Authentication with JWT token in Spring Boot - Step-by-Step Execution

Concept Flow - Authentication with JWT token

User sends login request

↓

Server verifies credentials

↓

Generate JWT token

↓

Send JWT token to user

↓

User sends requests with JWT

↓

Server validates JWT token

↓

Allow access

This flow shows how a user logs in, receives a JWT token, and uses it for authenticated requests.

Execution Sample

Spring Boot

POST /login {username, password}
-> Server checks credentials
-> If valid, create JWT token
-> Return token to user
User sends request with Authorization: Bearer <token>
Server validates token and grants access

This code simulates login, token creation, and token validation for authentication.

Execution Table

Step	Action	Input/Condition	Result	Next Step
1	Receive login request	username=alice, password=1234	Credentials checked	2
2	Verify credentials	Are username and password correct?	Yes	3
3	Generate JWT token	User info encoded in token	Token created with expiry	4
4	Send token to user	Token sent in response	User receives token	5
5	User sends request with token	Authorization header with token	Server extracts token	6
6	Validate token	Is token signature valid and not expired?	Yes	7
7	Grant access	Token valid	User allowed to access resource	End
8	If credentials invalid	No	Reject login with error	End
9	If token invalid	No	Reject request with 401 Unauthorized	End

💡 Execution stops when user is granted access or denied due to invalid credentials or token.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
username	null	alice	alice	alice	alice
password	null	1234	1234	1234	1234
credentialsValid	false	true	true	true	true
jwtToken	null	null	tokenString	tokenString	tokenString
tokenValid	false	false	false	true	true

Key Moments - 3 Insights

Why does the server reject access if the token is expired even if it was originally valid?

Because the execution_table row 6 shows token validation checks expiry and signature. Expired tokens fail validation and lead to rejection at step 9.

What happens if the user sends a request without a token?

The server cannot validate the token (step 6), so it treats it as invalid and denies access (step 9). This is implied by the token extraction and validation steps.

Why do we generate a token only after verifying credentials?

As shown in steps 2 and 3, token generation happens only if credentials are valid to ensure only authenticated users get tokens.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'credentialsValid' after step 2?

A true

B false

C null

D undefined

💡 Hint

Check variable_tracker column 'After Step 2' for 'credentialsValid'

At which step does the server send the JWT token to the user?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look at execution_table row describing 'Send token to user'

If the token is invalid, which step in the execution_table shows the server denying access?

A Step 7

B Step 9

C Step 8

D Step 6

💡 Hint

Check the rows where token validation fails and access is denied

Concept Snapshot

Authentication with JWT token in Spring Boot:
- User logs in with credentials
- Server verifies credentials
- If valid, server creates JWT token with user info
- Token sent to user for future requests
- User sends token in Authorization header
- Server validates token signature and expiry
- If valid, access granted; else denied
- Tokens allow stateless, secure authentication

Full Transcript

This visual execution shows how authentication with JWT token works in Spring Boot. First, the user sends a login request with username and password. The server checks if these credentials are correct. If they are, the server generates a JWT token that encodes user information and an expiry time. This token is sent back to the user. Later, when the user sends requests, they include this token in the Authorization header. The server extracts the token and validates its signature and expiry. If the token is valid, the server grants access to the requested resource. If the credentials or token are invalid, the server rejects the request. This process allows secure, stateless authentication without storing session data on the server.