package org.springframework.security.config.oauth2.client;
    
    public enum CommonOAuth2Provider {
    	...
    
    	GITHUB {
    
    		@Override
    		public Builder getBuilder(String registrationId) {
    			ClientRegistration.Builder builder = getBuilder(registrationId,
    					ClientAuthenticationMethod.BASIC, DEFAULT_REDIRECT_URL);
    			builder.scope("read:user");
    			builder.authorizationUri("https://github.com/login/oauth/authorize");
    			builder.tokenUri("https://github.com/login/oauth/access_token");
    			builder.userInfoUri("https://api.github.com/user");
    			builder.userNameAttributeName("id");
    			builder.clientName("GitHub");
    			return builder;
    		}
    	},
    
    	...
    }

    @EnableWebSecurity
    public class SecurityConfig extends WebSecurityConfigurerAdapter {
    
        @Override
        protected void configure(final HttpSecurity http) throws Exception {
            http.authorizeRequests()
                    .antMatchers("/").permitAll()
                    .anyRequest().authenticated()
                .and()
                    .oauth2Login();            // 기본 oauth login 적용
        }
    }

    package org.springframework.security.oauth2.client.endpoint;
    
    // Access Token 발급 과정
    public class DefaultAuthorizationCodeTokenResponseClient {
    	...
    
    	@Override
    	public OAuth2AccessTokenResponse getTokenResponse(OAuth2AuthorizationCodeGrantRequest authorizationCodeGrantRequest) {
    		...
    		// post request 생성
    		// public T convert(S s) {
    		//	...
    		//	// 기존에 Spring Security가 설정한 token uri 값을 불러와 uri 경로로 설정
    		//	URI uri = UriComponentsBuilder.fromUriString(clientRegistration.getProviderDetails().getTokenUri())
    		//		.build()
    		//		.toUri();
    		//	return new RequestEntity<>(formParameters, headers, HttpMethod.POST, uri);
    		// }
    		RequestEntity<?> request = this.requestEntityConverter.convert(authorizationCodeGrantRequest);
    
    		ResponseEntity<OAuth2AccessTokenResponse> response;
    		try {
    			// RestOperation을 이용한 통신을 진행
    			response = this.restOperations.exchange(request, OAuth2AccessTokenResponse.class);
    		} catch (RestClientException ex) {
    			OAuth2Error oauth2Error = new OAuth2Error(INVALID_TOKEN_RESPONSE_ERROR_CODE,
    					"An error occurred while attempting to retrieve the OAuth 2.0 Access Token Response: " + ex.getMessage(), null);
    			throw new OAuth2AuthorizationException(oauth2Error, ex);
    		}
    
    		OAuth2AccessTokenResponse tokenResponse = response.getBody();
    
    		...
    	}
    
    	...
    }

    package org.springframework.security.oauth2.client.userinfo;
    
    public class DefaultOAuth2UserService implements OAuth2UserService<OAuth2UserRequest, OAuth2User> {
    	...
    	@Override
    	public OAuth2User loadUser(OAuth2UserRequest userRequest) throws OAuth2AuthenticationException {
    		...
    		// convert() 로 request 요청 생성
    		RequestEntity<?> request = this.requestEntityConverter.convert(userRequest);
    
    		ResponseEntity<Map<String, Object>> response;
    
    		try {
    			// Authorization Server와 통신
    			response = this.restOperations.exchange(request, PARAMETERIZED_RESPONSE_TYPE);
    		} catch (OAuth2AuthorizationException ex) {
    			...
    		}
    
    		Map<String, Object> userAttributes = response.getBody();
    		Set<GrantedAuthority> authorities = new LinkedHashSet<>();
    		authorities.add(new OAuth2UserAuthority(userAttributes));
    		OAuth2AccessToken token = userRequest.getAccessToken();
    		for (String authority : token.getScopes()) {
    			authorities.add(new SimpleGrantedAuthority("SCOPE_" + authority));
    		}
    
    		return new DefaultOAuth2User(authorities, userAttributes, userNameAttributeName);
    	}
    
    	...
    }

    package org.springframework.security.oauth2.client.userinfo;
    
    public class OAuth2UserRequestEntityConverter {
    	...
    
    	@Override
    	public RequestEntity<?> convert(OAuth2UserRequest userRequest) {
    		ClientRegistration clientRegistration = userRequest.getClientRegistration();
    
    		...
    
    		HttpHeaders headers = new HttpHeaders();
    		headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
    
    		// 기존에 Spring Security가 설정한 user-info-endpoint의 값을 불러와 uri 경로 설정
    		URI uri = UriComponentsBuilder.fromUriString(clientRegistration.getProviderDetails().getUserInfoEndpoint().getUri())
    				.build()
    				.toUri();
    
    		...
    
    		return request;
    	}
    	...
    }

    package org.springframework.security.oauth2.client.authentication;
    
    public class OAuth2LoginAuthenticationProvider implements AuthenticationProvider {
    	...
    	
    	@Override
    	public Authentication authenticate(Authentication authentication) throws AuthenticationException {
    		
    		...
                OAuth2AccessTokenResponse accessTokenResponse;
		try {
			OAuth2AuthorizationExchangeValidator.validate(
					authorizationCodeAuthentication.getAuthorizationExchange());
                        
                        // access token 발급
			accessTokenResponse = this.accessTokenResponseClient.getTokenResponse(
					new OAuth2AuthorizationCodeGrantRequest(
							authorizationCodeAuthentication.getClientRegistration(),
							authorizationCodeAuthentication.getAuthorizationExchange()));

		} catch (OAuth2AuthorizationException ex) {
			OAuth2Error oauth2Error = ex.getError();
			throw new OAuth2AuthenticationException(oauth2Error, oauth2Error.toString());
		}

                // 위에서 받아온 accessToken 추출
    		OAuth2AccessToken accessToken = accessTokenResponse.getAccessToken();
    		Map<String, Object> additionalParameters = accessTokenResponse.getAdditionalParameters();
    
    		// 인가된 사용자 정보 가져옴 with accessToken
    		OAuth2User oauth2User = this.userService.loadUser(new OAuth2UserRequest(
    				authorizationCodeAuthentication.getClientRegistration(), accessToken, additionalParameters));
    
    		...
    
    		// 위에서 가져온 정보 저장
    		OAuth2LoginAuthenticationToken authenticationResult = new OAuth2LoginAuthenticationToken(
    			authorizationCodeAuthentication.getClientRegistration(),
    			authorizationCodeAuthentication.getAuthorizationExchange(),
    			oauth2User,
    			mappedAuthorities,
    			accessToken,
    			accessTokenResponse.getRefreshToken());
    		authenticationResult.setDetails(authorizationCodeAuthentication.getDetails());
    
    		return authenticationResult;
    	}
    
    	...
    }

    package com.gaejangmo.apiserver;
    
    @ResController
    public class Controller {
    
      @GetMapping("/access_token")
      public String index(OAuth2AuthenticationToken authenticationToken) {

    	// SecurityContextHolder에 저장된 사용자 정보 사용
        log.info("authenticationToken {}", authenticationToken);
    
    	return null;
      }
    }