Understanding the Differences Between @ Controller and @ RestController in Spring Boot

Explore when to use `@ Controller` vs `@ RestController` in Spring Boot, and gain insights into the underlying concepts that matter for effective REST API development.
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Understanding the Differences Between @ Controller and @ RestController in Spring Boot

When starting your journey with Spring Boot, especially for web applications, you may find yourself facing a common dilemma: when should you use @ Controller and when is it more appropriate to use @ RestController? This decision can be crucial in determining how your application behaves, especially in how it responds to user requests. In this post, we will explore the differences between these two annotations and explain in detail the concept that will help you make the right choice for your application.

The Core Observations

Before diving into the nuances of these annotations, let’s clarify their basic functionalities:

@ Controller: This annotation is used in traditional MVC (Model-View-Controller) applications. When using it, you typically return views (HTML pages), and the framework looks for a view (like JSP, Thymeleaf, etc.) to render the data returned from the controller methods. This means that the response often includes a full HTML page presented to the user.

@ RestController: On the other hand, this is a specialized version of @ Controller. It is mainly used for building RESTful web services. Here, the main focus is on returning data instead of views. The return value from the methods would typically be in JSON format, making it better suited for applications that interact with APIs rather than web pages.

Understanding the Underlying Concepts

So, what’s happening behind the scenes that differentiates these two annotations? The key lies in how they handle the return values from their methods.

1. Composition of @ RestController

One important point is that @ RestController is built on top of @ Controller and @ ResponseBody. This means that a class annotated with @ RestController implicitly includes the behavior of both annotations. To break it down further:

@ Controller: Will process returned values as those of a view. If a method returns a string, that string will be interpreted as the name of the view to render.

@ ResponseBody: Forces the method return value to be used as the HTTP response body. Therefore, string or object returned will be sent directly to the client.

2. Handling of Return Values

When you are defining your controller methods, the handling of the return types is what primarily differentiates the two annotations:

Using @ RestController: The framework directly converts the returned object (e.g., a Java object) into JSON or XML, and sends it as the response body. This is ideal for API functionalities where the client expects a lightweight data format.

Using @ Controller: You end up needing additional steps for serializing your objects into a proper format, typically requiring a combination of view resolution and other handlers that manage how data is rendered and displayed.

Practical Implications

Understanding these differences not only allows you to choose the right annotation but also informs how you construct your applications. Here are a few quick guidelines:

Use @ Controller when:

Your application is primarily a web application with HTML views.

You need to render server-side views with model data.

Use @ RestController when:

Your application provides RESTful services that require returning JSON or XML data.

You are building an API that will be consumed by different clients (web or mobile).

Conclusion

Choosing between @ Controller and @ RestController in Spring Boot boils down to understanding the nature of the application you're building. Whether you're developing a dynamic web application with server-rendered views or an API service focuse

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