@RestController annotation in Spring Boot - Step-by-Step Execution

Concept Flow - @RestController annotation

Start Spring Boot App

↓

Scan for @RestController

↓

Create Controller Bean

↓

Map HTTP Requests to Methods

↓

Execute Method on Request

↓

Return Data as JSON

↓

Send JSON Response to Client

The Spring Boot app starts, finds classes with @RestController, creates them, maps HTTP requests to their methods, executes those methods, and returns JSON responses.

Execution Sample

Spring Boot

@RestController
public class HelloController {
  @GetMapping("/hello")
  public Map<String, String> sayHello() {
    return Map.of("message", "Hello World");
  }
}

Defines a REST controller that responds to GET /hello with JSON {"message":"Hello World"}.

Execution Table

Step	Action	Evaluation	Result
1	Start Spring Boot app	App starts	Spring context initializes
2	Scan classes for @RestController	Find HelloController	Create HelloController bean
3	Map HTTP GET /hello	Method sayHello found	Mapping stored
4	Receive HTTP GET /hello request	Match to sayHello method	Invoke sayHello()
5	Execute sayHello()	Return Map.of("message", "Hello World")	Response body set to {"message":"Hello World"}
6	Send response	Response body sent as JSON	Client receives {"message":"Hello World"}
7	No more requests	App waits	Idle

💡 No more HTTP requests; app remains running waiting for new requests.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	Final
HelloController bean	null	Created instance	Instance ready	Instance method executed	Instance ready
HTTP Request	none	none	GET /hello received	Handled	none
Response Body	empty	empty	empty	{"message":"Hello World"}	{"message":"Hello World"}

Key Moments - 2 Insights

Why does the method return a Map but the client receives JSON?

Because @RestController tells Spring to convert the return value to JSON automatically, as shown in execution_table step 5 where the Map is serialized to JSON for the response body.

What happens if we remove @RestController but keep @GetMapping?

Spring won't create the controller bean or map requests properly, so no response is sent. This is shown in execution_table step 2 where the bean creation depends on @RestController.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 3?

A The app sends the response to the client

B Spring maps the HTTP GET /hello to the sayHello method

C The HelloController bean is created

D The app starts running

💡 Hint

Check the 'Action' and 'Result' columns at step 3 in execution_table

At which step does the method sayHello() actually run?

A Step 5

B Step 2

C Step 4

D Step 6

💡 Hint

Look for 'Execute sayHello()' in the 'Action' column in execution_table

If the @RestController annotation is missing, what changes in the execution_table?

A Step 4 would still invoke sayHello()

B Step 5 would return JSON anyway

C Step 2 bean creation would not happen

D Step 6 would send a different response

💡 Hint

Refer to key_moments about bean creation depending on @RestController

Concept Snapshot

@RestController marks a class as a REST API controller.
Spring creates a bean and maps HTTP requests to its methods.
Returned values are automatically converted to JSON.
Use @GetMapping, @PostMapping etc. inside it.
No need for @ResponseBody on each method.
Ideal for building RESTful web services.

Full Transcript

The @RestController annotation in Spring Boot marks a class as a REST API controller. When the app starts, Spring scans for classes with this annotation and creates beans for them. It then maps HTTP requests like GET or POST to the methods inside these classes. When a request comes in, Spring calls the matching method and takes its return value. Because of @RestController, Spring automatically converts this return value to JSON and sends it back as the HTTP response. This makes it easy to build web APIs that return data in JSON format without extra code. For example, a method returning a Map will send that Map serialized as a JSON response. If @RestController is missing, Spring won't create the controller bean or map requests, so the API won't work. This flow helps developers quickly create RESTful services.

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