title: xmlns:th="http://www.thymeleaf.org" doesn't work
body: <p>Thanks！I've finish this problem! I add<br>
  protected void addResourceHandlers(ResourceHandlerRegistry registry) 
  {
    registry.addResourceHandler("/static/**").addResourceLocations(ResourceUtils.CLASSPATH_URL_PREFIX + "/static/");
          super.addResourceHandlers(registry);
  }
  in application, then the bug disappears.</p>
  
  <p>Recently, in learning spring boot through the book "spring-boot-in-action", I encountered the following problems</p>
  
  <p>When the program runs, I open the webpage and find that the line <code>"Xmlns: th= 'http://www.thymeleaf.org'"</code> has not been executed, and the CSS file has not been invoked.</p>
  
  <p>Attached below are HTML source code, build. gradle configuration and web controller:</p>
  
  <p>HTML source code:</p>
  
  <pre><code>&lt;!DOCTYPE html SYSTEM "http://www.thymeleaf.org/dtd/xhtml1-strict-thymeleaf-4.dtd"&gt;
  
  &lt;html xmlns="http://www.w3.org/1999/xhtml"
        xmlns:th="http://www.thymeleaf.org"&gt;
  
  &lt;head&gt;
      &lt;title&gt;Reading List&lt;/title&gt;
      &lt;link rel="stylesheet"  th:href="@{/style.css}"&gt;&lt;/link&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
  &lt;h2&gt;Your Reading List&lt;/h2&gt;
  &lt;div th:unless="${#lists.isEmpty(books)}"&gt;
      &lt;dl th:each="book : ${books}"&gt;
          &lt;dt class="bookHeadline"&gt;
              &lt;span th:text="${book.title}"&gt;Title&lt;/span&gt; by
              &lt;span th:text="${book.author}"&gt;Author&lt;/span&gt;
              (ISBN: &lt;span th:text="${book.isbn}"&gt;ISBN&lt;/span&gt;)
  
          &lt;/dt&gt;
          &lt;dd class="bookDescription"&gt;
            &lt;span th:if="${book.description}"
                  th:text="${book.description}"&gt;Description&lt;/span&gt;
              &lt;span th:if="${book.description eq null}"&gt;
                  No description available&lt;/span&gt;
          &lt;/dd&gt;
      &lt;/dl&gt;
  &lt;/div&gt;
  &lt;div th:if="${#lists.isEmpty(books)}"&gt;
      &lt;p&gt;You have no books in your book list&lt;/p&gt;
  &lt;/div&gt;
  
  &lt;hr/&gt;
  
  &lt;h3&gt;Add a book&lt;/h3&gt;
  &lt;form method="POST"&gt;
      &lt;label for="title"&gt;Title:&lt;/label&gt;
      &lt;input type="text" name="title" size="50"&gt;&lt;/input&gt;&lt;br/&gt;
      &lt;label for="author"&gt;Author:&lt;/label&gt;
      &lt;input type="text" name="author" size="50"&gt;&lt;/input&gt;&lt;br/&gt;
      &lt;label for="isbn"&gt;ISBN:&lt;/label&gt;
      &lt;input type="text" name="isbn" size="15"&gt;&lt;/input&gt;&lt;br/&gt;
      &lt;label for="description"&gt;Description:&lt;/label&gt;&lt;br/&gt;
      &lt;textarea name="description" cols="80" rows="5"&gt;
          &lt;/textarea&gt;&lt;br/&gt;
      &lt;input type="submit"&gt;&lt;/input&gt;
  &lt;/form&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>gradle configuration </p>
  
  <pre><code>dependencies {
  
      implementation 'org.springframework.boot:spring-boot-starter-data-jpa'
      implementation 'org.springframework.boot:spring-boot-starter-thymeleaf'
      implementation 'org.springframework.boot:spring-boot-starter-web'
      runtimeOnly 'com.h2database:h2'
      testImplementation 'org.springframework.boot:spring-boot-starter-test'
  }
  </code></pre>
  
  <p>web controller prtSc:(I'm not allowed to put a picture, sorry, so I put a link)</p>
  
  <pre><code>&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
  </code></pre>
  
  <p>I can find this code, but </p>
  
  <pre><code> xmlns:th="http://www.thymeleaf.org" 
  </code></pre>
  
  <p>disappeared.</p>
  
  <p><a href="https://i.stack.imgur.com/pt5Nj.png" rel="nofollow noreferrer">https://i.stack.imgur.com/pt5Nj.png</a></p>