@RestController annotation in Spring Boot - Mini Project: Build & Apply

Using the @RestController Annotation in Spring Boot

📖 Scenario: You are building a simple web service that responds with greeting messages. This service will use Spring Boot to create a REST API endpoint.

🎯 Goal: Create a Spring Boot REST controller using the @RestController annotation that returns a greeting message when accessed via HTTP GET.

📋 What You'll Learn

Create a Spring Boot application class

Create a controller class annotated with @RestController

Add a method that handles GET requests at path /greet

Return a plain text greeting message from the method

💡 Why This Matters

🌍 Real World

REST controllers are used to build web services that communicate over HTTP, commonly used in web and mobile apps.

💼 Career

Understanding @RestController is essential for backend developers working with Spring Boot to create APIs.

1

Create the Spring Boot application class

Create a class called GreetingApplication annotated with @SpringBootApplication and add the main method that runs SpringApplication.run(GreetingApplication.class, args).

Spring Boot

// Create the GreetingApplication class with @SpringBootApplication and main method
// Your code here

Hint

The main class needs the @SpringBootApplication annotation and a main method to start the app.

2

Create the REST controller class

Create a class called GreetingController and annotate it with @RestController.

Spring Boot

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

// Create the GreetingController class annotated with @RestController
// Your code here

Hint

The controller class must have the @RestController annotation above the class declaration.

3

Add a GET method to return a greeting

Inside the GreetingController class, add a method called greet annotated with @GetMapping("/greet") that returns the string "Hello, Spring Boot!".

Spring Boot

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

// Add a method greet() annotated with @GetMapping("/greet") that returns "Hello, Spring Boot!"
// Your code here

Hint

The method should be public, return a String, and have the @GetMapping("/greet") annotation.

4

Complete the REST controller setup

Ensure the GreetingController class imports org.springframework.web.bind.annotation.GetMapping and that the class and method are properly structured to serve the greeting at /greet.

Spring Boot

// Verify imports and class structure are complete
// Your code here

Hint

Make sure all necessary imports are present and the class and method annotations are correct.

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