@RestController annotation in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Spring Boot controller method?

Consider this Spring Boot controller class annotated with @RestController. What will be the HTTP response body when accessing /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 HTTP response will be a redirect to /hello.html

B The HTTP response will be a JSON object {"message": "Hello, World!"}

C The HTTP response will be empty with status 200 OK

D The HTTP response body will be the string "Hello, World!"

💡 Hint

Remember that @RestController combines @Controller and @ResponseBody.

The @RestController annotation tells Spring Boot to write the return value of the method directly to the HTTP response body. Since the method returns a String, the response body will be exactly that string.

📝 Syntax

intermediate

Which option correctly uses @RestController to handle a POST request?

Select the code snippet that correctly defines a Spring Boot controller method to handle POST requests at path /submit and returns a confirmation string.

A

@RestController
public class SubmitController {
    @GetMapping("/submit")
    public String submit() {
        return "Submitted!";
    }
}

B

@RestController
public class SubmitController {
    @PostMapping("/submit")
    public String submit() {
        return "Submitted!";
    }
}

C

@Controller
public class SubmitController {
    @PostMapping("/submit")
    public String submit() {
        return "Submitted!";
    }
}

D

@RestController
public class SubmitController {
    @PostMapping("submit")
    public String submit() {
        return "Submitted!";
    }
}

💡 Hint

Check the HTTP method annotation and the path format.

Option B correctly uses @PostMapping("/submit") with the leading slash and @RestController. Option B uses GET instead of POST. Option B uses @Controller without @ResponseBody, so it won't return the string as response body. Option B misses the leading slash in the path, which is required.

🔧 Debug

advanced

Why does this @RestController method return a 404 error?

Given this controller, why does accessing /api/data return 404 Not Found?

Spring Boot

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

@RestController
public class DataController {
    @GetMapping("api/data")
    public String getData() {
        return "Data";
    }
}

A The path in @GetMapping is missing a leading slash; it should be "/api/data"

B The method must return a ResponseEntity<String> instead of String

C The class must be annotated with @Controller instead of @RestController

D The method must be public static to be accessible

💡 Hint

Check the path string format in the mapping annotation.

Spring expects the path in mapping annotations to start with a leading slash. Without it, the mapping does not match the requested URL, causing 404.

🧠 Conceptual

advanced

What is the main difference between @Controller and @RestController?

Choose the statement that best describes the difference between @Controller and @RestController in Spring Boot.

A @RestController combines @Controller and @ResponseBody, so methods return data directly as HTTP response body

B @Controller automatically serializes return values to JSON, while @RestController does not

C @RestController is used only for WebSocket endpoints, @Controller for HTTP

D @Controller is deprecated and replaced by @RestController in Spring Boot

💡 Hint

Think about how return values are handled in HTTP responses.

@RestController is a shortcut for @Controller plus @ResponseBody. It means the return value of methods is written directly to the HTTP response body, usually as JSON or plain text. @Controller is used for MVC views and requires @ResponseBody to return data directly.

❓ state_output

expert

What is the HTTP response when this @RestController method throws an exception?

Given this controller method, what will be the HTTP response status and body when /error is accessed?

Spring Boot

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

@RestController
public class ErrorController {
    @GetMapping("/error")
    public String error() {
        throw new IllegalStateException("Something went wrong");
    }
}

A HTTP 500 Internal Server Error with default error HTML page

B HTTP 200 OK with body "Something went wrong"

C HTTP 500 Internal Server Error with JSON body containing error details

D HTTP 404 Not Found with empty body

💡 Hint

Spring Boot's default error handling for REST controllers returns JSON error responses.

By default, Spring Boot returns a JSON error response with HTTP status 500 when an exception is thrown in a @RestController method. The JSON includes error details like timestamp, status, error, and message.