Spring Security: un ejemplo de aplicación web de autorización OAuth2 a través de BitBucket

@Configuration
public class SecurityConfig {

    @Value("${client_id}")
    private String clientId;

    @Value("${client_secret}")
    private String clientSecret;

    @Bean
    public ClientRegistration clientRegistration() {
        return ClientRegistration
                .withRegistrationId("bitbucket")
                .clientId(clientId)
                .clientSecret(clientSecret)
                .userNameAttributeName("username")
                .clientAuthenticationMethod(BASIC)
                .authorizationGrantType(AUTHORIZATION_CODE)
                .userInfoUri("https://api.bitbucket.org/2.0/user")
                .tokenUri("https://bitbucket.org/site/oauth2/access_token")
                .authorizationUri("https://bitbucket.org/site/oauth2/authorize")
                .redirectUriTemplate("{baseUrl}/login/oauth2/code/{registrationId}")
                .build();
    }

    @Bean
    @Autowired
    public MyOAuth2UserService oAuth2userService(UserService userService) {
        return new MyOAuth2UserService(userService);
    }

    @Bean
    @Autowired
    public ClientRegistrationRepository clientRegistrationRepository(List<ClientRegistration> registrations) {
        return new InMemoryClientRegistrationRepository(registrations);
    }

    @Configuration
    @EnableWebSecurity
    public static class AuthConfig extends WebSecurityConfigurerAdapter {

        private final MyOAuth2UserService userService;

        @Autowired
        public AuthConfig(MyOAuth2UserService userService) {
            this.userService = userService;
        }

        @Override
        protected void configure(HttpSecurity http) throws Exception {
            http
                    .authorizeRequests(authorizeRequests -> authorizeRequests
                            .anyRequest().authenticated()
                    )
                    .oauth2Login(oauth2Login -> oauth2Login
                            .userInfoEndpoint(userInfoEndpoint -> userInfoEndpoint.userService(userService))
                    );
        }
    }
}

public class MyOAuth2UserService implements OAuth2UserService<OAuth2UserRequest, OAuth2User> {

    private static final String MISSING_USER_INFO_URI_ERROR_CODE = "missing_user_info_uri";

    private static final String INVALID_USER_INFO_RESPONSE_ERROR_CODE = "invalid_user_info_response";

    private static final String MISSING_USER_NAME_ATTRIBUTE_ERROR_CODE = "missing_user_name_attribute";

    private static final ParameterizedTypeReference<Map<String, Object>> PARAMETERIZED_RESPONSE_TYPE = new ParameterizedTypeReference<Map<String, Object>>() {
    };

    private final UserService userService;

    private final RestOperations restOperations;

    private final Converter<OAuth2UserRequest, RequestEntity<?>> requestEntityConverter = new OAuth2UserRequestEntityConverter();

    public MyOAuth2UserService(UserService userService) {
        this.userService = requireNonNull(userService);
        this.restOperations = createRestTemplate();
    }

    @Override
    public OAuth2User loadUser(OAuth2UserRequest userRequest) throws OAuth2AuthenticationException {
        checkNotNull(userRequest, "userRequest cannot be null");
        if (!StringUtils.hasText(userRequest.getClientRegistration().getProviderDetails().getUserInfoEndpoint().getUri())) {
            OAuth2Error oauth2Error = new OAuth2Error(MISSING_USER_INFO_URI_ERROR_CODE, "Missing required UserInfo Uri in UserInfoEndpoint for Client Registration: " + userRequest.getClientRegistration().getRegistrationId(), null);
            throw new OAuth2AuthenticationException(oauth2Error, oauth2Error.toString());
        }

        String registrationId = userRequest.getClientRegistration().getRegistrationId();
        String userNameAttributeName = userRequest.getClientRegistration().getProviderDetails().getUserInfoEndpoint().getUserNameAttributeName();
        if (!StringUtils.hasText(userNameAttributeName)) {
            OAuth2Error oauth2Error = new OAuth2Error(
                    MISSING_USER_NAME_ATTRIBUTE_ERROR_CODE,
                    "Missing required \"user name\" attribute name in UserInfoEndpoint for Client Registration: " + registrationId, null);
            throw new OAuth2AuthenticationException(oauth2Error, oauth2Error.toString());
        }

        ResponseEntity<Map<String, Object>> response;
        try {
            // OAuth2UserRequestEntityConverter cannot return null values.
            //noinspection ConstantConditions
            response = this.restOperations.exchange(requestEntityConverter.convert(userRequest), PARAMETERIZED_RESPONSE_TYPE);
        } catch (OAuth2AuthorizationException ex) {
            OAuth2Error oauth2Error = ex.getError();
            StringBuilder errorDetails = new StringBuilder();
            errorDetails.append("Error details: [");
            errorDetails.append("UserInfo Uri: ").append(
                    userRequest.getClientRegistration().getProviderDetails().getUserInfoEndpoint().getUri());
            errorDetails.append(", Error Code: ").append(oauth2Error.getErrorCode());
            if (oauth2Error.getDescription() != null) {
                errorDetails.append(", Error Description: ").append(oauth2Error.getDescription());
            }
            errorDetails.append("]");
            oauth2Error = new OAuth2Error(INVALID_USER_INFO_RESPONSE_ERROR_CODE,
                    "An error occurred while attempting to retrieve the UserInfo Resource: " + errorDetails.toString(), null);
            throw new OAuth2AuthenticationException(oauth2Error, oauth2Error.toString(), ex);
        } catch (RestClientException ex) {
            OAuth2Error oauth2Error = new OAuth2Error(INVALID_USER_INFO_RESPONSE_ERROR_CODE,
                    "An error occurred while attempting to retrieve the UserInfo Resource: " + ex.getMessage(), null);
            throw new OAuth2AuthenticationException(oauth2Error, oauth2Error.toString(), ex);
        }

        Map<String, Object> userAttributes = emptyIfNull(response.getBody());

        Set<GrantedAuthority> authorities = new LinkedHashSet<>();
        authorities.add(new OAuth2UserAuthority(userAttributes));

        for (String authority : userRequest.getAccessToken().getScopes()) {
            authorities.add(new SimpleGrantedAuthority("SCOPE_" + authority));
        }

        //     ,   
        //          ,
        //    
        User user = findOrCreate(userAttributes);
        userAttributes.put(MyOAuth2User.ID_ATTR, user.getId());
        return new MyOAuth2User(userNameAttributeName, userAttributes, authorities);
    }

    private User findOrCreate(Map<String, Object> userAttributes) {
        String login = (String) userAttributes.get("username");
        String username = (String) userAttributes.get("display_name");

        Optional<User> userOpt = userService.findByLogin(login);
        if (!userOpt.isPresent()) {
            User user = new User();
            user.setLogin(login);
            user.setName(username);
            return userService.create(user);
        }
        return userOpt.get();
    }

    private RestTemplate createRestTemplate() {
        RestTemplate restTemplate = new RestTemplate();
        restTemplate.setErrorHandler(new OAuth2ErrorResponseErrorHandler());
        return restTemplate;
    }
}

@Path("/")
public class WelcomeEndpoint {

    @Autowired
    private UserService userService;

    @GET
    public String welcome() {
        User currentUser = getCurrentUser();
        return String.format("Welcome, %s! (user id: %s, user login: %s)",
                currentUser.getName(), currentUser.getId(), currentUser.getLogin());
    }

    public User getCurrentUser() {
        return userService.findByLogin(getAuthenticatedUser().getName()).orElseThrow(() -> new RuntimeException("No user logged in."));
    }

    private Authentication getAuthentication() {
        return SecurityContextHolder.getContext().getAuthentication();
    }

    private MyOAuth2User getAuthenticatedUser() {
        return (MyOAuth2User) getAuthentication().getPrincipal();
    }
}