Stateless JWT Authentication with Spring Security

Business Scenario

In this lab, we will enhance the Medicare backend by replacing HTTP Basic Authentication with stateless JWT (JSON Web Token) authentication.

Pre-Lab Preparation

Phase 9 — Stateless JWT Authentication

Module:

1) Mastering Springboot Security

Now we replace HTTP Basic Authentication with the authentication approach our React frontend will eventually use:

Register / Login
       ↓
JWT generated

       ↓
Frontend stores JWT
       ↓
Every protected request sends:

Authorization: Bearer <token>
       ↓
JWT Filter
       ↓
Token validated
       ↓
SecurityContextHolder
       ↓
Controller

What changes from Phase 8 ?

Every Request
     ↓
Email + Password
     ↓
Basic Authentication

Phase 8

Phase 9:

LOGIN ONCE
Email + Password
     ↓
    JWT

Then:

Every Request
     ↓
JWT only

The underlying concepts from Phase 8 remain:

AuthenticationManager
UserDetailsService
PasswordEncoder
DaoAuthenticationProvider
SecurityContextHolder

We are not replacing Spring Security.

We're only changing how authentication is carried between requests.

Phase 9 Target APIs

We will introduce:

POST /api/auth/register
POST /api/auth/login

Then protected APIs use:

Authorization: Bearer <JWT>

Important Security Decision

Public registration will create only:

{
    "role": "PATIENT"
}

The frontend will not send:

{
    "role": "DOCTOR"
}

Otherwise someone could simply register themselves as a doctor.

/api/auth/register
        ↓
Always PATIENT

Add JWT Dependencies

<!-- JWT API -->
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-api</artifactId>
    <version>0.13.0</version>
</dependency>

<!-- JWT implementation -->
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-impl</artifactId>
    <version>0.13.0</version>
    <scope>runtime</scope>
</dependency>

<!-- JWT JSON processing -->
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-gson</artifactId>
    <version>0.13.0</version>
    <scope>runtime</scope>
</dependency>

<!-- JWT JSON processing -->
<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt-gson</artifactId>
    <version>0.13.0</version>
    <scope>runtime</scope>
</dependency>

Then: Maven → Update Project

Understand JWT Structure

A JWT looks like:

xxxxx.yyyyy.zzzzz

Conceptually:

HEADER.PAYLOAD.SIGNATURE

Example payload:

{
    "sub": "rahul@example.com",
    "iat": 1789970000,
    "exp": 1790056400
}

We're keeping the token minimal.

For now: subject = user's email

JWT Secret

A signed JWT needs a cryptographic signing key.

Do not write something like:

jwt.secret=mysecret123

JJWT specifically recommends using proper cryptographic key material rather than treating an arbitrary password string as an HMAC key.

It provides secure key generation and Base64 encoding for storage.

Use a one-time Java snippet to generate a key:

import javax.crypto.SecretKey;

import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.io.Encoders;

public class JwtSecretGenerator {
    public static void main(String[] args) {

        SecretKey key = Jwts.SIG.HS256.key().build();
        String secret = Encoders.BASE64.encode(key.getEncoded());
        System.out.println(secret);
    }
}

Run once.

You'll receive a long Base64 value.

Do not commit that value to Git.

Configure JWT Properties

Inside: application.properties

add:

# JWT Configuration
jwt.secret=${JWT_SECRET}

# 24 hours in milliseconds
jwt.expiration=86400000

# 24 hours in milliseconds
jwt.expiration=86400000

For local development, set: JWT_SECRET

inside STS/Eclipse:

Run Configurations
→ Environment
→ New

Name: JWT_SECRET
Value: your-generated-base64-key

Now: Secret stays outside source code

Create Authentication DTOs

Create :

dto/
├── RegisterRequest.java
├── LoginRequest.java
└── AuthResponse.java

Create RegisterRequest

package com.medicare.dto;
import jakarta.validation.constraints.Email;

import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.Pattern;
import jakarta.validation.constraints.Size;

public class RegisterRequest {
    @NotBlank(message = "Name is required")
    @Size(min = 3,max = 100,message = "Name must be between 3 and 100 characters")
    private String name;

    @NotBlank(message = "Email is required")
    @Email(message = "Please provide a valid email")
    private String email;

    @NotBlank(message = "Phone number is required")
    @Pattern(regexp = "^[0-9]{10,15}$",
             message = "Phone number must contain 10 to 15 digits"
    )
    private String phone;

    @NotBlank(message = "Password is required")
    @Size(min = 8,max = 100,message = "Password must be between 8 and 100 characters")
    private String password;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

Create LoginRequest

package com.medicare.dto;
import jakarta.validation.constraints.Email;
import jakarta.validation.constraints.NotBlank;

public class LoginRequest {

    @NotBlank(message = "Email is required")
    @Email(message = "Please provide a valid email")
    private String email;

    @NotBlank(message = "Password is required")
    private String password;

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

Create AuthResponse

After successful login, React needs:

JWT
User ID
Name
Email
Role

package com.medicare.dto;
import com.medicare.entity.Role;


public class AuthResponse {

    private String token;
    private String tokenType;

    private Long userId;
    private String name;
    private String email;
    private Role role;

    public AuthResponse(
            String token,
            Long userId,
            String name,
            String email,
            Role role) {

        this.token = token;
        this.tokenType = "Bearer";
        this.userId = userId;
        this.name = name;
        this.email = email;
        this.role = role;
    }

    public String getToken() {
        return token;
    }

    public String getTokenType() {
        return tokenType;
    }

    public Long getUserId() {
        return userId;
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public Role getRole() {
        return role;
    }
}

Update UserRepository

Use case-insensitive email checks consistently.

package com.medicare.repository;
import java.util.Optional;
import org.springframework.data.jpa.repository.JpaRepository;
import com.medicare.entity.User;

public interface UserRepository
        extends JpaRepository<User, Long> {

    boolean existsByEmailIgnoreCase(String email);

    Optional<User> findByEmailIgnoreCase(String email);
}

Create EmailAlreadyExistsException

exception/
└── EmailAlreadyExistsException.java

Create :

package com.medicare.exception;

public class EmailAlreadyExistsExceptionextends RuntimeException {

    public EmailAlreadyExistsException(String message) {

        super(message);
    }
}

Create JwtService

Create :

security/
└── JwtService.java

package com.medicare.security;
import java.util.Date;
import javax.crypto.SecretKey;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.stereotype.Service;
import io.jsonwebtoken.Claims;
import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.io.Decoders;
import io.jsonwebtoken.security.Keys;

This class handles:

Generate JWT
Read JWT
Extract Username
Check Expiry
Validate JWT

@Service
public class JwtService {

    @Value("${jwt.secret}")
    private String jwtSecret;

    @Value("${jwt.expiration}")
    private long jwtExpiration;

    public String generateToken(v  UserDetails userDetails) {

        long currentTime = System.currentTimeMillis();

        return Jwts.builder().subject(userDetails.getUsername())
                .issuedAt(new Date(currentTime))
                .expiration(new Date(currentTime+ jwtExpiration))
                .signWith(getSigningKey()).compact();
    }

    public String extractUsername(String token) {
        return extractAllClaims(token).getSubject();
    }

    public boolean isTokenValid(String token,UserDetails userDetails) {

        String username = extractUsername(token);

        return username.equals(userDetails.getUsername())
                && !isTokenExpired(token);

    }

    private boolean isTokenExpired(String token) {

        Date expiration = extractAllClaims(token).getExpiration();

        return expiration.before(new Date());
    }

    private Claims extractAllClaims(String token) {
        return Jwts.parser().verifyWith(getSigningKey())
                .build().parseSignedClaims(token).getPayload();
    }

    private SecretKey getSigningKey() {

        byte[] keyBytes = Decoders.BASE64.decode(jwtSecret);

        return Keys.hmacShaKeyFor(keyBytes);
    }
}

JJWT's current API recommends signWith(key) for signing and verifyWith(secretKey) plus parseSignedClaims(...) for reading signed claim JWTs.

Understand Token Generation

This:

.subject(userDetails.getUsername())

means: JWT subject = user email

For Rahul, the JWT subject is set to rahul@example.com, which identifies the user associated with the token.

The .issuedAt(...) method records the date and time when the token was created.

The .expiration(...) method specifies when the token becomes invalid and can no longer be used for authentication.

The .signWith(getSigningKey()) method digitally signs the token using a secret key to protect its integrity

If someone modifies the email in the token from rahul@example.com to admin@example.com, the signature verification fails because the token has been tampered with.

Create AuthService

service/
└── AuthService.java

package com.medicare.service;
import org.springframework.security.authentication.AuthenticationManager;
import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.stereotype.Service;
import com.medicare.dto.AuthResponse;
import com.medicare.dto.LoginRequest;
import com.medicare.dto.RegisterRequest;
import com.medicare.dto.UserResponse;
import com.medicare.entity.Role;
import com.medicare.entity.User;
import com.medicare.exception.EmailAlreadyExistsException;
import com.medicare.exception.UserNotFoundException;
import com.medicare.repository.UserRepository;

service/
└── AuthService.java

import com.medicare.security.JwtService;
@Service
public class AuthService {

    private final UserRepository userRepository;
    private final PasswordEncoder passwordEncoder;
    private final AuthenticationManager authenticationManager;
    private final JwtService jwtService;

    public AuthService(UserRepository userRepository,PasswordEncoder passwordEncoder,
            AuthenticationManager authenticationManager,JwtService jwtService) {

        this.userRepository = userRepository;
        this.passwordEncoder = passwordEncoder;
        this.authenticationManager = authenticationManager;
        this.jwtService = jwtService;
    }

    public UserResponse register(RegisterRequest request) {

        String email = request.getEmail().trim().toLowerCase();

        if (userRepository.existsByEmailIgnoreCase(email)) {
            throw new EmailAlreadyExistsException("Email is already registered");
        }

        User user = new User();

        user.setName(request.getName().trim());

service/
└── AuthService.java

        user.setEmail(email);

        user.setPhone(request.getPhone());

        user.setPassword(passwordEncoder.encode(request.getPassword()));

        /*
         * Public registration always creates
         * a PATIENT account.
         */
        user.setRole(Role.PATIENT);
        User savedUser = userRepository.save(user);

        return mapToUserResponse(savedUser);
    }

    public AuthResponse login(LoginRequest request) {

        String email = request.getEmail().trim().toLowerCase();

        /*
         * AuthenticationManager checks
         * email and password.
         */
        var authentication = authenticationManager.authenticate(
                new UsernamePasswordAuthenticationToken(email,
                       request.getPassword()));

        UserDetails userDetails = (UserDetails)authentication.getPrincipal();

service/
└── AuthService.java

        User user = userRepository.findByEmailIgnoreCase(userDetails.getUsername())
                        .orElseThrow(() -> new UserNotFoundException(
                                        "User not found"));

        String token = jwtService.generateToken(userDetails);

        return new AuthResponse(
                token,user.getId(),user.getName(),user.getEmail(),user.getRole()
        );
    }

    private UserResponse mapToUserResponse(User user) {

        return new UserResponse(
                user.getId(),
                user.getName(),
                user.getEmail(),
                user.getPhone(),
                user.getRole()
        );
    }
}

Create AuthController

Create

service/
└── AuthService.java

package com.medicare.controller;
import org.springframework.http.H ttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import com.medicare.dto.AuthResponse;
import com.medicare.dto.LoginRequest;
import com.medicare.dto.RegisterRequest;
import com.medicare.dto.UserResponse;
import com.medicare.service.AuthService;

import jakarta.validation.Valid;

@RestController
@RequestMapping("/api/auth")
public class AuthController {

    private final AuthService authService;

controller/
└── AuthController.java

service/
└── AuthService.java

    public AuthController(AuthService authService) {
        this.authService = authService;
    }

    @PostMapping("/register")
    public ResponseEntity<UserResponse>
            register(@Valid @RequestBody RegisterRequest request) {

        UserResponse response = authService.register(request);
        return ResponseEntity.status(HttpStatus.CREATED).body(response);
    }

    @PostMapping("/login")
    public ResponseEntity<AuthResponse>
            login( @Valid @RequestBody LoginRequest request) {
        return ResponseEntity.ok(authService.login(request)
        );
    }
}

Remove Public User Creation

Previously we temporarily had: POST /api/users for Phase 8.

Now remove the POST method from UserController

Registration now happens only through: POST /api/auth/register

Also remove UserRequest.java if it is no longer used.

Create JWT Authentication Filter

Create

security/
└── JwtAuthenticationFilter.java

package com.medicare.security;
import java.io.IOException;
import org.springframework.http.HttpHeaders;
import org.springframework.security.authentication.
UsernamePasswordAuthenticationToken;
import org.springframework.security.core.context.SecurityContextHolder;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.web.authentication.
WebAuthenticationDetailsSource;
import org.springframework.stereotype.Component;
import org.springframework.web.filter.OncePerRequestFilter;
import io.jsonwebtoken.JwtException;
import jakarta.servlet.FilterChain;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

Create

security/
└── JwtAuthenticationFilter.java

@Component
public class JwtAuthenticationFilter extends OncePerRequestFilter {

    private final JwtService jwtService;

    private final CustomUserDetailsService userDetailsService;

    public JwtAuthenticationFilter( JwtService jwtService,
          CustomUserDetailsService userDetailsService) {

        this.jwtService = jwtService;

        this.userDetailsService = userDetailsService;
    }

    @Override
    protected void doFilterInternal( HttpServletRequest request, 
         HttpServletResponse response,FilterChain filterChain)
            throws ServletException, IOException {

        String authHeader = request.getHeader( HttpHeaders.AUTHORIZATION);
        /*
         * No Bearer token?
         * Continue the filter chain.
         */
        if (authHeader == null || !authHeader.startsWith("Bearer ")) {
            filterChain.doFilter(request,response);
            return;
        }

Create

security/
└── JwtAuthenticationFilter.java

        String token = authHeader.substring(7);

        try {
            String username = jwtService.extractUsername(token);

            if (username != null && SecurityContextHolder
                            .getContext().getAuthentication() == null) {

                UserDetails userDetails = userDetailsService
                     .loadUserByUsername(username);

                if (jwtService.isTokenValid(token,userDetails)) {

                    UsernamePasswordAuthenticationToken
                            authenticationToken = 
                                new UsernamePasswordAuthenticationToken(
                                    userDetails,null,userDetails
                                            .getAuthorities()
                            );

                    authenticationToken.setDetails(
                            new WebAuthenticationDetailsSource()
                                    .buildDetails(request)
                    );

                    SecurityContextHolder.getContext()
                            .setAuthentication(authenticationToken);
                }
            }

        } catch ( JwtException | IllegalArgumentException exception) {
            /*
             * Invalid / expired / malformed token.
             * Leave the user unauthenticated.
             */
            SecurityContextHolder.clearContext();
        }
        filterChain.doFilter(request,response);
    }
}

Spring Security supports custom filters through addFilterBefore, addFilterAfter, or addFilterAt, and OncePerRequestFilter is specifically designed for filters that should run once per request.

Understand Bearer

Request: Authorization: Bearer eyJhbGciOi...

The filter checks: authHeader.startsWith("Bearer ")

Then: authHeader.substring(7) removes: Bearer leaving only: eyJhbGciOi...

Filter Logic

Request
   ↓
Authorization header exists?
   │
   ├── NO → continue
   │
   └── YES
        ↓
Starts with Bearer?
        ↓
Extract JWT
        ↓

Extract Email
        ↓
Load UserDetails
        ↓
Validate Signature
        ↓
Validate Expiration
        ↓
Valid?
   ┌────┴────┐
   NO       YES
   │         │
   ▼         ▼
No Auth   Authentication object
             ↓
      SecurityContextHolder

Update SecurityConfig

package com.medicare.config;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.HttpStatus;
import org.springframework.security.authentication.AuthenticationManager;

Now remove: .httpBasic(...) -> We don't want HTTP Basic anymore.

Add:

STATELESS session management
JWT filter
Public auth endpoints

Final relevant configuration:

import org.springframework.security.authentication.AuthenticationProvider;
import org.springframework.security.authentication.dao.DaoAuthenticationProvider;
import org.springframework.security.config.annotation.authentication.configuration.
AuthenticationConfiguration;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.http.SessionCreationPolicy;
import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.security.web.SecurityFilterChain;
import org.springframework.security.web.authentication.HttpStatusEntryPoint;
import org.springframework.security.web.authentication.
UsernamePasswordAuthenticationFilter;
import com.medicare.security.CustomUserDetailsService;
import com.medicare.security.JwtAuthenticationFilter;

@Configuration
public class SecurityConfig {
    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder();
    }

    @Bean
    public AuthenticationProvider authenticationProvider(
            CustomUserDetailsService userDetailsService,
            PasswordEncoder passwordEncoder) {

        DaoAuthenticationProvider provider =new DaoAuthenticationProvider(
                        userDetailsService );

        provider.setPasswordEncoder(passwordEncoder);

        return provider;
    }

    @Bean
    public AuthenticationManager authenticationManager(
            AuthenticationConfiguration configuration)
            throws Exception {

        return configuration.getAuthenticationManager();
    }

    @Bean
    public SecurityFilterChain securityFilterChain(
            HttpSecurity http,
            AuthenticationProvider authenticationProvider,
            JwtAuthenticationFilter jwtAuthenticationFilter)
            throws Exception {
        http.csrf(csrf ->csrf.disable())

                /*
                 * JWT authentication is stateless.
                 * Spring Security should not create
                 * an authentication session.
                 */
                .sessionManagement(session ->session.sessionCreationPolicy(
                                SessionCreationPolicy.STATELESS)
                )

                .authenticationProvider(authenticationProvider)

                .authorizeHttpRequests(auth ->auth

                                .requestMatchers("/api/health").permitAll()

                          .requestMatchers("/api/auth/register","/api/auth/login")
                          .permitAll().anyRequest().authenticated()
                )

                /*
                 * Return 401 when authentication
                 * is missing or invalid.
                 */
                .exceptionHandling(exception -> exception.authenticationEntryPoint(
                               new HttpStatusEntryPoint(HttpStatus.UNAUTHORIZED)
                                )
                )
                /*
                 * JWT must be checked before
                 * username/password authentication filter.
                 */
                .addFilterBefore(jwtAuthenticationFilter,
                 UsernamePasswordAuthenticationFilter.class
                );
        return http.build();
    }
}

Filter Logic

Traditional session:

Login
 ↓
Server creates Session
 ↓
Server remembers User

JWT:

Login
 ↓
Server creates JWT
 ↓
Client holds JWT
 ↓
Server validates JWT on every request

So: Server-side login session

That is why: SessionCreationPolicy.STATELESS is important.

Update Global Exception Handler

@ExceptionHandler(EmailAlreadyExistsException.class)

public ResponseEntity<ErrorResponse> handleEmailAlreadyExists(
                EmailAlreadyExistsException exception,
                HttpServletRequest request) {

    ErrorResponse errorResponse = new ErrorResponse(
                    LocalDateTime.now(),
                    HttpStatus.CONFLICT.value(),
                    exception.getMessage(),
                    request.getRequestURI()
            );

    return ResponseEntity
            .status(HttpStatus.CONFLICT).body(errorResponse);
}

Handle duplicate email.

Handle Wrong Login Credentials

@ExceptionHandler( BadCredentialsException.class)
public ResponseEntity<ErrorResponse>
        handleBadCredentials(
                BadCredentialsException exception,
                HttpServletRequest request) {

    ErrorResponse errorResponse = new ErrorResponse(
                    LocalDateTime.now(),
                    HttpStatus.UNAUTHORIZED.value(),
                    "Invalid email or password",
                    request.getRequestURI()
            );

    return ResponseEntity
            .status(HttpStatus.UNAUTHORIZED).body(errorResponse);
}

AuthenticationManager will throw an authentication exception when credentials are wrong.

Now: Wrong login → 401 rather than some uncontrolled server error.

Test Registration

POST http://localhost:8080/api/auth/register

Body :

{
    "name": "Rahul Shah",
    "email": "rahul@example.com",
    "phone": "9876543210",
    "password": "Rahul@123"
}

Notice: role is not supplied.

Expected: 201 CREATED

Response:

{
    "id": 1,
    "name": "Rahul Shah",
    "email": "rahul@example.com",
    "phone": "9876543210",
    "role": "PATIENT"
}

Public users cannot choose their own role anymore.

Check Password in Database

Run :

SELECT id, email, password, role FROM users;

Expected:

email: rahul@example.com

password: $2a$10$....

role: PATIENT

Password must still be BCrypt encoded.

Duplicate Registration

Expected: 409 CONFLICT

{
    "timestamp": "...",
    "status": 409,
    "message": "Email is already registered",
    "path": "/api/auth/register"
}

Send the same email again.

Expected: 409 CONFLICT

Test Login

POST http://localhost:8080/api/auth/login

{
    "email": "rahul@example.com",
    "password": "Rahul@123"
}

Body:

Expected: 200 OK

Test Wrong password

{
    "email": "rahul@example.com",
    "password": "WrongPassword"
}

{
    "status": 401,
    "message": "Invalid email or password"
}

Expected: 401 UNAUTHORIZED

Protected API Without JWT

GET http://localhost:8080/api/doctors

No Authorization header.

Expected: 401 UNAUTHORIZED

Protected API With JWT

Authorization
→ Bearer Token

Paste JWT.

Then:

Postman :

GET http://localhost:8080/api/doctors

Expected: 200 OK

Modify the Token

For example: eyJ...

Change one character in the JWT.

to: eyX...

Then request:

GET /api/doctors

Expected: 401 UNAUTHORIZED

Because: Signature verification fails

Test Expiration

Normally: jwt.expiration=86400000 means about 24 hours.

For testing only, temporarily change to : jwt.expiration=60000

Request protected API.

Expected: 401 UNAUTHORIZED

Then return: jwt.expiration=86400000

We are done with this lab. The latest source code has been uploaded to GitHub. You can access the latest commit using the link below:

Github_repository_link

Great job!

You have successfully implemented stateless JWT authentication in the Medicare backend.

Checkpoint

Git Push

git push origin branchName

Next-Lab Preparation

Module:

1) Mastering Springboot Security

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