Rest client config with springboot 3.3.3 to use virtual Threads

[SzymonZasada]

In Spring Boot 3.3.3, I have virtual threads enabled (spring.threads.virtual.enabled=true). However, I'm having trouble configuring RestClient to use virtual threads instead of platform threads.
Could someone share how to configure RestClient to use virtual threads? Here's my current configuration:

@Bean
public RestClient restClient(RestClient.Builder builder) {
    return builder
        .baseUrl("http://api.example.com")
        .build();
}

What's the proper way to configure RestClient to utilize virtual threads rather than platform threads?

[bclozel]

RestClient does not manage threads in the first place and wraps underlying HTTP client libraries.
Which HTTP client library are you using? Are you aware of any virtual threads support there?

[SzymonZasada]

I'm using the default JDK HttpClient that comes with Spring MVC RestClient (no Netty or other HTTP clients). Here's my current service:

@Service
@AllArgsConstructor
public class MicroserviceCommunicationService {
    private final RestClient restClient;

    private <T> T callOtherMicroservice(ServiceNameEnum serviceName, String endpoint, UUID companyId, UUID userId, Class<T> responseType) {
        return restClient.get()
                .uri("http://" + serviceName.getServiceName() + endpoint)
                .header(HttpUserHeaders.COMPANY_ID.getHeaderName(), companyId.toString())
                .header(HttpUserHeaders.USER_ID.getHeaderName(), userId.toString())
                .retrieve()
                .body(responseType);
    }
}

just don't know how to just create a config to make it usevirtual threads. I have quite a few calls and a better option is us virtual threads than webflux.

[bclozel]

I see.
You can configure the JDK HttpClient as you wish, like so:

HttpClient httpClient = HttpClient.newBuilder()
		.executor(Executors.newVirtualThreadPerTaskExecutor())
		.build();
RestClient client = RestClient
		.builder()
		.requestFactory(new JdkClientHttpRequestFactory(httpClient))
		.build();

In a Spring Boot app, you could contribute the following to be applied to the global builder:

@Bean
public RestClientCustomizer virtualThreadsCustomizer() {
	HttpClient httpClient = HttpClient.newBuilder()
			.executor(Executors.newVirtualThreadPerTaskExecutor())
			.build();
	return (client) -> {
		client.requestFactory(new JdkClientHttpRequestFactory(httpClient));
	};
}

I don't think we can offer at this time a high level option at the RestClient level for this, since HTTP libraries have different approaches there and do not align.

Thanks!