Authentication with JWT token in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this JWT token validation method?

Consider this Spring Boot method that validates a JWT token. What will be the returned value if the token is expired?

Spring Boot

public boolean validateToken(String token) {
    try {
        Jwts.parserBuilder().setSigningKey(secretKey).build().parseClaimsJws(token);
        return true;
    } catch (ExpiredJwtException e) {
        return false;
    } catch (JwtException e) {
        return false;
    }
}

A false

B Throws ExpiredJwtException

C true

D null

💡 Hint

Think about what happens when the token is expired and how exceptions are handled.

The method catches ExpiredJwtException and returns false when the token is expired. So the output is false.

📝 Syntax

intermediate

Which option correctly creates a JWT token with a subject and expiration?

Select the code snippet that correctly creates a JWT token with subject "user123" and expiration 1 hour from now using jjwt library.

A String token = Jwts.builder().setSubject("user123").setExpiration(new Date(System.currentTimeMillis() + 3600000)).signWith(key).compact();

B String token = Jwts.builder().setSubject("user123").setExpiration(System.currentTimeMillis() + 3600000).signWith(key).compact();

C String token = Jwts.builder().setSubject("user123").setExpiration(new Date().getTime() + 3600000).signWith(key).compact();

D String token = Jwts.builder().setSubject("user123").setExpiration(new Date(System.currentTimeMillis() - 3600000)).signWith(key).compact();

💡 Hint

Expiration requires a Date object representing a future time.

Option A correctly sets expiration as a Date object 1 hour ahead. Option A passes a long instead of Date, causing error. Option A adds millis to a long, not Date. Option A sets expiration in the past.

🔧 Debug

advanced

Why does this JWT token parsing code throw a SignatureException?

Given this code snippet, why does parsing the JWT token throw a SignatureException?

Spring Boot

Jwts.parserBuilder().setSigningKey(wrongKey).build().parseClaimsJws(token);

A The token is null or empty.

B The token is expired and cannot be parsed.

C The token format is invalid and missing parts.

D The signing key used to parse the token does not match the key used to sign it.

💡 Hint

Think about what the signature in JWT ensures.

SignatureException occurs when the signature does not match. This happens if the key used to parse is different from the signing key.

❓ state_output

advanced

What is the value of 'username' after extracting from this JWT token?

Assuming the JWT token contains a claim 'sub' with value 'alice', what will be the value of 'username' after this code runs?

Spring Boot

Claims claims = Jwts.parserBuilder().setSigningKey(secretKey).build().parseClaimsJws(token).getBody();
String username = claims.getSubject();

A null

B "alice"

C Throws NullPointerException

D "sub"

💡 Hint

The 'sub' claim is accessed by getSubject() method.

The getSubject() method returns the value of the 'sub' claim, which is 'alice'.

🧠 Conceptual

expert

Which statement best explains why JWT tokens are stateless in authentication?

Why are JWT tokens considered stateless in authentication systems?

A Because the server stores all tokens in a database to track user sessions.

B Because JWT tokens expire immediately after creation, forcing re-authentication.

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

D Because JWT tokens require a server to validate each token against a session store.

💡 Hint

Think about where the user data lives in JWT authentication.

JWT tokens carry user data and claims inside themselves, so servers do not need to keep session state, making them stateless.