Thymeleafテンプレートが表示されない。【Spring Boot】

##状況

layout:fragmentを用いてhtmlテンプレート機能を実装しようとしていました。

しかし、テンプレート部分のhtmlが画面に表示されず困っています。

##ソースコード

layout.html(テンプレート部分)

<!DOCTYPE html>
<html  xmlns:th="http://www.thymeleaf.org"
       xmlns:layout="http://www.ultraq.net.nz/web/thymeleaf/layout">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
  <title>layout</title>
</head>
<body>
  <div class="container">
    <h1 class="text-center text-primary mt-5">顧客管理システム</h1>
    <div>
    </div>
    <div layout:fragment="content">
      Fake Content
    </div>
  </div>
</body>
</html>

list.html(コンテンツ部分)

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org/" xmlns:layout="http://www.ultraq.net.nz/web/thymeleaf/layout"
  layout:decorator="~{layout}">

<head>
  <title>顧客一覧</title>
</head>

<body>
  <div layout:fragment="content" class="col-sm-12">
    <form th:action="@{/customers/create}" th:object="${customerForm}" method="POST">
      <dl>
        <dt><label for="lastName">姓</label></dt>
        <dd>
          <input type="text" id="lastName" name="lastName" th:field="*{lastName}" th:errorclass="error-input"
            value="山田" />
          <span th:if="${#fields.hasErrors('lastName')}" th:errors="*{lastName}" class="error-messages">error!</span>
        </dd>
      </dl>
      <dl>
        <dt><label for="firstName">名</label></dt>
        <dd>
          <input type="text" id="firstName" name="firstName" th:field="*{firstName}" th:errorclass="error-input"
            value="太郎" />
          <span th:if="${#fields.hasErrors('firstName')}" th:errors="*{firstName}" class="error-messages">error!</span>
        </dd>
      </dl>
      <input type="submit" value="作成">
    </form>
    <table>
      <tr th:each="customer : ${customers}">
        <td th:text="${customer.id}">100</td>
        <td th:text="${customer.lastName}">山田</td>
        <td th:text="${customer.firstName}">太郎</td>
        <td>
          <form th:action="@{/customers/edit}" method="GET">
            <input type="submit" name="form" value="編集">
            <input type="hidden" name="id" th:value="${customer.id}" />
          </form>
        </td>
        <td>
          <form th:action="@{/customers/delete}" method="POST">
            <input type="submit" value="削除">
            <input type="hidden" name="id" th:value="${customer.id}" />
          </form>
        </td>
      </tr>
    </table>
  </div>
</body>

</html>

Contoroller

Java
1package com.example.thymeleaf_test.web;
2
3import java.util.List;
4import org.springframework.beans.BeanUtils;
5import org.springframework.beans.factory.annotation.Autowired;
6import org.springframework.stereotype.Controller;
7import org.springframework.ui.Model;
8import org.springframework.validation.BindingResult;
9import org.springframework.validation.annotation.Validated;
10import org.springframework.web.bind.annotation.GetMapping;
11import org.springframework.web.bind.annotation.ModelAttribute;
12import org.springframework.web.bind.annotation.PostMapping;
13import org.springframework.web.bind.annotation.RequestMapping;
14import org.springframework.web.bind.annotation.RequestParam;
15
16import com.example.thymeleaf_test.domain.Customer;
17import com.example.thymeleaf_test.service.CustomerService;
18
19@Controller
20@RequestMapping("customers")
21public class CustomerController {
22	@Autowired
23	CustomerService customerService;
24	
25	@ModelAttribute
26	CustomerForm steUpForm() {
27		return new CustomerForm();
28	}
29	
30	@GetMapping
31	String list(Model model) {
32		List<Customer> customers  = customerService.findAll();
33		model.addAttribute("customers", customers);
34		return "customers/list";
35	}
36
37	@PostMapping(path = "create")
38	String create(@Validated CustomerForm form, BindingResult result, Model model) {
39		if (result.hasErrors()) {
40			return list(model);
41		}
42		Customer customer = new Customer();
43		BeanUtils.copyProperties(form, customer);
44		customerService.create(customer);
45		return "redirect:/customers";
46	}
47
48	@GetMapping(path = "edit", params = "form")
49	String editForm(@RequestParam Integer id, CustomerForm form) {
50		Customer customer = customerService.findOne(id);
51		BeanUtils.copyProperties(customer, form);
52		System.out.println(form);
53		return "customers/edit";	
54	}
55
56	@PostMapping(path = "edit")
57	String edit(@RequestParam Integer id, @Validated CustomerForm form, BindingResult result) {
58		if (result.hasErrors()) {
59			return editForm(id, form);
60		}
61		Customer customer = new Customer();
62		BeanUtils.copyProperties(form, customer);
63		customerService.update(customer);
64		return "redirect:/customers";
65	}
66
67	@PostMapping(path = "edit", params = "goToTop")
68	String goToTop() {
69		return "redirect:/customers";
70	}
71	
72	@PostMapping(path = "delete")
73	String delete(@RequestParam Integer id) {
74		customerService.delete(id);
75		return "redirect:/customers";
76	}
77
78	
79}