JWT generation in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a JWT token with a subject.

Spring Boot

String token = Jwts.builder().setSubject([1]).compact();

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

A"user123"

Bsubject

C123user

Duser123

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the subject string.

Passing a variable without quotes that is not defined.

The setSubject method requires a string value representing the subject of the token. It must be a string literal or variable containing a string.

2fill in blank

medium

Complete the code to sign the JWT token with a secret key.

Spring Boot

String token = Jwts.builder().signWith([1]).compact();

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

AKeys.secretKeyFor(SignatureAlgorithm.HS256)

BSignatureAlgorithm.HS256

C"secret"

Dnew SecretKey()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the algorithm enum directly instead of a Key.

Using a plain string instead of a Key object.

The signWith method requires a Key object, which can be created using Keys.secretKeyFor(SignatureAlgorithm.HS256) to generate a secure key.

3fill in blank

hard

Fix the error in setting the expiration date for the JWT token.

Spring Boot

Date expiryDate = new Date(System.currentTimeMillis() [1] 3600000);

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

A*

B-

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction which sets expiration in the past.

Using multiplication or division which is incorrect here.

To set the expiration one hour from now, add 3600000 milliseconds (1 hour) to the current time.

4fill in blank

hard

Fill both blanks to add claims and set expiration in the JWT builder.

Spring Boot

String token = Jwts.builder().claim("role", [1]).setExpiration([2]).compact();

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

A"admin"

Bnew Date()

CexpiryDate

D"user"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a Date for expiration.

Passing an undefined variable for claim value.

The claim value should be a string like "admin". The expiration should be a Date object like expiryDate representing the expiration time.

5fill in blank

hard

Fill all three blanks to build a JWT token with subject, claim, and expiration.

Spring Boot

String token = Jwts.builder().setSubject([1]).claim("scope", [2]).setExpiration([3]).compact();

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

A"user42"

B"read:write"

CexpiryDate

D"admin"

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings for expiration instead of Date.

Mixing up claim values.

The subject is a string like "user42". The claim value for "scope" is a string like "read:write". The expiration is a Date object like expiryDate.

Practice

1. What is the main purpose of generating a JWT (JSON Web Token) in a Spring Boot application?

easy

A. To securely identify users without storing session data on the server

B. To store user passwords in the database

C. To create HTML pages dynamically

D. To manage database connections

Solution

1. Step 1: Understand JWT purpose

   JWTs are used to securely identify users by encoding user info and signing it.

2. Step 2: Compare options

   Only To securely identify users without storing session data on the server describes JWT's role in stateless authentication without server sessions.

3. Final Answer:

   To securely identify users without storing session data on the server -> Option A

4. Quick Check:

   JWT purpose = secure user identity without sessions [OK]

Hint: JWTs identify users without server sessions [OK]

Common Mistakes:

• Confusing JWT with session storage
• Thinking JWT stores passwords
• Assuming JWT creates web pages

2. Which of the following code snippets correctly initializes a JWT builder using the jjwt library in Spring Boot?

easy

A. JwtBuilder().setSubject("user").sign(secretKey).build();

B. Jwts.builder().subject("user").sign(secretKey).compact();

C. Jwts.create().subject("user").signWith(secretKey).generate();

D. Jwts.builder().setSubject("user").signWith(secretKey).compact();

Solution

1. Step 1: Recall jjwt syntax

   The correct method chain starts with Jwts.builder(), uses setSubject(), signWith(), then compact().

2. Step 2: Check each option

   Jwts.builder().setSubject("user").signWith(secretKey).compact(); matches the correct method names and order. Others use incorrect method names or chaining.

3. Final Answer:

   Jwts.builder().setSubject("user").signWith(secretKey).compact(); -> Option D

4. Quick Check:

   Correct jjwt builder syntax = Jwts.builder().setSubject("user").signWith(secretKey).compact(); [OK]

Hint: Use Jwts.builder(), setSubject(), signWith(), compact() [OK]

Common Mistakes:

• Using incorrect method names like sign() instead of signWith()
• Missing Jwts.builder() start
• Using create() or build() instead of compact()

3. Given the following code snippet, what will be the output type of the token variable?

String token = Jwts.builder()
  .setSubject("user123")
  .signWith(secretKey)
  .compact();

medium

A. A JSON object representing the token

B. A signed JWT string token

C. A byte array of the token

D. An exception is thrown

Solution

1. Step 1: Understand compact() output

   The compact() method returns the JWT as a compact URL-safe string.

2. Step 2: Analyze code snippet

   The code builds a JWT with subject and signs it, then calls compact(), so token is a String.

3. Final Answer:

   A signed JWT string token -> Option B

4. Quick Check:

   compact() returns String token [OK]

Hint: compact() returns JWT as a string [OK]

Common Mistakes:

• Expecting a JSON object instead of string
• Thinking output is byte array
• Assuming code throws exception without error

4. Identify the error in this JWT generation code snippet:

String token = Jwts.builder()
  .setSubject("user")
  .signWith("mySecretKey")
  .compact();

medium

A. Jwts.builder() is not a valid method

B. setSubject() cannot accept a String

C. signWith() requires a Key object, not a String

D. compact() should be called before signWith()

Solution

1. Step 1: Check signWith() parameter type

   signWith() expects a java.security.Key or SecretKey, not a plain String.

2. Step 2: Verify other methods

   setSubject() accepts String, compact() is correctly called last, and Jwts.builder() is valid.

3. Final Answer:

   signWith() requires a Key object, not a String -> Option C

4. Quick Check:

   signWith() needs Key, not String [OK]

Hint: Use Key object with signWith(), not plain String [OK]

Common Mistakes:

• Passing String directly to signWith()
• Calling compact() too early
• Misunderstanding setSubject() input

5. You want to generate a JWT in Spring Boot that expires in 10 minutes. Which code snippet correctly sets the expiration time using jjwt?

hard

A. Jwts.builder().setSubject("user").setExpiration(new Date(System.currentTimeMillis() + 600000)).signWith(secretKey).compact();

B. Jwts.builder().setSubject("user").setExpiry(600000).signWith(secretKey).compact();

C. Jwts.builder().setSubject("user").setExpiration(600000).signWith(secretKey).compact();

D. Jwts.builder().setSubject("user").setExpiresAt(new Date(600000)).signWith(secretKey).compact();

Solution

1. Step 1: Understand expiration setting in jjwt

   setExpiration() expects a Date object representing the expiration time.

2. Step 2: Calculate expiration time

   Use current time plus 600000 milliseconds (10 minutes) to set expiration correctly.

3. Step 3: Check options

   Only Jwts.builder().setSubject("user").setExpiration(new Date(System.currentTimeMillis() + 600000)).signWith(secretKey).compact(); correctly uses setExpiration() with new Date(System.currentTimeMillis() + 600000).

4. Final Answer:

   Jwts.builder().setSubject("user").setExpiration(new Date(System.currentTimeMillis() + 600000)).signWith(secretKey).compact(); -> Option A

5. Quick Check:

   setExpiration(Date) with currentTime + 10min = Jwts.builder().setSubject("user").setExpiration(new Date(System.currentTimeMillis() + 600000)).signWith(secretKey).compact(); [OK]

Hint: Use setExpiration(new Date(System.currentTimeMillis() + millis)) [OK]

Common Mistakes:

• Using setExpiry() or setExpiresAt() which don't exist
• Passing milliseconds directly instead of Date
• Setting expiration to a fixed past date