Why REST controllers are essential in Spring Boot - Performance Evidence

Performance: Why REST controllers are essential

MEDIUM IMPACT

REST controllers impact server response time and client rendering speed by structuring API endpoints efficiently.

Handling API requests efficiently in a Spring Boot application

Spring Boot

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.List;

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

Using @RestController returns JSON directly, reducing server processing and speeding up client rendering.

📈 Performance GainReduces server response time by 100-200ms and improves LCP by delivering lightweight JSON

Handling API requests efficiently in a Spring Boot application

Spring Boot

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
public class UserController {
    @RequestMapping("/users")
    public String getUsers() {
        // returns view name, not JSON data
        return "users";
    }
}

Using @Controller returns views instead of JSON, causing unnecessary server-side rendering and slower client response.

📉 Performance CostBlocks rendering until server-side view is processed, increasing LCP by 200-300ms

Performance Comparison

Pattern	Server Processing	Network Payload	Client Rendering	Verdict
@Controller returning views	High (renders HTML)	Large (full HTML pages)	Slower (parsing HTML)	[X] Bad
@RestController returning JSON	Low (serializes data)	Small (JSON payload)	Faster (direct data use)	[OK] Good

Rendering Pipeline

REST controllers handle HTTP requests and return JSON responses directly, minimizing server-side rendering and enabling faster client-side rendering.

→Server Processing

→Network Transfer

→Client Rendering

⚠️ BottleneckServer Processing when controllers return views instead of JSON

Core Web Vital Affected

LCP

REST controllers impact server response time and client rendering speed by structuring API endpoints efficiently.

Optimization Tips

1Use @RestController to return JSON directly for faster server responses.

2Avoid returning server-rendered views when building APIs to reduce processing time.

3Smaller JSON payloads improve network transfer and client rendering speed.

Performance Quiz - 3 Questions

What is the main performance benefit of using @RestController in Spring Boot?

A It compresses images automatically.

B It returns JSON directly, reducing server processing and speeding up client rendering.

C It caches HTML views to speed up rendering.

D It delays response to batch requests.

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by XHR/fetch, and inspect API response size and timing.

What to look for: Look for smaller JSON payloads and faster response times indicating efficient REST controller usage.

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