I’m developing a web application using Spring WebFlux and Spring Security with OAuth2 with KeyClock as third-party authorization. When trying to access any endpoint with a valid JWT Token, as it is a valid token, I don’t get forbidden, instead, I get a CodecException related to a type definition error saying that the ServerHttpResponse already committed a response. Here is the error log:

Error Log

2024-06-20T10:52:35.370-03:00 ERROR 27432 --- [demo] [     parallel-2] o.s.w.s.adapter.HttpWebHandlerAdapter    : [53e68de1] Error [org.springframework.core.codec.CodecException: Type definition error: [simple type, class org.springframework.web.reactive.function.server.DefaultEntityResponseBuilder$DefaultEntityResponse]] for HTTP GET "/api/demo", but ServerHttpResponse already committed (200 OK)

Security Bean
I have set up my security configuration as follows:

@Configuration
@EnableWebFluxSecurity
public class SecurityConfig {

    @Bean
    public SecurityWebFilterChain securityFilterChain(final ServerHttpSecurity serverHttpSecurity) {
        return serverHttpSecurity
                .authorizeExchange(exchanges -> exchanges.anyExchange().authenticated())
                .oauth2ResourceServer(securityOAuth2Specs -> {
                    securityOAuth2Specs.jwt(Customizer.withDefaults());
                })
                .csrf(ServerHttpSecurity.CsrfSpec::disable)
                .build();
    }
}

Controller
It is very simple, just an endpoint that will only be called with a valid JWT.

@RestController
@RequestMapping("/api")
public class DemoController {

    @GetMapping("/demo")
    public Mono<ServerResponse> doSomething() {
        return ServerResponse.ok().bodyValue("Hello World");
    }
}

Maven Dependencies:

I’m using Spring boot 3.3.0

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.3.0</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-oauth2-authorization-server</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-security</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.security</groupId>
            <artifactId>spring-security-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

Question:
How can I resolve this CodecException? It seems like the error occurs after the response has already been committed. What could be causing this, and how can I fix it to ensure the endpoint functions as expected?

Attempts:

Trying to use ResponseEntity but that is out of webflux purpose.