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。
本课
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;
}
}
和这个
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;
}
}
和这个
@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="${criteria.expirationDate}" />" disabled="disabled" class="form-control"/>
</div>
</div>
</c:forEach>
</form:form>
控制器：
/**
* @throws Exception
*
*/
@RequestMapping(value = { "/newdesign/manage/device/{appId}",
"/newdesign/manage/device/{appId}/"}, method = {RequestMethod.GET})
public String viewDevicesWithStatus(
@ModelAttribute("deviceForm") DeviceForm deviceForm,
@PathVariable Long appId,
HttpServletRequest request,
Model model ) throws Exception {
Device device = manageLicenseService.getDeviceById(appId, true);
if (device.getCriteria()==null) {
device.setCriteria(device.getProductGroup().getCriteria().get(0));
}
deviceForm.setDevice(device);
fillModel (model, request, device);
return "cbViewDeviceInfo";
}
/**
* @throws Exception
*
*/
@RequestMapping(value = { "/newdesign/manage/device/{appId}",
"/newdesign/manage/device/{appId}/"}, method = {RequestMethod.POST})
public String saveDevicesWithStatus(
@ModelAttribute("deviceForm") DeviceForm deviceForm,
@PathVariable Long appId,
HttpServletRequest request,
Model model ) throws Exception {
Device device = manageLicenseService.getDeviceById(deviceForm.getDevice().getId());
if (device.getCriteria()==null) {
device.setCriteria(device.getProductGroup().getCriteria().get(0));
}
//TODO: audit
device.updateDevice(deviceForm.getDevice());
manageLicenseService.saveDevice(device);
if (device.getCriteria()==null) {
device.setCriteria(device.getProductGroup().getCriteria().get(0));
}
deviceForm.setDevice(device);
fillModel (model, request, device);
return "cbViewDeviceInfo";
}
但是当我提交表单时出现了以下错误，在GET方法中我得到了相同的页面而没有错误
org.springframework.beans.NullValueInNestedPathException: Invalid property 'device.criteria' of bean class [com.tdk.iot.controller.newdesign.manage.DeviceForm]: Could not instantiate property type [com.tdk.iot.domain.criteria.BaseCriteria] to auto-grow nested property path: java.lang.InstantiationException
答案 0 :(得分：1)
您收到错误，因为您的表单中包含以下内容：
<form:radiobutton path="device.criteria.id" value="${criteria.id}"/>
并在你的POST处理程序中有这个：
public String saveDevicesWithStatus(@ModelAttribute("deviceForm") DeviceForm deviceForm){
}
这意味着MVC框架将尝试自动设置属性
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。
你可能会认为你在GET处理程序中创建并填充的DeviceForm会被使用但是Spring MVC是无状态的，所以你 需要将它存储在会话范围之间，以便重新使用它或者重新修改你的逻辑。
....鉴于上面的例子，实例可以来自哪里？那里 有几种选择..... [在没有任何其他选择的情况下] 可能 使用其默认构造函数
进行实例化
更好的方法是将表单更改为：
<form:radiobutton path="device.criteria" value="${criteria.id}"/>
并注册一个转换器，它将转换提交的参数并绑定相应的实体实例。
@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
}
}