Spring Security: JWT from Scratch, thesarfo

Start with the security configuration:

@Configuration
@EnableWebSecurity
public class SecurityConfigurer {

    private final JwtRequestFilter jwtRequestFilter;

    // the below is a hardcoded user
    @Bean
    public UserDetailsService userDetailsService() {
        return username -> new User("foo", new BCryptPasswordEncoder().encode("foo"), new ArrayList<>());
    }

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

    @Bean
    public AuthenticationManager authenticationManagerBean() throws Exception {
        return super.authenticationManagerBean();
    }

    @Bean
    public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
        http
                .authorizeHttpRequests(authz -> authz
                        .requestMatchers("/login").permitAll()
                        .anyRequest().authenticated()
                        .and()
                        .sessionManagement()
                                // we need the session stateless since we're using JWT, but a filter must
                                // intercept requests and validate the token before that — hence
                                // addFilterBefore below, running the JwtRequestFilter ahead of the
                                // default UsernamePasswordAuthenticationFilter
                                .sessionCreationPolicy(SessionCreationPolicy.STATELESS)
                        .addFilterBefore(jwtRequestFilter(), UsernamePasswordAuthenticationFilter.class)
                )
                .httpBasic(Customizer.withDefaults());
        return http.build();
    }
}

Customize the user details service to load a user by username:

@Service
public class MyUserDetailsService implements UserDetailsService
{
    private final PasswordEncoder passwordEncoder;

    public MyUserDetailsService(PasswordEncoder passwordEncoder) {
        this.passwordEncoder = passwordEncoder;
    }

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        return new User("foo", passwordEncoder.encode("foo"), new ArrayList<>());
    }
}

Add the jjwt dependency (a Java implementation similar to jwt.io) and jaxb-api:

<dependency>
    <groupId>io.jsonwebtoken</groupId>
    <artifactId>jjwt</artifactId>
    <version>0.9.1</version>
</dependency>

<dependency>
    <groupId>javax.xml.bind</groupId>
    <artifactId>jaxb-api</artifactId>
    <version>2.3.1</version>
</dependency>

A JwtUtil class abstracts all the JWT-related work — creating a JWT from an Authentication, extracting the username, checking expiry, and validating tokens:

import io.jsonwebtoken.Claims;
import io.jsonwebtoken.Jwts;
import io.jsonwebtoken.SignatureAlgorithm;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.stereotype.Service;

import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;

@Service
public class JwtUtil {

    private String SECRET_KEY = "secret";

    public String extractUsername(String token) {
        return extractClaim(token, Claims::getSubject);
    }

    public Date extractExpiration(String token) {
        return extractClaim(token, Claims::getExpiration);
    }

    public <T> T extractClaim(String token, Function<Claims, T> claimsResolver) {
        final Claims claims = extractAllClaims(token);
        return claimsResolver.apply(claims);
    }

    private Claims extractAllClaims(String token) {
        return Jwts.parser().setSigningKey(SECRET_KEY).parseClaimsJws(token).getBody();
    }

    private Boolean isTokenExpired(String token) {
        return extractExpiration(token).before(new Date());
    }

    public String generateToken(UserDetails userDetails) {
        Map<String, Object> claims = new HashMap<>();
        return createToken(claims, userDetails.getUsername());
    }

    private String createToken(Map<String, Object> claims, String subject) {
        return Jwts
                .builder()
                .setClaims(claims)
                .setSubject(subject)
                .setIssuedAt(new Date(System.currentTimeMillis()))
                .setExpiration(new Date(System.currentTimeMillis() + 1000 * 60 * 60 * 10))
                .signWith(SignatureAlgorithm.HS256, SECRET_KEY)
                .compact();
    }

    public Boolean validateToken(String token, UserDetails userDetails) {
        final String username = extractUsername(token);
        return (username.equals(userDetails.getUsername()) && !isTokenExpired(token));
    }
}

DTOs for the login request/response:

public class AuthenticationRequest {

    private String username;
    private String password;

    public AuthenticationRequest() {
    }

    public AuthenticationRequest(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

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

public class AuthenticationResponse {
    private final String jwt;

    public AuthenticationResponse(String jwt) {
        this.jwt = jwt;
    }
}

A login controller taking username/password as the request body:

private final AuthenticationManager authenticationManager; // authenticates the request body
private final MyUserDetailsService userDetailsService; // loads the user by username

@PostMapping("/login")
public ResponseEntity<?> createAuthenticationToken(@RequestBody AuthenticationRequest authenticationRequest) throws Exception {
    try {
        authenticationManager.authenticate(
                new UsernamePasswordAuthenticationToken(authenticationRequest.getUsername(), authenticationRequest.getPassword())
        );
    } catch (BadCredentialsException e) {
        throw new Exception("Incorrect username or password", e);
    }
    // if authentication succeeds, generate a jwt token
    final UserDetails userDetails = userDetailsService.loadUserByUsername(authenticationRequest.getUsername());
    final String jwt = jwtUtil.generateToken(userDetails);

    return ResponseEntity.ok(new AuthenticationResponse(jwt));
}

A POST to /login with valid credentials returns a JWT you can use to authenticate future requests. But first you need a filter that intercepts requests and validates the token — treating anything with a Bearer prefix as a JWT:

@Component
public class JwtRequestFilter extends OncePerRequestFilter {

    private final MyUserDetailsService userDetailsService;
    private JwtUtil jwtUtil;

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain)
            throws ServletException, IOException {
        // runs once per request, checking the Authorization header
        final String authorizationHeader = request.getHeader("Authorization");

        String username = null;
        String jwt = null;

        if (authorizationHeader != null && authorizationHeader.startsWith("Bearer ")) {
            jwt = authorizationHeader.substring(7);
            username = jwtUtil.extractUsername(jwt);
        }

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

            UserDetails userDetails = this.userDetailsService.loadUserByUsername(username);

            if (jwtUtil.validateToken(jwt, userDetails)) {

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

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

                SecurityContextHolder.getContext().setAuthentication(usernamePasswordAuthenticationToken);
            }
        }
        chain.doFilter(request, response);
    }
}