Below is a sample code to implement ResponseInterceptor for openfeign with spring-boot.

import feign.InvocationContext;
import feign.ResponseInterceptor;
import org.springframework.stereotype.Component;

@Component
public class FeignClientInterceptor implements ResponseInterceptor {

    @Override
    public Object aroundDecode(InvocationContext invocationContext) {

        feign.Response response= invocationContext.response();
        feign.Request request=response.request();

        //Do something with request and response        

        return invocationContext.proceed();
    }
}

Dependency used for this example:

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>2022.0.1</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-openfeign</artifactId>
    </dependency>
</dependencies>

You don't really need your own implementation of the FeignRequestInterceptor as there is already BasicAuthRequestInterceptor in the feign.auth package that does exactly the same.

That being said, you basically have almost everything set up already. All is left to do is to define the basicAuthRequestInterceptor bean with specific username and password:

@Bean
public RequestInterceptor basicAuthRequestInterceptor() {
    return new BasicAuthRequestInterceptor("username", "password");
}

A practical example of how to intercept the response in a Spring Cloud OpenFeign.

1. Create a custom Client by extending Client.Default as shown below:

public class CustomFeignClient extends Client.Default {


    public CustomFeignClient(SSLSocketFactory sslContextFactory, HostnameVerifier hostnameVerifier) {
        super(sslContextFactory, hostnameVerifier);
    }

    @Override
    public Response execute(Request request, Request.Options options) throws IOException {

        Response response = super.execute(request, options);
        InputStream bodyStream = response.body().asInputStream();

        String responseBody = StreamUtils.copyToString(bodyStream, StandardCharsets.UTF_8);

        //TODO do whatever you want with the responseBody - parse and modify it

        return response.toBuilder().body(responseBody, StandardCharsets.UTF_8).build();
    }
}

2. Then use the custom Client in a configuration class:

public class FeignClientConfig {


    public FeignClientConfig() { }

    @Bean
    public Client client() {
        return new CustomFeignClient(null, null);
    }

}

3. Finally, use the configuration class in a FeignClient:

@FeignClient(name = "api-client", url = "${api.base-url}", configuration = FeignClientConfig.class)
public interface ApiClient {

}

Good luck