Add condition for ClientHttpConnectorBuilder detection

Update `ClientHttpConnectorAutoConfiguration` with a condition to ensure
that `ClientHttpConnectorBuilder.detect` will return a result. Prior
to this commit, when using a JDK without `java.net.http.HttpClient`
access the auto-configuration would fail.

Fixes gh-45955

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/http/client/reactive/ClientHttpConnectorAutoConfiguration.java

@@ -34,6 +34,7 @@ import org.springframework.boot.http.client.reactive.ClientHttpConnectorSettings
 import org.springframework.boot.ssl.SslBundles;
 import org.springframework.boot.util.LambdaSafe;
 import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.context.annotation.Lazy;
@@ -48,6 +49,7 @@ import org.springframework.http.client.reactive.ClientHttpConnector;
  */
 @AutoConfiguration(after = SslAutoConfiguration.class)
 @ConditionalOnClass({ ClientHttpConnector.class, Mono.class })
+@Conditional(ConditionalOnClientHttpConnectorBuilderDetection.class)
 @EnableConfigurationProperties(HttpReactiveClientProperties.class)
 public class ClientHttpConnectorAutoConfiguration implements BeanClassLoaderAware {
 

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/http/client/reactive/ConditionalOnClientHttpConnectorBuilderDetection.java

@@ -0,0 +1,44 @@
+/*
+ * 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.http.client.reactive;
+
+import org.springframework.boot.autoconfigure.condition.ConditionOutcome;
+import org.springframework.boot.autoconfigure.condition.SpringBootCondition;
+import org.springframework.boot.http.client.reactive.ClientHttpConnectorBuilder;
+import org.springframework.context.annotation.Condition;
+import org.springframework.context.annotation.ConditionContext;
+import org.springframework.core.type.AnnotatedTypeMetadata;
+
+/**
+ * {@link Condition} that checks that {@link ClientHttpConnectorBuilder} can be detected.
+ *
+ * @author Phillip Webb
+ */
+class ConditionalOnClientHttpConnectorBuilderDetection extends SpringBootCondition {
+
+	@Override
+	public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
+		try {
+			ClientHttpConnectorBuilder.detect(context.getClassLoader());
+			return ConditionOutcome.match("Detected ClientHttpConnectorBuilder");
+		}
+		catch (IllegalStateException ex) {
+			return ConditionOutcome.noMatch("Unable to detect ClientHttpConnectorBuilder");
+		}
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/http/client/reactive/ClientHttpConnectorAutoConfigurationTests.java

@@ -41,6 +41,7 @@ import org.springframework.http.client.ReactorResourceFactory;
 import org.springframework.http.client.reactive.ClientHttpConnector;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -143,6 +144,17 @@ class ClientHttpConnectorAutoConfigurationTests {
 			});
 	}
 
+	@Test
+	void shouldBeConditionalOnAtLeastOneHttpConnectorClass() {
+		FilteredClassLoader classLoader = new FilteredClassLoader(reactor.netty.http.client.HttpClient.class,
+				org.eclipse.jetty.client.HttpClient.class, org.apache.hc.client5.http.impl.async.HttpAsyncClients.class,
+				java.net.http.HttpClient.class);
+		assertThatIllegalStateException().as("enough filtering")
+			.isThrownBy(() -> ClientHttpConnectorBuilder.detect(classLoader));
+		this.contextRunner.withClassLoader(classLoader)
+			.run((context) -> assertThat(context).doesNotHaveBean(ClientHttpConnectorSettings.class));
+	}
+
 	private List<String> sslPropertyValues() {
 		List<String> propertyValues = new ArrayList<>();
 		String location = "classpath:org/springframework/boot/autoconfigure/ssl/";