JWT generation in Spring Boot - Step-by-Step Execution

Concept Flow - JWT generation

Start JWT Generation

↓

Create JWT Header

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Create JWT Payload

↓

Sign JWT with Secret Key

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Combine Header, Payload, Signature

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Return JWT Token

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End

This flow shows how a JWT token is created step-by-step: header and payload are prepared, then signed with a secret key, and finally combined into a token string.

Execution Sample

Spring Boot

String jwt = Jwts.builder()
  .setSubject("user123")
  .setIssuedAt(new Date())
  .setExpiration(new Date(System.currentTimeMillis() + 3600000))
  .signWith(key, SignatureAlgorithm.HS256)
  .compact();

This code builds a JWT token with a subject, issue time, expiration, signs it, and returns the compact token string.

Execution Table

Step	Action	Data Created/Evaluated	Result
1	Create JWT Header	{"alg":"HS256","typ":"JWT"}	Header JSON object
2	Create JWT Payload	{"sub":"user123","iat":<now>,"exp":<now+1h>}	Payload JSON object
3	Sign JWT	Header + Payload + Secret Key	Signature string
4	Combine parts	Base64Url(Header).Base64Url(Payload).Signature	JWT token string
5	Return JWT	JWT token string	Token ready for client use
6	End	Process complete	JWT generation finished

💡 JWT token is fully generated and returned after combining header, payload, and signature.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
header	null	{"alg":"HS256","typ":"JWT"}	{"alg":"HS256","typ":"JWT"}	{"alg":"HS256","typ":"JWT"}	{"alg":"HS256","typ":"JWT"}	{"alg":"HS256","typ":"JWT"}
payload	null	null	{"sub":"user123","iat":<now>,"exp":<now+1h>}	{"sub":"user123","iat":<now>,"exp":<now+1h>}	{"sub":"user123","iat":<now>,"exp":<now+1h>}	{"sub":"user123","iat":<now>,"exp":<now+1h>}
signature	null	null	null	signatureString	signatureString	signatureString
jwtToken	null	null	null	null	header.payload.signature	header.payload.signature

Key Moments - 3 Insights

Why do we need to sign the JWT after creating header and payload?

Signing ensures the token is secure and cannot be tampered with. The signature is created from header and payload plus a secret key, as shown in step 3 of the execution_table.

What does the 'compact()' method do in JWT generation?

The 'compact()' method combines the Base64Url encoded header, payload, and signature into a single string token, as shown in step 4 of the execution_table.

Why do we set an expiration time in the payload?

Expiration limits how long the token is valid for security reasons. The payload includes 'exp' field with a timestamp, as seen in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is created at step 2?

A JWT Header JSON object

B JWT Payload JSON object

C JWT Signature string

D Final JWT token string

💡 Hint

Check the 'Data Created/Evaluated' column at step 2 in the execution_table.

At which step is the JWT token string formed?

A Step 4

B Step 1

C Step 3

D Step 5

💡 Hint

Look for the step where header, payload, and signature are combined into one string in the execution_table.

If the secret key changes, which step's output will be different?

A Step 1 - Header creation

B Step 2 - Payload creation

C Step 3 - Signing

D Step 4 - Combining parts

💡 Hint

Signing uses the secret key, so check the 'Sign JWT' step in the execution_table.

Concept Snapshot

JWT generation in Spring Boot:
- Build header and payload JSON objects
- Set claims like subject, issuedAt, expiration
- Sign with secret key using signWith()
- Use compact() to get token string
- Token = header.payload.signature
- Token is secure and time-limited

Full Transcript

JWT generation in Spring Boot involves creating a header and payload with claims like subject and expiration. Then the token is signed with a secret key to ensure security. Finally, the parts are combined into a compact string token that can be sent to clients. This process ensures the token is valid and tamper-proof for a limited time.