Sprintbootで@ModelAttributeでモデルを受け取れません。StringタイプのフィールドがLongタイプに強制変換されます。

Question

SpringBootで掲示板を作りたいです。
タイトルとコンテンツだけをPostしたいですが、実装中に以下のエラーメッセージが発生しました。

↓は画面側のエラーです。Controllerまでいかないです。

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Thu Oct 03 21:52:42 JST 2019
There was an unexpected error (type=Bad Request, status=400).
Failed to convert value of type 'java.lang.String' to required type 'com.cms.video.yoocms.Entity.Contents'; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [java.lang.Long] for value 'this is contents'; nested exception is java.lang.NumberFormatException: For input string: "thisiscontents"
org.springframework.beans.TypeMismatchException: Failed to convert value of type 'java.lang.String' to required type 'com.cms.video.yoocms.Entity.Contents'; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [java.lang.Long] for value 'this is contents'; nested exception is java.lang.NumberFormatException: For input string: "thisiscontents"
	at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:79)
	at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:53)
	at org.springframework.validation.DataBinder.convertIfNecessary(DataBinder.java:693)
	at org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor.createAttributeFromRequestValue(ServletModelAttributeMethodProcessor.java:141)
	at org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor.createAttribute(ServletModelAttributeMethodProcessor.java:77)
	at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.resolveArgument(ModelAttributeMethodProcessor.java:139)
	at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:127)
	at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:167)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:134)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:892)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797)

...省略...

Caused by: org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [java.lang.Long] for value 'this is contents'; nested exception is java.lang.NumberFormatException: For input string: "thisiscontents"
	at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:47)
	at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:191)
	at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:174)
	at org.springframework.data.repository.support.DomainClassConverter$ToEntityConverter.convert(DomainClassConverter.java:168)
	at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:41)
	at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:191)
	at org.springframework.beans.TypeConverterDelegate.convertIfNecessary(TypeConverterDelegate.java:129)
	at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:73)
	... 94 more
Caused by: java.lang.NumberFormatException: For input string: "thisiscontents"
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Long.parseLong(Long.java:589)
	at java.lang.Long.valueOf(Long.java:803)
	at org.springframework.util.NumberUtils.parseNumber(NumberUtils.java:214)
	at org.springframework.core.convert.support.StringToNumberConverterFactory$StringToNumber.convert(StringToNumberConverterFactory.java:62)
	at org.springframework.core.convert.support.StringToNumberConverterFactory$StringToNumber.convert(StringToNumberConverterFactory.java:49)
	at org.springframework.core.convert.support.GenericConversionService$ConverterFactoryAdapter.convert(GenericConversionService.java:436)
	at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:41)
	... 101 more

該当のソースコード

package com.cms.video.yoocms.Entity;

import lombok.Getter;
import lombok.Setter;
import org.hibernate.annotations.CreationTimestamp;
import org.hibernate.annotations.UpdateTimestamp;

import javax.persistence.*;
import java.sql.Timestamp;

@Entity
@Getter
@Setter
public class Contents {

    @Id
    @GeneratedValue
    private Long id;

    @Column
    private String title;

    @Column
    private String contents;

    @Column
    @CreationTimestamp
    private Timestamp createTime;

    @Column
    @UpdateTimestamp
    private Timestamp updateTime;
}

package com.cms.video.yoocms.Controller;

import com.cms.video.yoocms.Entity.Contents;
import com.cms.video.yoocms.Service.ContentsService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.*;

import javax.validation.Valid;


@Controller
@RequestMapping(value = "/contents/**")
public class ContentsController {

    @Autowired
    ContentsService contentsService;

    @GetMapping("/create")
    public String createContents(Model model){
        model.addAttribute("contents",new Contents());
        return "contents_create";
    }

    @PostMapping("/create")
    public String createContents(@Valid @ModelAttribute Contents contents, BindingResult bindingResult){
        //contentsService.createContents(contents);
        return "redirect:/contents/list";
    }

    @GetMapping("/list")
    public String list(){
        return "contents_list";
    }

}

<!doctype html>
<html lang="en" xmlns:th="http://www.thymeleaf.org">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
<form action="#" th:action="@{/contents/create}" method="post">

  <input type="text" name="title" value="this is title">
  <input type="text" name="contents" value="this is contents">
  <input type="submit" value="OK">
</form>
</body>
</html>

試したこと

contentsフィールドのタイプがStringなのに、submitするとLongタイプに変換しているようです。
画面側で文字列を入力したら上記エラーが発生しますが、
数字を入力したらPostでcreateContentsメソッドに届きます。

@RequestParam String title, @RequestParam String contents　で受けると正常ですが、@ModelAttributeで上記エラーになる原因が知りたいです。

補足情報（FW/ツールのバージョンなど）

springboot 2.1.8.RELEASE
java se 8

Answer 1

Longに変換……というよりも
Contentsクラスの引数の名前が『contents』なので
HTML側のFormの中身一式ではなくname="contents"の中身の文字列が単独で反応して結果落ちてしまっているのではないでしょうか？
Long変換が発生してるのは先頭にあるフィールド「id」がLongだからそこに入れ込もうとしているのかも