thymeleaf

Thymeleaf - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: thymeleaf for comprehensive documentation.

Pattern Essenziali

Basic Syntax

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
    <title th:text="${title}">Default</title>
</head>
<body>
    <h1 th:text="${message}">Hello</h1>
    <p th:text="'Welcome, ' + ${name}">Welcome, User</p>
</body>
</html>

Variables & Loops

<!-- Variable -->
<p th:text="${user.name}">Name</p>

<!-- Loop -->
<tr th:each="user : ${users}">
    <td th:text="${user.name}">Name</td>
    <td th:text="${user.email}">Email</td>
</tr>

<!-- Conditionals -->
<p th:if="${user.active}">Active</p>
<p th:unless="${user.active}">Inactive</p>

Email Service

@Service
@RequiredArgsConstructor
public class EmailService {

    private final TemplateEngine templateEngine;
    private final JavaMailSender mailSender;

    public void sendWelcome(User user) {
        Context ctx = new Context();
        ctx.setVariable("userName", user.getName());
        ctx.setVariable("actionUrl", "https://app.com/verify");

        String html = templateEngine.process("emails/welcome", ctx);

        MimeMessage msg = mailSender.createMimeMessage();
        MimeMessageHelper helper = new MimeMessageHelper(msg, true);
        helper.setTo(user.getEmail());
        helper.setSubject("Welcome!");
        helper.setText(html, true);
        mailSender.send(msg);
    }
}

Fragments & Layouts

<!-- fragments/common.html -->
<nav th:fragment="navigation">
    <ul>
        <li><a th:href="@{/}">Home</a></li>
        <li><a th:href="@{/about}">About</a></li>
    </ul>
</nav>

<footer th:fragment="footer(year)">
    <p th:text="'© ' + ${year} + ' My Company'">© 2024 My Company</p>
</footer>

<!-- Usage: th:replace vs th:insert -->
<!-- th:replace - replaces host tag completely -->
<div th:replace="~{fragments/common :: navigation}"></div>

<!-- th:insert - inserts fragment inside host tag -->
<div th:insert="~{fragments/common :: navigation}"></div>

<!-- With parameters -->
<div th:replace="~{fragments/common :: footer(${#dates.year(#dates.createNow())})}"></div>

Layout Dialect

<!-- pom.xml -->
<dependency>
    <groupId>nz.net.ultraq.thymeleaf</groupId>
    <artifactId>thymeleaf-layout-dialect</artifactId>
</dependency>

<!-- layouts/main.html -->
<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org"
      xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout">
<head>
    <title layout:title-pattern="$CONTENT_TITLE - $LAYOUT_TITLE">My App</title>
    <link rel="stylesheet" th:href="@{/css/main.css}"/>
</head>
<body>
    <header th:replace="~{fragments/common :: navigation}"></header>

    <main layout:fragment="content">
        <!-- Page content goes here -->
    </main>

    <footer th:replace="~{fragments/common :: footer}"></footer>

    <script layout:fragment="scripts"></script>
</body>
</html>

<!-- pages/home.html -->
<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org"
      xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout"
      layout:decorate="~{layouts/main}">
<head>
    <title>Home</title>
</head>
<body>
    <main layout:fragment="content">
        <h1>Welcome!</h1>
        <p th:text="${message}">Content</p>
    </main>

    <th:block layout:fragment="scripts">
        <script th:src="@{/js/home.js}"></script>
    </th:block>
</body>
</html>

Form Binding

<form th:action="@{/users}" th:object="${userForm}" method="post">
    <div>
        <label for="name">Name:</label>
        <input type="text" id="name" th:field="*{name}"
               th:classappend="${#fields.hasErrors('name')} ? 'error' : ''"/>
        <span th:if="${#fields.hasErrors('name')}"
              th:errors="*{name}" class="error-msg">Name error</span>
    </div>

    <div>
        <label for="email">Email:</label>
        <input type="email" id="email" th:field="*{email}"/>
        <span th:if="${#fields.hasErrors('email')}"
              th:errors="*{email}" class="error-msg">Email error</span>
    </div>

    <div>
        <label for="role">Role:</label>
        <select id="role" th:field="*{role}">
            <option value="">-- Select --</option>
            <option th:each="role : ${roles}"
                    th:value="${role}"
                    th:text="${role.displayName}">Role</option>
        </select>
    </div>

    <button type="submit">Save</button>
</form>

JavaScript Inlining

<script th:inline="javascript">
    // Natural templates with fallback
    const user = /*[[${user}]]*/ { name: 'default' };
    const userId = /*[[${user.id}]]*/ 0;
    const isAdmin = /*[[${user.admin}]]*/ false;

    // Array
    const items = /*[[${items}]]*/ [];

    // Conditional in JS
    /*[# th:if="${user != null}"]*/
    console.log('User:', user.name);
    /*[/]*/
</script>

<!-- CSS inlining -->
<style th:inline="css">
    .user-bg {
        background-color: [[${user.favoriteColor}]];
    }
</style>

Utility Objects

<!-- Dates -->
<p th:text="${#dates.format(user.createdAt, 'dd/MM/yyyy HH:mm')}">Date</p>
<p th:text="${#dates.dayOfWeekName(date)}">Monday</p>

<!-- Strings -->
<p th:text="${#strings.toUpperCase(name)}">NAME</p>
<p th:text="${#strings.abbreviate(text, 100)}">Truncated...</p>
<p th:text="${#strings.isEmpty(value) ? 'N/A' : value}">Value</p>
<p th:text="${#strings.listJoin(items, ', ')}">a, b, c</p>

<!-- Numbers -->
<p th:text="${#numbers.formatDecimal(price, 1, 2)}">10.50</p>
<p th:text="${#numbers.formatCurrency(amount)}">$1,234.56</p>

<!-- Lists -->
<p th:text="${#lists.size(users)}">Count</p>
<p th:if="${#lists.isEmpty(users)}">No users</p>
<p th:text="${#lists.contains(roles, 'ADMIN')}">Has admin</p>

<!-- Aggregates -->
<p th:text="${#aggregates.sum(prices)}">Total</p>
<p th:text="${#aggregates.avg(scores)}">Average</p>

Switch/Case

<div th:switch="${user.role}">
    <p th:case="'ADMIN'">Administrator</p>
    <p th:case="'MANAGER'">Manager</p>
    <p th:case="'USER'">Regular User</p>
    <p th:case="*">Unknown Role</p>
</div>

Iteration Status

<tr th:each="user, stat : ${users}"
    th:class="${stat.odd} ? 'odd' : 'even'">
    <td th:text="${stat.index}">0</td>
    <td th:text="${stat.count}">1</td>
    <td th:text="${user.name}">Name</td>
    <td th:if="${stat.first}">First!</td>
    <td th:if="${stat.last}">Last!</td>
</tr>

Expressions

Expr	Uso
${var}	Variable
*{prop}	Selection (con th:object)
@{/url}	Link URL
#{msg}	Message i18n
~{frag}	Fragment

Best Practices

Do	Don't
Use th:text for escaped output	Use th:utext with user input (XSS)
Use fragments for reusable components	Duplicate HTML across templates
Keep templates simple	Put complex logic in templates
Use i18n messages	Hardcode text strings
Use layout dialects	Repeat layout in every page

When NOT to Use This Skill

• REST APIs - Use spring-rest skill for JSON responses
• SPA frontends - Use React, Vue, or Angular
• PDF reports - Use JasperReports or iText
• High-traffic APIs - Consider static frontends

Anti-Patterns

Anti-Pattern	Problem	Solution
th:utext with user input	XSS vulnerability	Use th:text for escaping
Complex logic in templates	Hard to test, maintain	Move logic to controller
Missing th namespace	Silent failures	Always declare xmlns:th
Inline styles everywhere	Inconsistent UI	Use CSS classes
No fragment reuse	Code duplication	Extract common fragments

Quick Troubleshooting

Problem	Diagnostic	Fix
Variables not rendering	Check th: prefix	Use th:text, th:value
Template not found	Check path	Place in templates/ folder
Iteration not working	Check th:each syntax	Verify collection is passed
Fragments not included	Check path syntax	Use ~{fragments/name :: fragment}
i18n not working	Check messages.properties	Verify file location and keys

Reference Documentation

• Thymeleaf Docs
• Spring + Thymeleaf