Refresh token pattern in Spring Boot - Step-by-Step Execution

Concept Flow - Refresh token pattern

User logs in

↓

Server issues Access Token + Refresh Token

↓

User uses Access Token to access resources

↓

Access Token expires?

No→Continue using Access Token

Yes↓

User sends Refresh Token to server

↓

Server validates Refresh Token

↓

Refresh Token valid?

No→Reject request, ask login

Yes↓

Server issues new Access Token (and optionally new Refresh Token)

↓

User continues with new Access Token

This flow shows how a user logs in, receives tokens, uses the access token until it expires, then uses the refresh token to get a new access token without logging in again.

Execution Sample

Spring Boot

POST /login -> issue accessToken + refreshToken
GET /resource with accessToken
If accessToken expired:
POST /refresh with refreshToken -> issue new accessToken

This code flow shows login issuing tokens, resource access with access token, and refreshing access token using refresh token.

Execution Table

Step	Action	Token State	Server Response	User Action
1	User logs in	No tokens	Issues accessToken + refreshToken	Stores tokens
2	User requests resource	accessToken valid	Resource data	Uses resource
3	Access token expires	accessToken expired	401 Unauthorized	Sends refreshToken
4	Server validates refreshToken	refreshToken valid	Issues new accessToken	Stores new accessToken
5	User requests resource again	new accessToken valid	Resource data	Uses resource
6	Refresh token invalid or expired	refreshToken invalid	401 Unauthorized	User must login again

💡 Process stops when refresh token is invalid or user logs out.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
accessToken	null	valid	valid	expired	new valid	valid	valid
refreshToken	null	valid	valid	valid	valid	valid	valid or invalid

Key Moments - 3 Insights

Why can't the user keep using the access token after it expires?

Because the access token is no longer valid (see Step 3 in execution_table), the server rejects requests with 401 Unauthorized to protect resources.

What happens if the refresh token is invalid or expired?

The server rejects the refresh request (Step 6), forcing the user to log in again to get new tokens.

Why do we need both access and refresh tokens?

Access tokens are short-lived for security; refresh tokens allow getting new access tokens without logging in again, improving user experience (see flow in concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table, what is the server response at Step 3 when the access token expires?

A Issues new access token

B 401 Unauthorized

C Resource data

D Redirect to login page

💡 Hint

Check the 'Server Response' column at Step 3 in execution_table

At which step does the user send the refresh token to the server?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'User Action' column in execution_table for when refreshToken is sent

If the refresh token becomes invalid, what must the user do next?

A Continue using expired access token

B Request new access token with refresh token

C Log in again to get new tokens

D Nothing, tokens auto-renew

💡 Hint

See Step 6 in execution_table and key_moments about invalid refresh token

Concept Snapshot

Refresh Token Pattern in Spring Boot:
- User logs in, server issues access + refresh tokens.
- Access token used for resource access, short-lived.
- When access token expires, user sends refresh token.
- Server validates refresh token, issues new access token.
- If refresh token invalid, user must log in again.
- Improves security and user experience.

Full Transcript

The refresh token pattern helps keep users logged in securely. When a user logs in, the server gives two tokens: an access token and a refresh token. The access token lets the user access resources but expires quickly for safety. When it expires, the user sends the refresh token to get a new access token without logging in again. If the refresh token is invalid or expired, the user must log in again. This flow balances security and convenience by limiting how long access tokens last but allowing easy renewal.

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