JWT generation in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does JWT stand for and what is its main purpose?

JWT stands for JSON Web Token. It is used to securely transmit information between parties as a JSON object, often for authentication and authorization.

beginner

In Spring Boot, which library is commonly used to create and verify JWT tokens?

The 'jjwt' library (io.jsonwebtoken) is commonly used in Spring Boot to create and verify JWT tokens easily.

beginner

What are the three parts of a JWT token?

A JWT token has three parts separated by dots: Header (token type and algorithm), Payload (claims or data), and Signature (to verify token integrity).

intermediate

Which Spring Boot component typically holds the secret key used for signing JWT tokens?

The secret key is usually stored in application.properties or environment variables and injected into the JWT utility class for signing tokens.

beginner

What is the purpose of the 'claims' in JWT generation?

Claims are pieces of information (like user ID, roles, expiration) stored in the JWT payload to convey user identity and permissions.

Which part of the JWT contains the user's data like username or roles?

A Header

B Payload

C Signature

D Secret key

The Payload part of the JWT contains the claims, which hold user data like username or roles.

In Spring Boot, which method is commonly used to sign a JWT token?

A encode()

B encrypt()

C hash()

D signWith()

The signWith() method from the jjwt library is used to sign the JWT token with a secret key.

What does the JWT signature ensure?

A The token is valid and untampered

B The token is encrypted

C The token expires

D The token contains user roles

The signature ensures the token has not been altered and is valid.

Where is the secret key for JWT signing usually stored in a Spring Boot app?

A In application.properties or environment variables

B In the JWT header

C In the JWT payload

D Hardcoded in the controller

The secret key is stored securely in application.properties or environment variables, not in the token itself.

Which algorithm is commonly used to sign JWT tokens in Spring Boot?

A AES

B MD5

C HS256

D SHA1

HS256 (HMAC with SHA-256) is a common algorithm used to sign JWT tokens.

Explain the process of generating a JWT token in a Spring Boot application.

Think about the steps from user info to a signed token string.

Describe the structure of a JWT token and the role of each part.

Remember the token has three parts separated by dots.

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