Refresh token pattern in Spring Boot - Performance & Optimization

Performance: Refresh token pattern

MEDIUM IMPACT

This pattern affects the responsiveness and load on the backend server during user authentication refresh cycles.

Handling user session renewal without blocking user actions

Spring Boot

Use silent background refresh of access tokens before expiry with asynchronous calls, allowing user actions to continue.

Reduces blocking calls and spreads token refresh load, improving responsiveness.

📈 Performance GainImproves INP by reducing input wait times; user actions are not blocked.

Handling user session renewal without blocking user actions

Spring Boot

On every API request, the client sends the refresh token to get a new access token synchronously before proceeding.

This causes blocking network calls and delays user interactions, increasing input latency.

📉 Performance CostBlocks rendering and user interaction for 100-300ms per request during token refresh.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous token refresh on every request	Minimal	0	0	[X] Bad
Asynchronous background token refresh	Minimal	0	0	[OK] Good
Storing refresh token in localStorage	N/A	N/A	N/A	[X] Bad
Storing refresh token in HttpOnly cookie	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Refresh token pattern mainly affects backend communication and user input responsiveness rather than direct rendering.

→Network

→JavaScript Execution

→Idle Time

⚠️ BottleneckBlocking synchronous token refresh calls increase input latency.

Core Web Vital Affected

INP

This pattern affects the responsiveness and load on the backend server during user authentication refresh cycles.

Optimization Tips

1Avoid synchronous token refresh calls that block user input.

2Use asynchronous background refresh to improve interaction speed.

3Store refresh tokens securely in HttpOnly cookies to reduce forced re-authentication.

Performance Quiz - 3 Questions

What is the main performance benefit of using asynchronous background refresh tokens?

A Blocks rendering until token refresh completes

B Increases server load by refreshing tokens more often

C Reduces blocking user input and improves interaction responsiveness

D Stores tokens in localStorage for faster access

DevTools: Performance

How to check: Record a session while triggering token refresh; look for blocking network calls and long scripting tasks.

What to look for: Long blocking times during token refresh indicate poor pattern; smooth interaction shows good pattern.

Practice

1.

What is the main purpose of using a refresh token in a Spring Boot authentication system?

easy

A. To encrypt user data in the database

B. To store user passwords securely

C. To log out users automatically after a timeout

D. To allow users to get a new access token without logging in again

Solution

1. Step 1: Understand the role of refresh tokens

   Refresh tokens are used to get new access tokens without asking the user to log in again.

2. Step 2: Compare options with this purpose

   Only To allow users to get a new access token without logging in again describes this purpose correctly; others describe unrelated functions.

3. Final Answer:

   To allow users to get a new access token without logging in again -> Option D

4. Quick Check:

   Refresh token purpose = renew access token [OK]

Hint: Refresh tokens renew access tokens without re-login [OK]

Common Mistakes:

• Confusing refresh token with password storage
• Thinking refresh token logs out users
• Assuming refresh token encrypts data

2.

Which of the following is the correct way to define a method in a Spring Boot controller to handle refresh token requests?

@PostMapping("/refresh")
public ResponseEntity<TokenResponse> refreshToken(@RequestBody RefreshRequest request) {
    // method body
}

easy

A. @GetMapping("/refresh") public TokenResponse refreshToken(@RequestBody RefreshRequest request)

B. @PostMapping("/refresh") public void refreshToken(String token)

C. @PostMapping("/refresh") public ResponseEntity<TokenResponse> refreshToken(@RequestBody RefreshRequest request)

D. @RequestMapping("/refresh") public String refreshToken()

Solution

1. Step 1: Check HTTP method and parameters

   The refresh token request should be a POST with a JSON body containing the refresh token.

2. Step 2: Match method signature

   @PostMapping("/refresh") public ResponseEntity<TokenResponse> refreshToken(@RequestBody RefreshRequest request) correctly uses @PostMapping, returns ResponseEntity<TokenResponse>, and accepts @RequestBody RefreshRequest.

3. Final Answer:

   @PostMapping("/refresh") public ResponseEntity<TokenResponse> refreshToken(@RequestBody RefreshRequest request) -> Option C

4. Quick Check:

   Correct POST method and request body = @PostMapping("/refresh") public ResponseEntity<TokenResponse> refreshToken(@RequestBody RefreshRequest request) [OK]

Hint: Refresh token requests use POST with @RequestBody [OK]

Common Mistakes:

• Using GET instead of POST
• Missing @RequestBody annotation
• Wrong return type or parameters

3.

Given the following Spring Boot service method, what will be the output if the refresh token is invalid?

public TokenResponse refreshAccessToken(String refreshToken) {
    if (!tokenRepository.existsByToken(refreshToken)) {
        throw new RuntimeException("Invalid refresh token");
    }
    // generate new access token
    return new TokenResponse("newAccessToken");
}

medium

A. Throws RuntimeException with message "Invalid refresh token"

B. Returns a new TokenResponse with "newAccessToken"

C. Returns null

D. Returns the old refresh token

Solution

1. Step 1: Analyze the token existence check

   The method checks if the refresh token exists in the repository; if not, it throws an exception.

2. Step 2: Determine behavior on invalid token

   Since the token is invalid, the method throws RuntimeException with the message "Invalid refresh token".

3. Final Answer:

   Throws RuntimeException with message "Invalid refresh token" -> Option A

4. Quick Check:

   Invalid token triggers exception = Throws RuntimeException with message "Invalid refresh token" [OK]

Hint: Invalid refresh token causes exception throw [OK]

Common Mistakes:

• Assuming method returns null on invalid token
• Thinking it returns old token instead
• Ignoring exception throwing

4.

Identify the error in this Spring Boot refresh token controller method:

@PostMapping("/refresh")
public ResponseEntity<TokenResponse> refreshToken(@RequestParam String refreshToken) {
    TokenResponse token = authService.refreshAccessToken(refreshToken);
    return ResponseEntity.ok(token);
}

What is the problem?

medium

A. Using @RequestParam instead of @RequestBody for refresh token

B. Missing @PostMapping annotation

C. Returning ResponseEntity instead of TokenResponse

D. Calling wrong service method

Solution

1. Step 1: Check parameter annotation

   The refresh token is usually sent in the request body as JSON, not as a query parameter.

2. Step 2: Identify correct annotation

   The method should use @RequestBody instead of @RequestParam to receive the refresh token properly.

3. Final Answer:

   Using @RequestParam instead of @RequestBody for refresh token -> Option A

4. Quick Check:

   Refresh token needs @RequestBody, not @RequestParam [OK]

Hint: Refresh token comes in body, use @RequestBody [OK]

Common Mistakes:

• Using query parameters for refresh token
• Confusing ResponseEntity with return type
• Missing or wrong annotations

5.

You want to implement a refresh token mechanism in Spring Boot that invalidates the old refresh token after use and issues a new one along with the access token. Which approach below correctly achieves this?

hard

A. Check refresh token validity, generate new access token, keep old refresh token unchanged

B. Generate new access token and new refresh token, save new refresh token, delete old refresh token

C. Generate new access token only, do not check refresh token validity

D. Delete refresh token without issuing new tokens

Solution

1. Step 1: Understand token rotation

   To improve security, the old refresh token should be invalidated and replaced with a new one after use.

2. Step 2: Identify correct token handling

   Generate new access token and new refresh token, save new refresh token, delete old refresh token correctly generates new access and refresh tokens, saves the new refresh token, and deletes the old one.

3. Final Answer:

   Generate new access token and new refresh token, save new refresh token, delete old refresh token -> Option B

4. Quick Check:

   Refresh token rotation = Generate new access token and new refresh token, save new refresh token, delete old refresh token [OK]

Hint: Rotate refresh tokens: new token saved, old token deleted [OK]

Common Mistakes:

• Not invalidating old refresh token
• Skipping refresh token validity check
• Deleting tokens without issuing new ones