Why REST controllers are essential in Spring Boot - See It in Action

Why REST controllers are essential

📖 Scenario: You are building a simple web service for a bookstore. You want to let users see the list of books and add new books through the web.

🎯 Goal: Create a REST controller in Spring Boot that handles HTTP requests to get all books and add a new book. This will show why REST controllers are essential for connecting your backend logic to the web.

📋 What You'll Learn

Create a list of books as initial data

Add a configuration variable for the base URL path

Write a REST controller class with methods to handle GET and POST requests

Use Spring Boot annotations to map URLs and HTTP methods

💡 Why This Matters

🌍 Real World

REST controllers are essential in web applications to expose backend services as APIs that frontend apps or other services can use.

💼 Career

Understanding REST controllers is key for backend developers working with Spring Boot to build scalable and maintainable web services.

1

Create initial book data

Create a List<String> called books with these exact entries: "Spring Boot Basics", "RESTful APIs", "Java Programming".

Spring Boot

# Create a List called books with 3 book titles
# Your code here

Hint

Use List.of() to create an immutable list with the given book titles.

2

Add base URL path configuration

Create a String variable called basePath and set it to "/api/books".

Spring Boot

List books = List.of("Spring Boot Basics", "RESTful APIs", "Java Programming");
# Create a String variable basePath with value "/api/books"
# Your code here

Hint

This string will be used as the base URL path for the REST controller.

3

Create REST controller class with GET and POST methods

Create a public class called BookController annotated with @RestController and @RequestMapping(basePath). Inside it, write a method getBooks() annotated with @GetMapping that returns the books list. Also, add a method addBook(String book) annotated with @PostMapping that returns a string "Added: " + book.

Spring Boot

List books = List.of("Spring Boot Basics", "RESTful APIs", "Java Programming");
String basePath = "/api/books";

// Create BookController class with @RestController and @RequestMapping(basePath)
// Add getBooks() method with @GetMapping returning books
// Add addBook(String book) method with @PostMapping returning "Added: " + book
# Your code here

Hint

Use Spring Boot annotations to map the class and methods to HTTP requests. The @RequestBody annotation is needed to get the POST request body.

4

Complete REST controller setup

Add the necessary import statements for List and org.springframework.web.bind.annotation.RequestBody at the top of the file to complete the REST controller code.

Spring Boot

import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
// Add import for java.util.List and org.springframework.web.bind.annotation.RequestBody

@RestController
@RequestMapping("/api/books")
public class BookController {
    List books = List.of("Spring Boot Basics", "RESTful APIs", "Java Programming");

    @GetMapping
    public List getBooks() {
        return books;
    }

    @PostMapping
    public String addBook(@RequestBody String book) {
        return "Added: " + book;
    }
}
# Your code here

Hint

Import statements are needed for the List type and the @RequestBody annotation.

Practice

1. Why are REST controllers essential in a Spring Boot application?

easy

A. They manage database connections directly.

B. They style the user interface of the application.

C. They handle HTTP requests and return data in formats like JSON.

D. They compile the Java code into bytecode.

Solution

1. Step 1: Understand the role of REST controllers

   REST controllers are designed to handle web requests and responses in a Spring Boot app.

2. Step 2: Identify what REST controllers return

   They typically return data in formats like JSON, which is easy for other apps to use.

3. Final Answer:

   They handle HTTP requests and return data in formats like JSON. -> Option C

4. Quick Check:

   REST controllers = handle HTTP + return JSON [OK]

Hint: REST controllers handle web requests and send JSON responses [OK]

Common Mistakes:

• Thinking REST controllers manage database connections
• Confusing REST controllers with UI styling components
• Believing REST controllers compile code

2. Which annotation is used to define a REST controller in Spring Boot?

easy

A. @RestController

B. @Controller

C. @Service

D. @Repository

Solution

1. Step 1: Recall Spring Boot annotations for controllers

   @Controller is for MVC controllers, but REST controllers need to return data directly.

2. Step 2: Identify the annotation that combines @Controller and @ResponseBody

   @RestController does both, making it perfect for REST APIs.

3. Final Answer:

   @RestController -> Option A

4. Quick Check:

   @RestController = REST API controller [OK]

Hint: Use @RestController to create REST APIs in Spring Boot [OK]

Common Mistakes:

• Using @Controller without @ResponseBody for REST APIs
• Confusing @Service or @Repository as controller annotations
• Omitting the annotation entirely

3. What will be the output when accessing the endpoint defined by this REST controller method?

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

medium

A. A JSON object {"message":"Hello, World!"}

B. Plain text response: Hello, World!

C. HTTP 404 Not Found error

D. An HTML page with 'Hello, World!' text

Solution

1. Step 1: Analyze the method return type and annotation

   The method returns a String and is inside a @RestController, so the string is sent as plain text.

2. Step 2: Understand @RestController behavior

   @RestController automatically adds @ResponseBody, so the return is the response body, not a view or JSON object.

3. Final Answer:

   Plain text response: Hello, World! -> Option B

4. Quick Check:

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

Hint: String return in @RestController sends plain text response [OK]

Common Mistakes:

• Expecting JSON output without wrapping string in an object
• Thinking it returns an HTML page
• Assuming 404 error without mapping

4. Identify the error in this REST controller code snippet:

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

Assuming User is a simple POJO with name and age fields.

medium

A. No error; this code will return user data as JSON

B. User class must be annotated with @Entity

C. The method should return String, not User

D. Missing @ResponseBody annotation on the method

Solution

1. Step 1: Check if @RestController handles response body automatically

   @RestController combines @Controller and @ResponseBody, so no need to add @ResponseBody on methods.

2. Step 2: Confirm if returning a POJO works

   Spring Boot automatically converts POJOs to JSON in REST responses.

3. Final Answer:

   No error; this code will return user data as JSON -> Option A

4. Quick Check:

   @RestController + POJO return = JSON response [OK]

Hint: @RestController auto-converts POJOs to JSON, no extra annotation needed [OK]

Common Mistakes:

• Adding unnecessary @ResponseBody on methods inside @RestController
• Thinking POJO must be @Entity for REST response
• Changing return type to String unnecessarily

5. You want to create a REST controller that returns a list of users as JSON. Which approach correctly ensures the response is JSON and accessible at "/users" endpoint?

@RestController
public class UserController {
  public List<User> getUsers() {
    return List.of(new User("Bob", 25), new User("Carol", 28));
  }
}

hard

A. Return a String with JSON manually instead of List<User>

B. Change @RestController to @Controller and add @ResponseBody on getUsers()

C. Annotate User class with @RestController

D. Add @GetMapping("/users") above getUsers() method

Solution

1. Step 1: Identify missing mapping annotation

   The method lacks @GetMapping, so it won't be exposed as an endpoint.

2. Step 2: Confirm correct annotation for REST endpoint

   Adding @GetMapping("/users") exposes the method at /users and returns JSON automatically.

3. Final Answer:

   Add @GetMapping("/users") above getUsers() method -> Option D

4. Quick Check:

   @GetMapping + @RestController = JSON endpoint [OK]

Hint: Use @GetMapping on method inside @RestController for JSON endpoint [OK]

Common Mistakes:

• Forgetting @GetMapping annotation on methods
• Misusing @Controller without @ResponseBody
• Trying to manually build JSON strings
• Annotating POJO class with @RestController