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5.1 @RestController and @RequestMapping

When building a web application with Spring Boot, let's learn about the 'Controller', which acts as the gatekeeper receiving the user's initial requests.

1. @Controller vs @RestController​

There are two main annotations for declaring a controller in Spring. In modern REST API servers, @RestController is primarily used.

  • @Controller: Mainly used in traditional server-side rendering environments (JSP, Thymeleaf). It finds an HTML View file based on the string returned by the method and sends it as a response.
  • @RestController: The standard for modern REST API environments communicating with frontends (React, Vue) or mobile apps. Instead of finding an HTML screen, it automatically converts the returned data into JSON format and injects it directly into the response body.

(@RestController is internally a combination of @Controller and @ResponseBody.)

2. URL Mapping with @RequestMapping​

Once you have set up a gatekeeper (controller), you must map paths so that a specific method is executed when a request comes to a particular URL address.

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController // Designates as a REST API controller (responses are JSON)
public class HelloController {

    // This method is executed when accessing the "/hello" address
    @RequestMapping("/hello")
    public String sayHello() {
        return "Hello, Spring Boot!"; // Only pure string data is sent back to the browser
    }
}

Class-level @RequestMapping​

When designing an API, the front part of the URL often overlaps. In this case, you can specify a Base Path by attaching @RequestMapping to the top of the class.

@RestController
@RequestMapping("/api/v1/users") // All methods in this class will start with this path
public class UserController {

    // The actual full URL called: "/api/v1/users/info"
    @RequestMapping("/info")
    public String getUserInfo() {
        return "User Info";
    }
}

Beyond just connecting addresses, we will learn about HTTP method mapping in the next chapter, which distinguishes the exact purpose (create, update, delete) of the incoming request.