Stateless authentication mental model in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to extract the token from the HTTP header.

Spring Boot

String token = request.getHeader([1]);

Drag options to blanks, or click blank then click option'

A"Authorization"

B"Cookie"

C"Content-Type"

D"User-Agent"

Attempts:

3 left

💡 Hint

Common Mistakes

Using Cookie header instead of Authorization.

Trying to get token from Content-Type or User-Agent headers.

The token is usually sent in the Authorization header in stateless authentication.

2fill in blank

medium

Complete the code to validate the JWT token signature.

Spring Boot

boolean isValid = jwtUtil.validateToken([1]);

Drag options to blanks, or click blank then click option'

Atoken

Brequest

Cauthentication

DuserDetails

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the whole request object instead of the token string.

Passing user details or authentication objects.

The token string is passed to the validation method to check its signature and claims.

3fill in blank

hard

Fix the error in the code to set the authentication in the security context.

Spring Boot

SecurityContextHolder.getContext().setAuthentication([1]);

Drag options to blanks, or click blank then click option'

AuserDetails

Btoken

CauthenticationToken

Drequest

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the raw token string instead of an Authentication object.

Passing the request or user details directly.

The authenticationToken object represents the authenticated user and must be set in the security context.

4fill in blank

hard

Fill both blanks to create a JWT token with a subject and expiration.

Spring Boot

String jwt = Jwts.builder().setSubject([1]).setExpiration([2]).compact();

Drag options to blanks, or click blank then click option'

Ausername

Bnew Date(System.currentTimeMillis() + 3600000)

Cnew Date()

Dpassword

Attempts:

3 left

💡 Hint

Common Mistakes

Using password as subject.

Setting expiration to current date (already expired).

The username is set as the subject, and the expiration is set to one hour from now.

5fill in blank

hard

Fill both blanks to extract username, check expiration, and get claims from JWT.

Spring Boot

Claims claims = Jwts.parser().setSigningKey(secretKey).parseClaimsJws([1]).getBody(); String username = claims.getSubject(); boolean expired = claims.getExpiration().[2](new Date());

Drag options to blanks, or click blank then click option'

Atoken

Bafter

Cbefore

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'after' instead of 'before' to check expiration.

Parsing something other than the token string.

The token is parsed to get claims. The expiration date is checked if it is before the current date to determine if expired.

Practice

1. What is the main idea behind stateless authentication in Spring Boot?

easy

A. The server does not keep user session data; clients send tokens each time.

B. The server stores all user sessions in memory for quick access.

C. The server uses cookies to remember users between requests.

D. The server requires users to log in for every single request manually.

Solution

1. Step 1: Understand stateless authentication concept

   Stateless means the server does not save any user session data between requests.

2. Step 2: Identify how user identity is maintained

   Clients send a token with each request to prove who they are without server memory.

3. Final Answer:

   The server does not keep user session data; clients send tokens each time. -> Option A

4. Quick Check:

   Stateless = No server session, token sent each time [OK]

Hint: Stateless means no server memory, token sent every request [OK]

Common Mistakes:

• Thinking server stores session data
• Confusing cookies with stateless tokens
• Assuming login required every request

2. Which of the following is the correct way to send a token in a stateless Spring Boot API request?

easy

A. Include the token in the request body as plain text.

B. Send the token in the Authorization header as a Bearer token.

C. Store the token in a server-side session variable.

D. Attach the token as a URL query parameter without encoding.

Solution

1. Step 1: Recall token transmission best practice

   Tokens are usually sent in the Authorization header using the Bearer scheme.

2. Step 2: Eliminate incorrect methods

   Request body is not standard for tokens; server-side session breaks statelessness; URL query parameters are insecure and not recommended.

3. Final Answer:

   Send the token in the Authorization header as a Bearer token. -> Option B

4. Quick Check:

   Token in Authorization header = correct [OK]

Hint: Tokens go in Authorization header as Bearer [OK]

Common Mistakes:

• Putting token in request body
• Using server session storage
• Sending token in URL query insecurely

3. Given this Spring Boot controller snippet using stateless authentication:

@GetMapping("/profile")
public ResponseEntity<String> getProfile(@RequestHeader("Authorization") String authHeader) {
    if (authHeader == null || !authHeader.startsWith("Bearer ")) {
        return ResponseEntity.status(401).body("Unauthorized");
    }
    String token = authHeader.substring(7);
    if (token.equals("valid-token")) {
        return ResponseEntity.ok("User Profile Data");
    } else {
        return ResponseEntity.status(403).body("Forbidden");
    }
}

What will be the response if the client sends header Authorization: Bearer valid-token?

medium

A. 401 Unauthorized

B. 403 Forbidden

C. 200 OK with 'User Profile Data'

D. 500 Internal Server Error

Solution

1. Step 1: Check Authorization header presence and format

   The header is present and starts with "Bearer ", so it passes the first check.

2. Step 2: Extract token and compare

   The token extracted is "valid-token", which matches the expected valid token.

3. Step 3: Determine response

   Since token is valid, the method returns 200 OK with "User Profile Data".

4. Final Answer:

   200 OK with 'User Profile Data' -> Option C

5. Quick Check:

   Valid token = 200 OK response [OK]

Hint: Valid Bearer token returns 200 OK [OK]

Common Mistakes:

• Confusing 401 and 403 status codes
• Ignoring token prefix check
• Assuming server stores session

4. Identify the bug in this stateless authentication filter code snippet:

public boolean isValidToken(String token) {
    if (token == null || token.isEmpty()) {
        return false;
    }
    // Token validation logic
    return token.equals("valid-token");
}

public void doFilter(HttpServletRequest req, HttpServletResponse res) {
    String auth = req.getHeader("Authorization");
    if (auth != null && auth.startsWith("Bearer ")) {
        String token = auth.substring(7);
        if (!isValidToken(token)) {
            res.setStatus(401);
        }
    }
    // Continue filter chain
}

What is the main issue?

medium

A. Possible NullPointerException if Authorization header is missing

B. Token validation logic is incorrect

C. Response status code should be 403 instead of 401

D. Filter does not extract token correctly

Solution

1. Step 1: Analyze header usage

   The original code calls auth.startsWith("Bearer ") without checking if auth is null.

2. Step 2: Identify risk

   If Authorization header is missing, auth is null, so calling startsWith causes NullPointerException.

3. Final Answer:

   Possible NullPointerException if Authorization header is missing -> Option A

4. Quick Check:

   Check null before startsWith to avoid error [OK]

Hint: Check for null before calling startsWith [OK]

Common Mistakes:

• Assuming header always present
• Mixing 401 and 403 status codes
• Ignoring null safety in Java

5. In a stateless Spring Boot app using JWT tokens, which approach best supports scaling across multiple servers?

hard

A. Store user sessions in a shared database accessed by all servers.

B. Cache user sessions in server memory for faster access.

C. Use sticky sessions to keep users on the same server.

D. Validate JWT tokens on each request without server session storage.

Solution

1. Step 1: Understand stateless scaling needs

   Scaling means any server can handle any request without shared session state.

2. Step 2: Evaluate options

   Storing sessions in DB or memory adds state and complexity; sticky sessions tie users to one server, limiting scaling.

3. Step 3: Identify best stateless method

   Validating JWT tokens on each request keeps servers stateless and allows easy scaling.

4. Final Answer:

   Validate JWT tokens on each request without server session storage. -> Option D

5. Quick Check:

   Stateless + JWT = validate token each request [OK]

Hint: Stateless scaling means no server session, validate tokens each time [OK]

Common Mistakes:

• Using sticky sessions limits scaling
• Storing sessions breaks statelessness
• Caching sessions in memory causes sync issues