Unit Testing Mustache Templates in Spring Boot

Unit Testing Mustache Templates in Spring Boot: Strategies for Effective Tests

Hey fellow Spring Boot devs! If you’re using Mustache templates in your projects, you’ve probably wondered how to test them without pulling your hair out. Today, I’m diving into unit testing Mustache templates in Spring Boot—something I’ve tackled plenty as a senior Java developer. Mustache’s logic-less nature makes it a breeze for templating, but ensuring it renders correctly with your data takes some strategy. In this post, I’ll share a working example, real-world use cases, and some battle-tested tips to make your tests effective. Let’s get testing!

Unit Testing Mustache Templates in Spring Boot

1. Introduction

Spring Boot and Mustache are a killer combo for server-side rendering—lightweight and straightforward. But here’s the catch: Mustache templates rely heavily on the data you pass from your controllers, and bugs can sneak in if that handshake goes wrong. Unit testing your templates ensures they display what you expect, whether it’s a list, a conditional block, or just plain text. With Spring Boot’s testing tools like MockMvc, you can verify this without firing up a browser. I’ll show you how to set up solid tests with a practical example—because who doesn’t love catching bugs before they hit production?

2. Usages

Why bother testing Mustache templates? Here’s where it’s saved my bacon:

Dynamic Lists: Making sure product catalogs or user lists render correctly, even when empty.
Conditional Rendering: Verifying that “Admin Only” sections show up—or don’t—based on user roles.
Error Messages: Ensuring fallback content (e.g., “No data found”) displays when it should.
UI Consistency: Checking that templates match designs across edge cases—like missing fields or special characters.

Testing keeps your app reliable and your users happy, no matter what data gets thrown at it.

3. Code Example

Let’s build a Spring Boot app with a Mustache template for a “task list” and write unit tests to verify it. We’ll test rendering, empty states, and conditionals.

Project Setup

Spring Boot version: 3.2.x
Dependencies: spring-boot-starter-mustache, spring-boot-starter-test

`pom.xml`

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-mustache</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

Application Properties (`application.yml`)

spring:
  mustache:
    suffix: .mustache

Model (`Task.java`)

public class Task {
    private String title;
    private boolean completed;

    public Task(String title, boolean completed) {
        this.title = title;
        this.completed = completed;
    }

    // Getters and setters
    public String getTitle() { return title; }
    public boolean isCompleted() { return completed; }
    public void setTitle(String title) { this.title = title; }
    public void setCompleted(boolean completed) { this.completed = completed; }
}

Controller (`TaskController.java`)

@Controller
public class TaskController {

    @GetMapping("/tasks")
    public String getTasks(Model model) {
        List<Task> tasks = Arrays.asList(
            new Task("Write blog", true),
            new Task("Review code", false)
        );
        model.addAttribute("tasks", tasks);
        return "tasks";
    }
}

Mustache Template (`src/main/resources/templates/tasks.mustache`)

<!DOCTYPE html>
<html>
<head>
    <title>Task List</title>
</head>
<body>
    <h1>Your Tasks</h1>
    {{#tasks}}
        <div class="task">
            <p>{{title}} - {{#completed}}Done{{/completed}}{{^completed}}Pending{{/completed}}</p>
        </div>
    {{/tasks}}
    {{^tasks}}
        <p>No tasks available.</p>
    {{/tasks}}
</body>
</html>

Unit Test (`TaskControllerTest.java`)

@SpringBootTest
@AutoConfigureMockMvc
public class TaskControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void testTasksRenderCorrectly() throws Exception {
        mockMvc.perform(get("/tasks"))
            .andExpect(status().isOk())
            .andExpect(content().string(containsString("Your Tasks")))
            .andExpect(content().string(containsString("Write blog - Done")))
            .andExpect(content().string(containsString("Review code - Pending")));
    }

    @Test
    public void testEmptyTasks() throws Exception {
        // Mock controller with empty list
        TaskController controller = new TaskController() {
            @Override
            public String getTasks(Model model) {
                model.addAttribute("tasks", Collections.emptyList());
                return "tasks";
            }
        };
        MockMvc customMockMvc = MockMvcBuilders.standaloneSetup(controller).build();

        customMockMvc.perform(get("/tasks"))
            .andExpect(status().isOk())
            .andExpect(content().string(containsString("No tasks available")));
    }
}

Main Application (`DemoApplication.java`)

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

Run the tests with mvn test—they’ll verify the template renders tasks and handles empty states!

4. Explanation

Here’s how this testing setup works:

Dependencies: spring-boot-starter-test brings in MockMvc for HTTP testing, while spring-boot-starter-mustache powers the templates.
Controller: The TaskController passes a task list to the model, which Mustache renders.
Template: {{#tasks}} loops over tasks, using {{#completed}} and {{^completed}} for status. {{^tasks}} handles the empty case.
Tests:
- testTasksRenderCorrectly: Uses MockMvc to hit /tasks, checking the response contains expected text like “Write blog - Done”.
- testEmptyTasks: Overrides the controller with an empty list, verifying “No tasks available” shows up.
Assertions: content().string(containsString()) checks the rendered HTML matches expectations.

This approach tests the full controller-to-template flow without a real server.

5. Best Practices

From my testing adventures, here’s what keeps Mustache tests solid:

Test Real Scenarios: Cover happy paths (data present), edge cases (empty lists), and errors (nulls or exceptions).
Use MockMvc: It’s lightweight and lets you test rendering without spinning up the app—fast and reliable.
Keep Tests Focused: Check specific strings or patterns in the output, not the entire HTML—less brittle that way.
Mock Data: Override controllers or use mocks to simulate different data states, like empty or malformed inputs.
SEO Check: If your templates are public-facing, test for meta tags and keywords (e.g., “Spring Boot Mustache testing”).

6. Conclusion

And there you have it—unit testing Mustache templates in Spring Boot doesn’t have to be a chore! With the right strategies, you can catch rendering bugs early and keep your app rock-solid. The task list example is a great starting point—adapt it for your own lists, conditionals, or error pages. I love how these tests give me confidence that my Mustache views will behave, no matter the data. Tried testing Mustache yet? Got a killer testing tip? Drop a comment—I’d love to swap stories! Happy coding, and catch you next time!

Unit Testing Mustache Templates in Spring Boot