Why REST controllers are essential in Spring Boot - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use REST controllers in Spring Boot?

What is the main purpose of REST controllers in a Spring Boot application?

A To style the user interface with CSS

B To manage database connections directly

C To handle HTTP requests and send responses in a RESTful way

D To compile Java code into bytecode

💡 Hint

Think about how web applications communicate with clients over the internet.

REST controllers in Spring Boot are designed to receive HTTP requests, process them, and return HTTP responses, usually in JSON or XML format. They follow REST principles to make APIs easy to use and scalable.

❓ component_behavior

intermediate

Behavior of a REST controller method

Given a Spring Boot REST controller method annotated with @GetMapping("/hello"), what will be the output when a client sends a GET request to /hello?

Spring Boot

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

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String sayHello() {
        return "Hello, World!";
    }
}

A The client receives the plain text response: Hello, World!

B The client receives a 404 Not Found error

C The client receives an HTML page with the text Hello, World!

D The client receives a JSON object with key 'message' and value 'Hello, World!'

💡 Hint

Look at the return type and annotations used.

The method returns a plain String and the class is annotated with @RestController, so Spring Boot sends the string as the HTTP response body directly as plain text.

📝 Syntax

advanced

Correct annotation for REST controller

Which annotation correctly defines a REST controller in Spring Boot that returns JSON responses by default?

A @Controller

B @RestController

C @Service

D @Component

💡 Hint

This annotation combines @Controller and @ResponseBody.

@RestController is a specialized version of @Controller that automatically adds @ResponseBody to all methods, so responses are serialized to JSON or XML by default.

🔧 Debug

advanced

Why does this REST controller method return 404?

Consider this Spring Boot REST controller method: @GetMapping("/greet") public String greet() { return "Greetings!"; } Why might a GET request to /greet return a 404 Not Found error?

Spring Boot

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

@RestController
public class GreetController {
    @GetMapping("/greet")
    public String greet() {
        return "Greetings!";
    }
}

A The application is not running or the controller is not scanned by Spring Boot

B The class is missing the @RestController annotation

C The method should return a ResponseEntity<String> instead of String

D The @GetMapping annotation should be @PostMapping

💡 Hint

Check if Spring Boot knows about this controller class.

If the controller class is not detected by Spring Boot (for example, it's outside the scan path or the app is not running), requests to its endpoints will return 404 errors.

❓ state_output

expert

Output of REST controller with path variable and JSON response

What is the JSON output of this Spring Boot REST controller method when a client sends a GET request to /user/42? @GetMapping("/user/{id}") public Map getUser(@PathVariable int id) { return Map.of("userId", id, "name", "User" + id); }

Spring Boot

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import java.util.Map;

@RestController
public class UserController {
    @GetMapping("/user/{id}")
    public Map<String, Object> getUser(@PathVariable int id) {
        return Map.of("userId", id, "name", "User" + id);
    }
}

A 404 Not Found error

B "userId=42,name=User42"

C {"id":42,"username":"User42"}

D {"userId":42,"name":"User42"}

💡 Hint

Look at the keys and values returned in the Map and how Spring Boot converts it to JSON.

The method returns a Map with keys 'userId' and 'name'. Spring Boot converts this Map to a JSON object with those keys and values. So the output is a JSON string with userId 42 and name 'User42'.