@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.

Practice

1. What is the main purpose of the @RestController annotation in Spring Boot?

easy

A. To create HTML views for web pages

B. To define a service class for business logic

C. To create a controller that returns data directly as JSON or XML

D. To configure database connections

Solution

1. Step 1: Understand the role of @RestController

   @RestController is used to create REST APIs that return data directly, usually in JSON or XML format.

2. Step 2: Compare with other options

   Creating HTML views is handled by MVC controllers, defining service classes by @Service, and configuring database connections elsewhere in Spring Boot.

3. Final Answer:

   To create a controller that returns data directly as JSON or XML -> Option C

4. Quick Check:

   @RestController = returns data directly [OK]

Hint: Remember: @RestController returns data, not HTML pages [OK]

Common Mistakes:

• Confusing @RestController with @Service
• Thinking it renders HTML views
• Assuming it configures database

2. Which of the following is the correct way to declare a Spring Boot REST controller?

easy

A. @Service public class MyController {}

B. @Controller public class MyController {}

C. @RestService public class MyController {}

D. @RestController public class MyController {}

Solution

1. Step 1: Identify the correct annotation for REST controllers

   @RestController is the correct annotation to create REST APIs in Spring Boot.

2. Step 2: Check other annotations

   @Controller is for MVC controllers returning views, @RestService is not a valid annotation, and @Service is for service classes.

3. Final Answer:

   @RestController public class MyController {} -> Option D

4. Quick Check:

   Correct REST controller syntax = @RestController public class MyController {} [OK]

Hint: Use @RestController for REST APIs, not @Controller [OK]

Common Mistakes:

• Using @Controller instead of @RestController
• Using non-existent @RestService annotation
• Confusing @Service with controller annotations

3. Given the following code, what will be the output when accessing /hello endpoint?

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

medium

A. An HTML page displaying "Hello, Spring!"

B. A plain text response with the string "Hello, Spring!"

C. A 404 Not Found error

D. A compilation error due to missing annotations

Solution

1. Step 1: Understand @RestController behavior

   @RestController returns the method's return value directly as the response body, usually JSON or plain text.

2. Step 2: Analyze the method return

   The method returns a String "Hello, Spring!" which will be sent as plain text in the response body.

3. Final Answer:

   A plain text response with the string "Hello, Spring!" -> Option B

4. Quick Check:

   @RestController + String return = plain text response [OK]

Hint: @RestController returns method data as response body [OK]

Common Mistakes:

• Expecting an HTML page instead of data
• Thinking it causes 404 without mapping
• Assuming compilation error without reason

4. Identify the error in this Spring Boot REST controller code:

@RestController
public class UserController {
    @GetMapping("/user")
    public User getUser() {
        User user = new User();
        user.setName("Alice");
        return user;
    }
}

Assume User is a simple POJO with a name field and getters/setters.

medium

A. No error; code will return User object as JSON

B. User class must be annotated with @Entity

C. Missing @ResponseBody annotation on getUser() method

D. Missing @RequestMapping on class level

Solution

1. Step 1: Check @RestController effect

   @RestController automatically adds @ResponseBody to all methods, so no need to add it explicitly.

2. Step 2: Verify User class requirements

   User is a POJO; it does not need @Entity unless used with database. Returning it will serialize to JSON automatically.

3. Final Answer:

   No error; code will return User object as JSON -> Option A

4. Quick Check:

   @RestController auto @ResponseBody = No error; code will return User object as JSON [OK]

Hint: @RestController adds @ResponseBody automatically [OK]

Common Mistakes:

• Adding unnecessary @ResponseBody on methods
• Confusing @Entity with serialization
• Thinking class-level @RequestMapping is mandatory

5. You want to create a REST API that returns a list of users as JSON. Which of the following code snippets correctly uses @RestController to achieve this?

hard

A. @RestController
public class UserApi {
@GetMapping("/users")
public List<String> getUsers() {
return List.of("Alice", "Bob", "Carol");
}
}

B. @RestController
public class UserApi {
@GetMapping("/users")
public String getUsers() {
return "Alice, Bob, Carol";
}
}

C. @Controller
public class UserApi {
@GetMapping("/users")
public List<String> getUsers() {
return List.of("Alice", "Bob", "Carol");
}
}

D. @RestController
public class UserApi {
@RequestMapping("/users")
public void getUsers() {
System.out.println("Alice, Bob, Carol");
}
}

Solution

1. Step 1: Check @RestController and return type

   @RestController public class UserApi { @GetMapping("/users") public List<String> getUsers() { return List.of("Alice", "Bob", "Carol"); } } uses @RestController and returns a List<String>, which Spring Boot converts to JSON automatically.

2. Step 2: Analyze other options

   The snippet returning String sends a plain string, not a JSON list. The snippet using @Controller attempts to resolve a view name, not return JSON data. The snippet with void return type sends no response body, only prints to console.

3. Final Answer:

   @RestController public class UserApi { @GetMapping("/users") public List<String> getUsers() { return List.of("Alice", "Bob", "Carol"); } } -> Option A

4. Quick Check:

   @RestController + List return = JSON list [OK]

Hint: Return List with @RestController for JSON array response [OK]

Common Mistakes:

• Using @Controller instead of @RestController for JSON
• Returning void or printing instead of returning data
• Returning plain string instead of list for JSON array