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/main/java/org/springframework/http/client/ReactorNettyClientRequestFactory.java

@@ -19,34 +19,63 @@ package org.springframework.http.client;
 import java.io.IOException;
 import java.net.URI;
 import java.time.Duration;
+import java.util.function.Function;
 
 import io.netty.channel.ChannelOption;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import reactor.netty.http.client.HttpClient;
+import reactor.netty.resources.ConnectionProvider;
+import reactor.netty.resources.LoopResources;
 
+import org.springframework.context.SmartLifecycle;
 import org.springframework.http.HttpMethod;
+import org.springframework.http.client.reactive.ReactorResourceFactory;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
  * Reactor-Netty implementation of {@link ClientHttpRequestFactory}.
  *
+ * <p>This class implements {@link SmartLifecycle} and can be optionally declared
+ * as a Spring-managed bean in order to support JVM Checkpoint Restore.
+ *
  * @author Arjen Poutsma
+ * @author Sebastien Deleuze
  * @since 6.1
  */
-public class ReactorNettyClientRequestFactory implements ClientHttpRequestFactory {
+public class ReactorNettyClientRequestFactory implements ClientHttpRequestFactory, SmartLifecycle {
 
-	private final HttpClient httpClient;
+	private static final Log logger = LogFactory.getLog(ReactorNettyClientRequestFactory.class);
+
+	private final static Function<HttpClient, HttpClient> defaultInitializer = client -> client.compress(true);
+
+
+	private HttpClient httpClient;
+
+	@Nullable
+	private final ReactorResourceFactory resourceFactory;
+
+	@Nullable
+	private final Function<HttpClient, HttpClient> mapper;
 
 	private Duration exchangeTimeout = Duration.ofSeconds(5);
 
 	private Duration readTimeout = Duration.ofSeconds(10);
 
+	private volatile boolean running = true;
+
+	private final Object lifecycleMonitor = new Object();
+
 
 	/**
 	 * Create a new instance of the {@code ReactorNettyClientRequestFactory}
 	 * with a default {@link HttpClient} that has compression enabled.
 	 */
 	public ReactorNettyClientRequestFactory() {
-		this(HttpClient.create().compress(true));
+		this.httpClient = defaultInitializer.apply(HttpClient.create());
+		this.resourceFactory = null;
+		this.mapper = null;
 	}
 
 	/**
@@ -57,6 +86,47 @@ public class ReactorNettyClientRequestFactory implements ClientHttpRequestFactor
 	public ReactorNettyClientRequestFactory(HttpClient httpClient) {
 		Assert.notNull(httpClient, "HttpClient must not be null");
 		this.httpClient = httpClient;
+		this.resourceFactory = null;
+		this.mapper = null;
+	}
+
+	/**
+	 * Constructor with externally managed Reactor Netty resources, including
+	 * {@link LoopResources} for event loop threads, and {@link ConnectionProvider}
+	 * for the connection pool.
+	 * <p>This constructor should be used only when you don't want the client
+	 * to participate in the Reactor Netty global resources. By default the
+	 * client participates in the Reactor Netty global resources held in
+	 * {@link reactor.netty.http.HttpResources}, which is recommended since
+	 * fixed, shared resources are favored for event loop concurrency. However,
+	 * consider declaring a {@link ReactorResourceFactory} bean with
+	 * {@code globalResources=true} in order to ensure the Reactor Netty global
+	 * resources are shut down when the Spring ApplicationContext is stopped or closed
+	 * and restarted properly when the Spring ApplicationContext is
+	 * (with JVM Checkpoint Restore for example).
+	 * @param resourceFactory the resource factory to obtain the resources from
+	 * @param mapper a mapper for further initialization of the created client
+	 */
+	public ReactorNettyClientRequestFactory(ReactorResourceFactory resourceFactory, Function<HttpClient, HttpClient> mapper) {
+		this.httpClient = createHttpClient(resourceFactory, mapper);
+		this.resourceFactory = resourceFactory;
+		this.mapper = mapper;
+	}
+
+
+	private static HttpClient createHttpClient(ReactorResourceFactory resourceFactory, Function<HttpClient, HttpClient> mapper) {
+		ConnectionProvider provider = resourceFactory.getConnectionProvider();
+		Assert.notNull(provider, "No ConnectionProvider: is ReactorResourceFactory not initialized yet?");
+		return defaultInitializer.andThen(mapper).andThen(applyLoopResources(resourceFactory))
+				.apply(HttpClient.create(provider));
+	}
+
+	private static Function<HttpClient, HttpClient> applyLoopResources(ReactorResourceFactory factory) {
+		return httpClient -> {
+			LoopResources resources = factory.getLoopResources();
+			Assert.notNull(resources, "No LoopResources: is ReactorResourceFactory not initialized yet?");
+			return httpClient.runOn(resources);
+		};
 	}
 
 
@@ -129,4 +199,52 @@ public class ReactorNettyClientRequestFactory implements ClientHttpRequestFactor
 		return new ReactorNettyClientRequest(this.httpClient, uri, httpMethod, this.exchangeTimeout, this.readTimeout);
 	}
 
+	@Override
+	public void start() {
+		synchronized (this.lifecycleMonitor) {
+			if (!isRunning()) {
+				if (this.resourceFactory != null && this.mapper != null) {
+					this.httpClient = createHttpClient(this.resourceFactory, this.mapper);
+				}
+				else {
+					logger.warn("Restarting a ReactorNettyClientRequestFactory bean is only supported with externally managed Reactor Netty resources");
+				}
+				this.running = true;
+			}
+		}
+	}
+
+	@Override
+	public void stop() {
+		synchronized (this.lifecycleMonitor) {
+			if (isRunning()) {
+				this.running = false;
+			}
+		}
+	}
+
+	@Override
+	public final void stop(Runnable callback) {
+		synchronized (this.lifecycleMonitor) {
+			stop();
+			callback.run();
+		}
+	}
+
+	@Override
+	public boolean isRunning() {
+		return this.running;
+	}
+
+	@Override
+	public boolean isAutoStartup() {
+		return false;
+	}
+
+	@Override
+	public int getPhase() {
+		// Start after ReactorResourceFactory
+		return 1;
+	}
+
 }