JWT generation in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this JWT generation snippet?

Consider this Spring Boot code that generates a JWT token. What will be the value of token after execution?

Spring Boot

String token = Jwts.builder()
  .setSubject("user123")
  .setIssuedAt(new Date())
  .setExpiration(new Date(System.currentTimeMillis() + 60000))
  .signWith(Keys.hmacShaKeyFor("mysecretkeymysecretkeymysecretkey12".getBytes()), SignatureAlgorithm.HS256)
  .compact();

A A valid JWT string with header, payload containing subject 'user123', and signature

B A plain string 'user123' without any JWT structure

C Throws a runtime exception due to invalid key length

D An empty string because expiration is set in the past

💡 Hint

Look at the signWith method and the key length used.

The code creates a JWT token with subject 'user123', sets issued and expiration times, and signs it with a valid HMAC SHA-256 key. The result is a valid JWT string.

📝 Syntax

intermediate

Which option correctly fixes the syntax error in this JWT builder code?

Identify the correct fix for the syntax error in this JWT generation snippet:

Jwts.builder()
  .setSubject("user")
  .signWith(Keys.hmacShaKeyFor("secretkeysecretkeysecretkey12".getBytes())
  .compact();

A Remove the .compact() call at the end

B Add a closing parenthesis after getBytes(): .signWith(Keys.hmacShaKeyFor("secretkeysecretkeysecretkey12".getBytes()))

C Change .setSubject("user") to .setSubject(user)

D Replace Keys.hmacShaKeyFor with new SecretKeySpec

💡 Hint

Count the parentheses carefully.

The method call to hmacShaKeyFor is missing a closing parenthesis, causing a syntax error. Adding it fixes the code.

🔧 Debug

advanced

Why does this JWT generation code throw an InvalidKeyException?

Examine this code snippet:

byte[] keyBytes = "shortkey".getBytes();
SecretKey key = Keys.hmacShaKeyFor(keyBytes);
String token = Jwts.builder()
  .setSubject("admin")
  .signWith(key, SignatureAlgorithm.HS256)
  .compact();

Why does it throw an InvalidKeyException?

A The key length is too short for HS256 algorithm

B The subject 'admin' is not allowed in JWT

C The SignatureAlgorithm HS256 is deprecated

D The getBytes() method returns null

💡 Hint

Check the required key length for HMAC SHA-256.

HS256 requires a key of at least 256 bits (32 bytes). The provided key is shorter, causing the exception.

❓ state_output

advanced

What is the expiration time of the generated JWT token?

Given this code snippet:

long now = System.currentTimeMillis();
String token = Jwts.builder()
  .setSubject("user")
  .setIssuedAt(new Date(now))
  .setExpiration(new Date(now + 300000))
  .signWith(Keys.hmacShaKeyFor("mysecretkeymysecretkeymysecretkey12".getBytes()), SignatureAlgorithm.HS256)
  .compact();

How long is the token valid after issuance?

A 30 seconds

B 1 hour

C 5 minutes

D No expiration set

💡 Hint

Look at the value added to now for expiration.

The expiration is set to 300000 milliseconds (5 minutes) after the issued time.

🧠 Conceptual

expert

Which option best explains why JWT tokens are stateless in Spring Boot applications?

Why are JWT tokens considered stateless when used for authentication in Spring Boot?

A Because JWT tokens require server memory to track each token's state

B Because Spring Boot stores JWT tokens in a centralized database

C Because JWT tokens expire immediately after creation

D Because all user information and claims are stored inside the token itself, no server-side session is needed

💡 Hint

Think about where the user data lives when using JWT.

JWT tokens carry all necessary user data and claims inside the token, so the server does not need to keep session state, making them stateless.