What is the role of the @Controller annotation in Thymeleaf applications?

Table of Contents

• Introduction
• The Role of the @Controller Annotation
  • 1. Handling HTTP Requests
    • Example:
  • 2. Rendering Thymeleaf Templates
  • 3. Binding Model Data
    • Example:
  • 4. Serving View Names
    • Example of Thymeleaf Template (userProfile.html):
• Practical Example: Controller and Thymeleaf Integration
  • Step 1: Create a Spring Boot Application
  • Step 2: Create a Controller with @Controller
  • Step 3: Create a Thymeleaf Template (home.html)
  • Step 4: Run the Application
• Conclusion

Introduction

In Spring Boot applications using Thymeleaf, the @Controller annotation plays a crucial role in handling HTTP requests and serving dynamic web content. The @Controller annotation is used to define a class as a controller, which is responsible for processing requests, adding data to the model, and returning a view name that corresponds to a Thymeleaf template.

This annotation marks the class as a Spring MVC controller, where you define methods to handle specific HTTP requests, and the controller interacts with the Thymeleaf template engine to render dynamic content.

In this guide, we will explore the significance of the @Controller annotation in Thymeleaf applications, how it works, and its role in the rendering of Thymeleaf templates.

The Role of the @Controller Annotation

The @Controller annotation is a central part of Spring MVC (Model-View-Controller) architecture, and in the context of Thymeleaf, it enables the integration between Spring Boot and the Thymeleaf template engine. Here’s how the @Controller annotation fits into the flow of a Thymeleaf application:

1. Handling HTTP Requests

A class annotated with @Controller is capable of handling HTTP requests. Within this class, methods are defined to process various types of HTTP requests (GET, POST, etc.) by using mapping annotations such as @GetMapping, @PostMapping, or @RequestMapping.

Example:

In this example, the @Controller annotation marks GreetingController as a Spring MVC controller. The greet method handles GET requests to the /greeting URL, adds a dynamic message attribute to the model, and returns the view name "greeting". This tells Spring Boot to look for a Thymeleaf template named greeting.html in the src/main/resources/templates/ directory.

2. Rendering Thymeleaf Templates

Once the controller processes the HTTP request, it returns a view name (such as "greeting" in the example above). Spring Boot, in conjunction with the Thymeleaf engine, uses this view name to locate the corresponding Thymeleaf template and render it dynamically, substituting variables in the template with data from the model.

3. Binding Model Data

The controller can add dynamic content to the model using Model or ModelAndView objects. This data is then available within the Thymeleaf template, allowing it to render content based on variables passed from the controller.

Example:

In this case, the controller adds a User object to the model with the key "user". The Thymeleaf template can now access this data and display it dynamically.

4. Serving View Names

A critical aspect of the @Controller annotation is that it enables the Spring MVC framework to return view names to be resolved by the Thymeleaf engine. When a method in a controller returns a view name, Spring Boot finds the corresponding Thymeleaf template file (e.g., userProfile.html) and renders it with the model data.

Example of Thymeleaf Template (userProfile.html):

Here, Thymeleaf will render the user’s firstName and lastName from the model, allowing for dynamic page rendering.

Practical Example: Controller and Thymeleaf Integration

Let's consider a complete example where a Spring Boot application uses the @Controller annotation to serve a Thymeleaf template:

Step 1: Create a Spring Boot Application

Step 2: Create a Controller with @Controller

Step 3: Create a Thymeleaf Template (home.html)

Step 4: Run the Application

When you run the application and visit http://localhost:8080/, Spring Boot will process the request, use the HomeController to populate the model with the greeting attribute, and render the home.html template with the appropriate dynamic content.

Conclusion

The @Controller annotation in Spring Boot applications using Thymeleaf plays a fundamental role in handling HTTP requests and serving dynamic content by rendering Thymeleaf templates. By defining methods in a class annotated with @Controller, developers can manage the flow of data, bind dynamic attributes to the model, and specify which Thymeleaf template to use for rendering the response. This seamless integration between Spring Boot and Thymeleaf allows for the development of dynamic, data-driven web applications with minimal configuration.