Add CRaC support to ReactorNettyClientRequestFactory

This commit adds a constructor with externally managed
Reactor Netty resources to ReactorNettyClientRequestFactory
and makes it lifecycle-aware in order to support Project CRaC.

Closes gh-31280
Closes gh-31281

spring-web/src/test/java/org/springframework/http/client/ReactorNettyClientHttpRequestFactoryTests.java

@@ -16,12 +16,19 @@
 
 package org.springframework.http.client;
 
+import java.util.function.Function;
+
 import org.junit.jupiter.api.Test;
+import reactor.netty.http.client.HttpClient;
 
 import org.springframework.http.HttpMethod;
+import org.springframework.http.client.reactive.ReactorResourceFactory;
+
+import static org.assertj.core.api.Assertions.assertThat;
 
 /**
  * @author Arjen Poutsma
+ * @author Sebastien Deleuze
  */
 public class ReactorNettyClientHttpRequestFactoryTests extends AbstractHttpRequestFactoryTests {
 
@@ -37,4 +44,44 @@ public class ReactorNettyClientHttpRequestFactoryTests extends AbstractHttpReque
 		assertHttpMethod("patch", HttpMethod.PATCH);
 	}
 
+	@Test
+	void restartWithDefaultConstructor() {
+		ReactorNettyClientRequestFactory requestFactory = new ReactorNettyClientRequestFactory();
+		assertThat(requestFactory.isRunning()).isTrue();
+		requestFactory.start();
+		assertThat(requestFactory.isRunning()).isTrue();
+		requestFactory.stop();
+		assertThat(requestFactory.isRunning()).isFalse();
+		requestFactory.start();
+		assertThat(requestFactory.isRunning()).isTrue();
+	}
+
+	@Test
+	void restartWithExternalResourceFactory() {
+		ReactorResourceFactory resourceFactory = new ReactorResourceFactory();
+		resourceFactory.afterPropertiesSet();
+		Function<HttpClient, HttpClient> mapper = Function.identity();
+		ReactorNettyClientRequestFactory requestFactory = new ReactorNettyClientRequestFactory(resourceFactory, mapper);
+		assertThat(requestFactory.isRunning()).isTrue();
+		requestFactory.start();
+		assertThat(requestFactory.isRunning()).isTrue();
+		requestFactory.stop();
+		assertThat(requestFactory.isRunning()).isFalse();
+		requestFactory.start();
+		assertThat(requestFactory.isRunning()).isTrue();
+	}
+
+	@Test
+	void restartWithHttpClient() {
+		HttpClient httpClient = HttpClient.create();
+		ReactorNettyClientRequestFactory requestFactory = new ReactorNettyClientRequestFactory(httpClient);
+		assertThat(requestFactory.isRunning()).isTrue();
+		requestFactory.start();
+		assertThat(requestFactory.isRunning()).isTrue();
+		requestFactory.stop();
+		assertThat(requestFactory.isRunning()).isFalse();
+		requestFactory.start();
+		assertThat(requestFactory.isRunning()).isTrue();
+	}
+
 }