@RestController annotation in Spring Boot

Introduction

The @RestController annotation tells Spring Boot to handle web requests and send back data, usually in JSON format, instead of a web page.

When building a web API that sends data to a client app like a mobile app or frontend JavaScript.

When you want to create a simple service that returns JSON or XML responses.

When you do not need to return HTML views but only data.

When you want to quickly create RESTful web services without extra configuration.

Syntax

Spring Boot

@RestController
public class MyController {
    // handler methods
}

It combines @Controller and @ResponseBody, so you don't need to add @ResponseBody on each method.

Use it on classes that handle HTTP requests and return data directly.

Examples

This example creates a simple endpoint /hello that returns plain text.

Spring Boot

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

This example returns a User object as JSON automatically.

Spring Boot

@RestController
public class UserController {
    @GetMapping("/user")
    public User getUser() {
        return new User("Alice", 30);
    }
}

Sample Program

This Spring Boot controller listens for GET requests at /greet and returns a simple greeting string as the response body.

Spring Boot

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

@RestController
public class GreetingController {

    @GetMapping("/greet")
    public String greet() {
        return "Hello from Spring Boot!";
    }
}

Important Notes

Remember that @RestController automatically converts return values to JSON or other formats using message converters.

If you want to return HTML views, use @Controller instead.

Use @GetMapping, @PostMapping, etc., inside the class to handle specific HTTP methods.

Summary

@RestController makes it easy to create REST APIs by returning data directly.

It combines controller and response body behavior in one annotation.

Use it when you want to send JSON or other data formats, not HTML pages.

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