Auto-configure rest client when virtual threads are enabled

Refine `RestClientAutoConfiguration` conditional so that it
applies in reactive web applications if virtual threads are
active and a task executor is configured.

See gh-44952

Signed-off-by: Dmitry Sulman <dmitry.sulman@gmail.com>

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/client/NotReactiveWebApplicationOrVirtualThreadsEnabledCondition.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2012-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.web.client;
+
+import org.springframework.boot.autoconfigure.condition.AnyNestedCondition;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnThreading;
+import org.springframework.boot.autoconfigure.condition.SpringBootCondition;
+import org.springframework.boot.autoconfigure.thread.Threading;
+import org.springframework.context.annotation.Conditional;
+
+/**
+ * {@link SpringBootCondition} that applies when running in a non-reactive web application
+ * or virtual threads are enabled.
+ *
+ * @author Dmitry Sulman
+ */
+class NotReactiveWebApplicationOrVirtualThreadsEnabledCondition extends AnyNestedCondition {
+
+	NotReactiveWebApplicationOrVirtualThreadsEnabledCondition() {
+		super(ConfigurationPhase.PARSE_CONFIGURATION);
+	}
+
+	@Conditional(NotReactiveWebApplicationCondition.class)
+	private static final class NotReactiveWebApplication {
+
+	}
+
+	@ConditionalOnThreading(Threading.VIRTUAL)
+	private static final class VirtualThreadsEnabled {
+
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/client/RestClientAutoConfiguration.java

@@ -53,7 +53,7 @@ import org.springframework.web.client.RestClient.Builder;
 @AutoConfiguration(after = { HttpClientAutoConfiguration.class, HttpMessageConvertersAutoConfiguration.class,
 		SslAutoConfiguration.class })
 @ConditionalOnClass(RestClient.class)
-@Conditional(NotReactiveWebApplicationCondition.class)
+@Conditional(NotReactiveWebApplicationOrVirtualThreadsEnabledCondition.class)
 public class RestClientAutoConfiguration {
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/client/RestClientAutoConfigurationTests.java

@@ -20,6 +20,8 @@ import java.time.Duration;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledForJreRange;
+import org.junit.jupiter.api.condition.JRE;
 
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
@@ -31,6 +33,8 @@ import org.springframework.boot.http.client.ClientHttpRequestFactorySettings.Red
 import org.springframework.boot.ssl.SslBundle;
 import org.springframework.boot.ssl.SslBundles;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
+import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
 import org.springframework.boot.web.client.RestClientCustomizer;
 import org.springframework.boot.web.codec.CodecCustomizer;
 import org.springframework.context.annotation.Bean;
@@ -260,6 +264,39 @@ class RestClientAutoConfigurationTests {
 			});
 	}
 
+	@Test
+	void whenReactiveWebApplicationRestClientIsNotConfigured() {
+		new ReactiveWebApplicationContextRunner()
+			.withConfiguration(AutoConfigurations.of(RestClientAutoConfiguration.class))
+			.run((context) -> {
+				assertThat(context).doesNotHaveBean(HttpMessageConvertersRestClientCustomizer.class);
+				assertThat(context).doesNotHaveBean(RestClientBuilderConfigurer.class);
+				assertThat(context).doesNotHaveBean(RestClient.Builder.class);
+			});
+	}
+
+	@Test
+	void whenServletWebApplicationRestClientIsConfigured() {
+		new WebApplicationContextRunner().withConfiguration(AutoConfigurations.of(RestClientAutoConfiguration.class))
+			.run((context) -> {
+				assertThat(context).hasSingleBean(HttpMessageConvertersRestClientCustomizer.class);
+				assertThat(context).hasSingleBean(RestClientBuilderConfigurer.class);
+				assertThat(context).hasSingleBean(RestClient.Builder.class);
+			});
+	}
+
+	@Test
+	@EnabledForJreRange(min = JRE.JAVA_21)
+	void whenReactiveWebApplicationAndVirtualThreadsAreEnabledOnJava21AndLaterRestClientIsConfigured() {
+		new ReactiveWebApplicationContextRunner().withPropertyValues("spring.threads.virtual.enabled=true")
+			.withConfiguration(AutoConfigurations.of(RestClientAutoConfiguration.class))
+			.run((context) -> {
+				assertThat(context).hasSingleBean(HttpMessageConvertersRestClientCustomizer.class);
+				assertThat(context).hasSingleBean(RestClientBuilderConfigurer.class);
+				assertThat(context).hasSingleBean(RestClient.Builder.class);
+			});
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class CodecConfiguration {