OAuth2 REST Client Support in Spring Security 6.4: A New Era of Simplified Configuration

Spring Security 6.4 introduces major improvements in OAuth2 client configuration, particularly with its new RestClient support. This update brings simplified configuration patterns and better alignment between servlet and reactive applications. Let’s dive into what this means for Spring developers.

The Evolution of OAuth2 Client Configuration

Spring Security’s journey through versions 6.2 and 6.3 has been marked by steady improvements in OAuth2 Client configuration. The framework now allows developers to publish beans that are automatically incorporated into the OAuth2 Client configuration during startup. Key improvements include:

• Extension grant types via OAuth2AuthorizedClientProvider beans
• Custom OAuth2 Access Token Request parameters through OAuth2AccessTokenResponseClient beans
• Automatic publication of OAuth2AuthorizedClientManager beans

Enter RestClient: A Game-Changing Addition

Spring Framework 6.1 introduced RestClient, a synchronous HTTP client with a fluent API similar to WebClient. The key difference? RestClient doesn’t depend on reactive libraries, making it a perfect fit for traditional servlet-based applications.

Why RestClient Matters

1. Simplified Configuration: No additional reactive dependencies required
2. Familiar API: Similar to WebClient, reducing the learning curve
3. Synchronous Operations: Better suited for traditional servlet applications
4. Unified Approach: Aligns servlet and reactive stacks on a common configuration model

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Implementing OAuth2 with RestClient

Here’s how to set up protected resource requests using RestClient:

First, add the required dependency:

implementation 'org.springframework.boot:spring-boot-starter-oauth2-client'

Configure your client registration:

spring:
  security:
    oauth2:
      client:
        registration:
          messaging-client:
            provider: spring
            client-id: client1
            client-secret: my-secret
            authorization-grant-type: authorization_code
            scope: message.read,message.write
        provider:
          spring:
            issuer-uri: http://localhost:9000

Set up RestClient configuration:

@Configuration
public class RestClientConfig {
    @Bean
    public RestClient restClient(RestClient.Builder builder, 
                               OAuth2AuthorizedClientManager authorizedClientManager) {
        OAuth2ClientHttpRequestInterceptor requestInterceptor =
            new OAuth2ClientHttpRequestInterceptor(authorizedClientManager);
        return builder.requestInterceptor(requestInterceptor).build();
    }
}

Advanced Features and Customizations

Parameter Customization

Spring Security 6.4 introduces powerful features for customizing OAuth2 Access Token requests:

1. Parameter Override: Use setParametersConverter() to modify default parameters
2. Parameter Omission: Utilize setParametersCustomizer() to remove unwanted parameters

Example for omitting client_id when using client assertions:

@Configuration
@EnableWebFluxSecurity
public class SecurityConfig {
    @Bean
    public ReactiveOAuth2AccessTokenResponseClient<OAuth2ClientCredentialsGrantRequest> 
            clientCredentialsAccessTokenResponseClient() {
        WebClientReactiveClientCredentialsTokenResponseClient client = 
            new WebClientReactiveClientCredentialsTokenResponseClient();
        
        client.setParametersCustomizer((parameters) -> {
            if (parameters.containsKey(OAuth2ParameterNames.CLIENT_ASSERTION)) {
                parameters.remove(OAuth2ParameterNames.CLIENT_ID);
            }
        });
        
        return client;
    }
}

Migration Path

For teams using RestTemplate, Spring Security 6.4 provides a smooth migration path:

• Convert existing RestTemplate to RestClient using RestClient.create(RestTemplate)
• Align configuration patterns with the reactive stack
• Prepare for Spring Security 7.0’s unified configuration model

Key Takeaways

1. Simplified Configuration: RestClient support reduces boilerplate and complexity
2. Better Integration: Seamless integration between servlet and reactive applications
3. Enhanced Customization: Greater control over OAuth2 Access Token requests
4. Future-Ready: Alignment with Spring Security’s future direction

Looking Forward

Spring Security 6.4’s RestClient support represents a significant step toward simplifying OAuth2 configuration while maintaining flexibility. Whether you’re building a new application or maintaining an existing one, these improvements offer clearer, more consistent ways to handle OAuth2 authentication and authorization.

The release is currently available as version 6.4.0-RC1, providing an excellent opportunity for developers to test these features and provide feedback to the Spring Security team.

For developers looking to get started, the Spring Security reference documentation and sample applications provide comprehensive guides and examples. The alignment between servlet and reactive stacks makes this an ideal time to standardize OAuth2 configuration patterns across your Spring applications.