feat: implement Google OAuth2 authentication flow

backend/src/main/java/de/szut/casino/security/GoogleController.java

@@ -0,0 +1,49 @@
+package de.szut.casino.security;
+
+import de.szut.casino.security.dto.AuthResponseDto;
+import de.szut.casino.security.dto.GithubCallbackDto;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+import org.springframework.web.servlet.view.RedirectView;
+
+@RestController
+@RequestMapping("/oauth2/google")
+public class GoogleController {
+    private static final Logger logger = LoggerFactory.getLogger(GoogleController.class);
+
+    @Value("${spring.security.oauth2.client.registration.google.client-id}")
+    private String clientId;
+
+    @Value("${spring.security.oauth2.client.provider.google.authorization-uri}")
+    private String authorizationUri;
+
+    @Value("${spring.security.oauth2.client.registration.google.redirect-uri}")
+    private String redirectUri;
+
+    @Autowired
+    private GoogleService googleService;
+
+    @GetMapping("/authorize")
+    public RedirectView authorizeGoogle() {
+        logger.info("Redirecting to Google for authorization");
+
+        String authUrl = authorizationUri +
+                "?client_id=" + clientId +
+                "&redirect_uri=" + redirectUri +
+                "&response_type=code" +
+                "&scope=email profile";
+
+        return new RedirectView(authUrl);
+    }
+
+    @PostMapping("/callback")
+    public ResponseEntity<AuthResponseDto> googleCallback(@RequestBody GithubCallbackDto callbackDto) {
+        String code = callbackDto.getCode();
+        AuthResponseDto response = googleService.processGoogleCode(code);
+        return ResponseEntity.ok(response);
+    }
+}

backend/src/main/java/de/szut/casino/security/GoogleService.java

@@ -0,0 +1,164 @@
+package de.szut.casino.security;
+
+import de.szut.casino.security.dto.AuthResponseDto;
+import de.szut.casino.security.jwt.JwtUtils;
+import de.szut.casino.user.AuthProvider;
+import de.szut.casino.user.UserEntity;
+import de.szut.casino.user.UserRepository;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
+import org.springframework.http.ResponseEntity;
+import org.springframework.security.authentication.AuthenticationManager;
+import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
+import org.springframework.security.core.Authentication;
+import org.springframework.security.crypto.password.PasswordEncoder;
+import org.springframework.stereotype.Service;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
+import org.springframework.web.client.RestTemplate;
+
+import java.math.BigDecimal;
+import java.util.*;
+
+@Service
+public class GoogleService {
+    private static final Logger logger = LoggerFactory.getLogger(GoogleService.class);
+
+    @Value("${spring.security.oauth2.client.registration.google.client-id}")
+    private String clientId;
+
+    @Value("${spring.security.oauth2.client.registration.google.client-secret}")
+    private String clientSecret;
+
+    @Value("${spring.security.oauth2.client.registration.google.redirect-uri}")
+    private String redirectUri;
+
+    @Value("${spring.security.oauth2.client.provider.google.token-uri}")
+    private String tokenUri;
+
+    @Value("${spring.security.oauth2.client.provider.google.user-info-uri}")
+    private String userInfoUri;
+
+    @Autowired
+    private AuthenticationManager authenticationManager;
+
+    @Autowired
+    private UserRepository userRepository;
+
+    @Autowired
+    private JwtUtils jwtUtils;
+
+    @Autowired
+    private PasswordEncoder oauth2PasswordEncoder;
+
+    public AuthResponseDto processGoogleCode(String code) {
+        try {
+            RestTemplate restTemplate = new RestTemplate();
+
+            HttpHeaders tokenHeaders = new HttpHeaders();
+            tokenHeaders.set("Content-Type", "application/x-www-form-urlencoded");
+            
+            MultiValueMap<String, String> tokenRequestBody = new LinkedMultiValueMap<>();
+            tokenRequestBody.add("client_id", clientId);
+            tokenRequestBody.add("client_secret", clientSecret);
+            tokenRequestBody.add("code", code);
+            tokenRequestBody.add("redirect_uri", redirectUri);
+            tokenRequestBody.add("grant_type", "authorization_code");
+
+            HttpEntity<MultiValueMap<String, String>> tokenRequestEntity = new HttpEntity<>(tokenRequestBody, tokenHeaders);
+
+            ResponseEntity<Map> tokenResponse = restTemplate.exchange(
+                    tokenUri,
+                    HttpMethod.POST,
+                    tokenRequestEntity,
+                    Map.class
+            );
+
+            Map<String, Object> tokenResponseBody = tokenResponse.getBody();
+
+            if (tokenResponseBody == null || tokenResponseBody.containsKey("error")) {
+                String error = tokenResponseBody != null ? (String) tokenResponseBody.get("error") : "Unknown error";
+                throw new RuntimeException("Google OAuth error: " + error);
+            }
+
+            String accessToken = (String) tokenResponseBody.get("access_token");
+            if (accessToken == null || accessToken.isEmpty()) {
+                throw new RuntimeException("Failed to receive access token from Google");
+            }
+
+            HttpHeaders userInfoHeaders = new HttpHeaders();
+            userInfoHeaders.set("Authorization", "Bearer " + accessToken);
+
+            HttpEntity<String> userInfoRequestEntity = new HttpEntity<>(null, userInfoHeaders);
+
+            ResponseEntity<Map> userResponse = restTemplate.exchange(
+                    userInfoUri,
+                    HttpMethod.GET,
+                    userInfoRequestEntity,
+                    Map.class
+            );
+
+            Map<String, Object> userAttributes = userResponse.getBody();
+            if (userAttributes == null) {
+                throw new RuntimeException("Failed to fetch user data from Google");
+            }
+
+            String googleId = (String) userAttributes.get("sub");
+            String email = (String) userAttributes.get("email");
+            String name = (String) userAttributes.get("name");
+            Boolean emailVerified = (Boolean) userAttributes.getOrDefault("email_verified", false);
+
+            if (email == null) {
+                throw new RuntimeException("Google account does not have an email");
+            }
+
+            String username = name != null ? name.replaceAll("\\s+", "") : email.split("@")[0];
+
+            Optional<UserEntity> userOptional = userRepository.findByProviderId(googleId);
+            UserEntity user;
+
+            if (userOptional.isPresent()) {
+                user = userOptional.get();
+            } else {
+                userOptional = userRepository.findByEmail(email);
+
+                if (userOptional.isPresent()) {
+                    user = userOptional.get();
+                    user.setProvider(AuthProvider.GOOGLE);
+                    user.setProviderId(googleId);
+                } else {
+                    user = new UserEntity();
+                    user.setEmail(email);
+                    user.setUsername(username);
+                    user.setProvider(AuthProvider.GOOGLE);
+                    user.setProviderId(googleId);
+                    user.setEmailVerified(emailVerified);
+                    
+                    user.setBalance(new BigDecimal("100.00"));
+                }
+            }
+
+            String randomPassword = UUID.randomUUID().toString();
+            user.setPassword(oauth2PasswordEncoder.encode(randomPassword));
+
+            userRepository.save(user);
+
+            Authentication authentication = authenticationManager.authenticate(
+                    new UsernamePasswordAuthenticationToken(user.getEmail(), randomPassword)
+            );
+
+            String token = jwtUtils.generateToken(authentication);
+
+            return new AuthResponseDto(token);
+
+        } catch (Exception e) {
+            logger.error("Failed to process Google authentication", e);
+            throw new RuntimeException("Failed to process Google authentication", e);
+        }
+    }
+}

backend/src/main/java/de/szut/casino/security/oauth2/GoogleOAuth2UserInfo.java

@@ -0,0 +1,25 @@
+package de.szut.casino.security.oauth2;
+
+import java.util.Map;
+
+public class GoogleOAuth2UserInfo extends OAuth2UserInfo {
+
+    public GoogleOAuth2UserInfo(Map<String, Object> attributes) {
+        super(attributes);
+    }
+
+    @Override
+    public String getId() {
+        return (String) attributes.get("sub");
+    }
+
+    @Override
+    public String getName() {
+        return (String) attributes.get("name");
+    }
+
+    @Override
+    public String getEmail() {
+        return (String) attributes.get("email");
+    }
+}

backend/src/main/java/de/szut/casino/security/oauth2/OAuth2UserInfoFactory.java

@@ -10,6 +10,8 @@ public class OAuth2UserInfoFactory {
     public static OAuth2UserInfo getOAuth2UserInfo(String registrationId, Map<String, Object> attributes) {
         if (registrationId.equalsIgnoreCase(AuthProvider.GITHUB.toString())) {
             return new GitHubOAuth2UserInfo(attributes);
+        } else if (registrationId.equalsIgnoreCase(AuthProvider.GOOGLE.toString())) {
+            return new GoogleOAuth2UserInfo(attributes);
         } else {
             throw new OAuth2AuthenticationProcessingException("Sorry! Login with " + registrationId + " is not supported yet.");
         }

backend/src/main/java/de/szut/casino/user/AuthProvider.java

@@ -2,5 +2,6 @@ package de.szut.casino.user;
 
 public enum AuthProvider {
     LOCAL,
-    GITHUB
+    GITHUB,
+    GOOGLE
 }

backend/src/main/resources/application.properties

@@ -41,3 +41,13 @@ spring.security.oauth2.client.provider.github.user-name-attribute=login
 # OAuth Success and Failure URLs
 app.oauth2.authorizedRedirectUris=${app.frontend-host}/auth/oauth2/callback
 
+# Google OAuth2 Configuration
+spring.security.oauth2.client.registration.google.client-id=${GOOGLE_CLIENT_ID:350791038883-c1r7v4o793itq8a0rh7dut7itm7uneam.apps.googleusercontent.com}
+spring.security.oauth2.client.registration.google.client-secret=${GOOGLE_CLIENT_SECRET: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}
+spring.security.oauth2.client.registration.google.redirect-uri=${app.frontend-host}/oauth2/callback/google
+spring.security.oauth2.client.registration.google.scope=email,profile
+spring.security.oauth2.client.provider.google.authorization-uri=https://accounts.google.com/o/oauth2/v2/auth
+spring.security.oauth2.client.provider.google.token-uri=https://oauth2.googleapis.com/token
+spring.security.oauth2.client.provider.google.user-info-uri=https://www.googleapis.com/oauth2/v3/userinfo
+spring.security.oauth2.client.provider.google.user-name-attribute=sub
+