以下是一个使用Java的示例代码，用于通过OAuth2客户端方式获取token和进行认证：

import org.springframework.security.oauth2.client.OAuth2AuthorizedClient;
import org.springframework.security.oauth2.client.OAuth2AuthorizedClientService;
import org.springframework.security.oauth2.client.registration.ClientRegistration;
import org.springframework.security.oauth2.client.registration.ClientRegistrationRepository;
import org.springframework.security.oauth2.core.OAuth2AccessToken;
import org.springframework.security.oauth2.core.OAuth2AuthenticationException;
import org.springframework.security.oauth2.core.user.OAuth2User;
import org.springframework.security.oauth2.core.user.OAuth2UserAuthority;
import org.springframework.security.oauth2.core.user.OAuth2UserRequest;
import org.springframework.security.oauth2.core.user.OAuth2UserRequestEntityConverter;
import org.springframework.security.oauth2.core.user.OAuth2UserRequestFactory;
import org.springframework.security.oauth2.core.user.OAuth2UserRequestFactoryProvider;
import org.springframework.security.oauth2.core.user.OAuth2UserRequestResolver;
import org.springframework.security.oauth2.core.user.OAuth2UserService;
import org.springframework.security.oauth2.core.user.OAuth2UserServiceException;
import org.springframework.security.oauth2.core.user.OAuth2UserToken;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenRepository;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenServices;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenServicesProvider;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidator;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorFactory;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorProvider;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorResolver;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenType;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenTypeResolver;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidator;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorFactory;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorProvider;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorResolver;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenType;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidator;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorFactory;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorProvider;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenValidatorResolver;
import org.springframework.security.oauth2.core.user.OAuth2UserTokenType;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

import java.util.Collections;
import java.util.Map;
import java.util.Optional;

@Service
public class OAuth2ClientService {

    private final ClientRegistrationRepository clientRegistrationRepository;
    private final OAuth2AuthorizedClientService authorizedClientService;
    private final OAuth2UserTokenServices userTokenServices;
    private final OAuth2UserService userService;
    private final OAuth2UserRequestFactoryProvider requestFactoryProvider;
    private final OAuth2UserRequestResolver requestResolver;
    private final OAuth2UserTokenValidatorResolver validatorResolver;
    private final OAuth2UserTokenTypeResolver tokenTypeResolver;

    public OAuth2ClientService(ClientRegistrationRepository clientRegistrationRepository,
                               OAuth2AuthorizedClientService authorizedClientService,
                               OAuth2UserTokenServices userTokenServices,
                               OAuth2UserService userService,
                               OAuth2UserRequestFactoryProvider requestFactoryProvider,
                               OAuth2UserRequestResolver requestResolver,
                               OAuth2UserTokenValidatorResolver validatorResolver,
                               OAuth2UserTokenTypeResolver tokenTypeResolver) {
        this.clientRegistrationRepository = clientRegistrationRepository;
        this.authorizedClientService = authorizedClientService;
        this.userTokenServices = userTokenServices;
        this.userService = userService;
        this.requestFactoryProvider = requestFactoryProvider;
        this.requestResolver = requestResolver;
        this.validatorResolver = validatorResolver;
        this.tokenTypeResolver = tokenTypeResolver;
    }

    public OAuth2AccessToken getToken(String clientRegistrationId, String code, String redirectUri) {
        ClientRegistration clientRegistration = clientRegistrationRepository.findByRegistrationId(clientRegistrationId);
        OAuth2UserRequestFactory requestFactory = requestFactoryProvider.getFactory(clientRegistration);
        OAuth2UserRequest request = requestResolver.resolve(requestFactory, clientRegistration, redirectUri, code);
        OAuth2AccessToken token = request.getAccessToken();
        OAuth2UserTokenValidator validator = validatorResolver.resolve(request.getTokenType());
        validator.validate(token);
        userTokenServices.saveToken(request.getTokenType(), request.getClientRegistration(), request.getPrincipalName(), token);
        return token;
    }

    public Optional<OAuth2User> getUser(String clientRegistrationId, String accessToken) {
        ClientRegistration clientRegistration = clientRegistrationRepository.findByRegistrationId(clientRegistrationId);
        RestTemplate restTemplate = new RestTemplate();
        restTemplate.getInterceptors().add((request, body, execution) -> {
            request.getHeaders().setBearerAuth(accessToken);
            return execution.execute(request, body);
        });
        OAuth2User user;
        try {
            user = userService.loadUser(new OAuth2UserRequest(clientRegistration, accessToken));
        } catch (OAuth2AuthenticationException e) {
            return Optional.empty();
        }
        OAuth2UserToken token = new OAuth2UserToken(OAuth2UserTokenType.ACCESS_TOKEN, clientRegistration, user.getName(), accessToken, Collections.emptyMap());
        userTokenServices.saveToken(token);
        return Optional.of(user);
    }

    public Optional<OAuth2AuthorizedClient> getAuthorizedClient(String clientRegistrationId, String principalName) {
        return Optional.ofNullable(authorizedClientService.loadAuthorizedClient(clientRegistrationId, principalName));
    }

    public void saveAuthorizedClient(OAuth2AuthorizedClient authorizedClient, OAuth2User user) {
        authorizedClientService.saveAuthorizedClient(authorizedClient, user);
    }

    public void removeAuthorizedClient(String clientRegistrationId, String principalName) {
        authorizedClientService.removeAuthorizedClient(clientRegistrationId, principalName);
    }

    public void removeToken(OAuth2UserToken token) {
        userTokenServices.removeToken(token);
    }

    public Map<String, Object> getUserInfo(String clientRegistrationId, String accessToken) {
        ClientRegistration clientRegistration = clientRegistrationRepository.findByRegistrationId(clientRegistrationId);
        RestTemplate restTemplate = new RestTemplate();
        restTemplate.getInterceptors().add((request, body, execution) -> {
            request.getHeaders().setBearerAuth(accessToken);
            return execution.execute(request, body);
        });
        return restTemplate.getForObject(clientRegistration.getProviderDetails().getUserInfoEndpoint().getUri(), Map.class);
    }

    public String getClientId(String clientRegistrationId) {
        return clientRegistrationRepository.findByRegistrationId(clientRegistrationId).getClientId();
    }
}

这个示例代码使用了Spring Security OAuth2库，并提供了一些方法来获取token、认证用户、获取已授权的客户端、保存已授权的客户端等。

在使用时，需要在应用程序中配置OAuth2客户端注册表，以便客户端可以使用它来获取token。您还需要提供一个授权端点和一个用户信息端点，在这些端点上，客户端可以使用token来访问受保护的资源。