForm deviceForm,
                                     @PathVariable Long appId,                                   
                                     HttpServletRequest request,
                                     Model model ) throws Exception {

        Device device =   manageLicenseService.getDeviceById(appId, true);

        if (device.getCriteria()==null) {
            device.setCriteria(device.getProductGroup().getCriteria().get(0));
        }

        deviceForm.setDevice(device);       
        fillModel (model, request, device);

        return "cbViewDeviceInfo";      
    }

    /**
     * @throws Exception    
     *                                 
     */
    @RequestMapping(value = {       "/newdesign/manage/device/{appId}",
                                    "/newdesign/manage/device/{appId}/"}, method = {RequestMethod.POST})
    public String saveDevicesWithStatus(                                
                                     @ModelAttribute("deviceForm") DeviceForm deviceForm,
                                     @PathVariable Long appId,                                   
                                     HttpServletRequest request,
                                     Model model ) throws Exception {           

        Device device =   manageLicenseService.getDeviceById(deviceForm.getDevice().getId());

        if (device.getCriteria()==null) {
            device.setCriteria(device.getProductGroup().getCriteria().get(0));
        }

        //TODO: audit
        device.updateDevice(deviceForm.getDevice());
        manageLicenseService.saveDevice(device);    

        if (device.getCriteria()==null) {
            device.setCriteria(device.getProductGroup().getCriteria().get(0));
        }

        deviceForm.setDevice(device);
        fillModel (model, request, device);


        return "cbViewDeviceInfo";      
    }

但是当我提交表单时出现以下错误，在 GET 方法上我得到了相同的页面而没有错误

org.springframework.beans.NullValueInNestedPathException: Invalid property 'device.criteria' of bean class [com.tdk.iot.controller.newdesign.manage.DeviceForm]: Could not instantiate property type [com.tdk.iot.domain.criteria.BaseCriteria] to auto-grow nested property path: java.lang.InstantiationException

Answer 1

您收到错误是因为您的表单中有以下内容：

<form:radiobutton path="device.criteria.id" value="${criteria.id}"/>

在你的 POST 处理程序中你有这个：

public String saveDevicesWithStatus(@ModelAttribute("deviceForm") DeviceForm deviceForm){

}

这意味着MVC框架会尝试自动设置属性

deviceForm.device.criteria.id.

现在，因为在任何范围内都没有现有的 DeviceForm，所以它会创建一个新的，当然device.getCriteria() 返回 null， 因此例外。

您可能认为您在 GET 处理程序中创建和填充的 DeviceForm 将被使用，但是 Spring MVC 是无状态的，因此您 需要在请求之间将其存储在 Session 范围内，以便重新使用或重新处理您的逻辑。

https://docs.spring.io/spring/docs/current/spring-framework-reference/html/mvc.html#mvc-ann-modelattrib-method-args

.... 鉴于上面的例子，实例来自哪里？那里 有几个选项.....[在没有任何其他选项的情况下] 可能 使用其默认构造函数进行实例化

然而，更好的方法是将您的表单更改为如下：

<form:radiobutton path="device.criteria" value="${criteria.id}"/>

并注册一个转换器，该转换器将转换提交的参数并绑定相应的实体实例。

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/validation.html#core-convert

@Component
public class StringToCriteriaConverter implements Converter<String, BaseCriteria> {

    @Autowired
    private CriteriaService service;

    //source is the ID passed by the page
    public BaseCriteria convert(String source) {
        // lookup and return item with corresponding ID from the database
    }
}