Stateless authentication mental model in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'stateless' mean in stateless authentication?

It means the server does not keep any information about the user's session between requests. Each request must carry all needed info to verify the user.

beginner

How does a server verify a user in stateless authentication?

The server checks a token sent with each request, like a JWT, to confirm the user's identity without storing session data.

intermediate

Why is stateless authentication good for scaling web apps?

Because servers don't store session info, any server can handle requests. This makes it easier to add more servers and balance load.

beginner

What is a common token format used in stateless authentication?

JSON Web Token (JWT) is commonly used. It contains user info and a signature to verify it hasn't been changed.

intermediate

What is a key difference between stateful and stateless authentication?

Stateful authentication stores session info on the server, while stateless authentication keeps no session data on the server and relies on tokens sent each time.

In stateless authentication, where is the user's session data stored?

A In the server's memory

B In a token sent with each request

C In a database session table

D In browser cookies only

Stateless authentication uses tokens (like JWT) sent with each request to carry session data, so the server does not store it.

Which of these is a benefit of stateless authentication?

A Easier to scale servers horizontally

B Server stores all user sessions

C Requires sticky sessions for load balancers

D Slower request processing

Because no session is stored on the server, any server can handle requests, making horizontal scaling easier.

What does JWT stand for?

A Java Web Token

B JavaScript Web Token

C JSON Web Token

D Joint Web Token

JWT stands for JSON Web Token, a compact token format used in stateless authentication.

In stateless authentication, what must each client request include?

A Authentication token

B Session ID stored on server

C User's password

D Server IP address

Each request must include an authentication token so the server can verify the user without stored session data.

Which is NOT true about stateless authentication?

A Each request is self-contained

B Uses tokens like JWT

C Improves scalability

D Server stores user session data

Stateless authentication does NOT store session data on the server; it relies on tokens sent with each request.

Explain how stateless authentication works in a web application.

Think about how the server knows who you are without remembering past requests.

Describe the advantages of using stateless authentication over stateful authentication.

Consider what happens when you add more servers to handle users.

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