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.