Spring对象转换[重复]

Question

我正在处理 Spring MVC 项目 spring 无法转换我的 Post 对象所需的用户对象。

我的 NewPost 对象：

public class NewPost {

private int postId;
@NotEmpty(message ="Title must not be empty!")
@Length(max = 50, message="Title must be less then 50 character")
private String title;
@NotEmpty(message ="Comment must not be empty!")
private String content;
@Length(max = 100, message="must be less then 100 character")
private String imagePath;
@DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME)
private LocalDateTime publishDate;
private User user;

我的控制器：

  //Get lates post from DB.
   @RequestMapping(value = "/displayBlogPage", method = RequestMethod.GET)
    public String displayLatesPost(Model model) {
    List<NewPost> displayAllPost = new ArrayList<>();
    displayAllPost = NPDao.getAllPost();
    model.addAttribute("displayAllPost", displayAllPost);

   //get newPost to the model.
    model.addAttribute("newPost", new NewPost());

    //get user object to the model
    model.addAttribute("user", userDao.getUserbyId(1));

    LocalDateTime timeStamp = LocalDateTime.now();
    model.addAttribute("timeStamp", timeStamp);

    return "NewPostPage";

}
//for add Post form
  @RequestMapping(value = "/newPost", method = RequestMethod.POST)
  public String createPost( @Valid @ModelAttribute("newPost") NewPost newPost,  BindingResult result) {

if(result.hasErrors()){
    return "NewPostPage";
}
    NPDao.addNewPost(newPost);

    return "redirect:NewPostPage";
}

我的 JSP

<sf:form  class="form-horizontal" 
 role="form"  method="POST" 
 action="newPost" modelAttribute="newPost" >
 <div class="col-md-12">
 <div class="form-group">
 <div class="col-md-6">
 <sf:input type="text" class="form-control" path="title" placeholder="title" />
 <sf:errors path="title" cssclass="error" ></sf:errors>
 </div>
 </div>
 <div class="form-group">
 <div class="col-md-6">
 <sf:input type="text" class="form-control" path="imagePath" placeholder="image" />
 <sf:errors path="imagePath" cssclass="error" ></sf:errors>
 <sf:input type="text" class="form-control" path="publishDate" placeholder="date" value="${timeStamp}" />
 <sf:errors type="date" path="publishDate" cssclass="error" ></sf:errors>
 <sf:input type="text" class="form-control" path="user" placeholder="user" value="${user}"/>
 <sf:errors path="user" cssclass="error" ></sf:errors>
 <sf:input type="text" class="form-control" path="postId" placeholder="postid"/>
 <sf:errors path="postId" cssclass="error" ></sf:errors>
 <div class="form-group">
 <div class="col-md-12">
 <textarea  type="text" class="form-control comment" name="comment"   placeholder="Comment" required ></textarea> 
 <input type="submit" id="add"class="btn btn-default" value="Submit Post"/>
 </div>
 </div>
 </div>
</sf:form>

这里是个例外：

无法将 java.lang.String 类型的属性值转换为属性用户所需的 com.sg.sophacms.Model.User 类型；嵌套异常是 java.lang.IllegalStateException：无法将 java.lang.String 类型的值转换为属性用户所需的 com.sg.sophacms.Model.User 类型：找不到匹配的编辑器或转换策略

Answer 1

在您的控制器中覆盖initBinder（或使用@InitBinder 注释任何方法）并为User 类和字段名称“user”提供PropertyEditor（尽管它可以用于此的任何字段类）与getAsText/setAsText:

@Override
protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder) throws Exception {  // or @InitBinde
    binder.registerCustomEditor(User.class, "user", new PropertyEditorSupport() {
        @Override
        public void setAsText(String text) {
            User user = new User(text); // Some kind of initialization of your object from string
            setValue(user);
        }

        @Override
        public String getAsText() {
            User user = getValue();
            return user.getId(); // Some kind of string from your User object
        }
    });
}

这个想法是你需要指定逻辑来翻译 ObjectString。在上面的示例中，我假设您可以从字符串构造User(str)，并且User.getId() 将返回一个字符串。这是您需要将对象绑定到文本字段的对象字符串翻译策略的示例。