Defer HttpHandler initialization

Update `ReactiveWebServerApplicationContext` so that the `HttpHandler`
bean is not longer created from `onRefresh`, but is instead created
only when the server starts.

Prior to this commit, the WebFlux hander would cause early
initialization of several beans, including Jackson Modules.

Closes gh-14666

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/reactive/context/ReactiveWebServerApplicationContext.java

@@ -16,6 +16,10 @@
 
 package org.springframework.boot.web.reactive.context;
 
+import java.util.function.Supplier;
+
+import reactor.core.publisher.Mono;
+
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.support.DefaultListableBeanFactory;
 import org.springframework.boot.web.context.ConfigurableWebServerApplicationContext;
@@ -23,6 +27,8 @@ import org.springframework.boot.web.reactive.server.ReactiveWebServerFactory;
 import org.springframework.boot.web.server.WebServer;
 import org.springframework.context.ApplicationContextException;
 import org.springframework.http.server.reactive.HttpHandler;
+import org.springframework.http.server.reactive.ServerHttpRequest;
+import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.util.StringUtils;
 
 /**
@@ -38,6 +44,8 @@ public class ReactiveWebServerApplicationContext
 
 	private volatile WebServer webServer;
 
+	private volatile DeferredHttpHandler httpHandler;
+
 	private String serverNamespace;
 
 	/**
@@ -78,6 +86,17 @@ public class ReactiveWebServerApplicationContext
 		}
 	}
 
+	private void createWebServer() {
+		WebServer localServer = this.webServer;
+		if (localServer == null) {
+			DeferredHttpHandler localHandler = new DeferredHttpHandler(
+					this::getHttpHandler);
+			this.webServer = getWebServerFactory().getWebServer(localHandler);
+			this.httpHandler = localHandler;
+		}
+		initPropertySources();
+	}
+
 	@Override
 	protected void finishRefresh() {
 		super.finishRefresh();
@@ -93,14 +112,6 @@ public class ReactiveWebServerApplicationContext
 		stopAndReleaseReactiveWebServer();
 	}
 
-	private void createWebServer() {
-		WebServer localServer = this.webServer;
-		if (localServer == null) {
-			this.webServer = getWebServerFactory().getWebServer(getHttpHandler());
-		}
-		initPropertySources();
-	}
-
 	/**
 	 * Returns the {@link WebServer} that was created by the context or {@code null} if
 	 * the server has not yet been created.
@@ -157,7 +168,12 @@ public class ReactiveWebServerApplicationContext
 
 	private WebServer startReactiveWebServer() {
 		WebServer localServer = this.webServer;
+		DeferredHttpHandler localHandler = this.httpHandler;
 		if (localServer != null) {
+			if (localHandler != null) {
+				localHandler.initialize();
+				this.httpHandler = null;
+			}
 			localServer.start();
 		}
 		return localServer;
@@ -186,4 +202,40 @@ public class ReactiveWebServerApplicationContext
 		this.serverNamespace = serverNamespace;
 	}
 
+	/**
+	 * {@link HttpHandler} that defers to a supplied handler which is initialized only
+	 * when the server starts.
+	 */
+	static class DeferredHttpHandler implements HttpHandler {
+
+		private Supplier<HttpHandler> factory;
+
+		private HttpHandler handler;
+
+		DeferredHttpHandler(Supplier<HttpHandler> factory) {
+			this.factory = factory;
+			this.handler = this::handleUninitialized;
+		}
+
+		public void initialize() {
+			this.handler = this.factory.get();
+		}
+
+		private Mono<Void> handleUninitialized(ServerHttpRequest request,
+				ServerHttpResponse response) {
+			throw new IllegalStateException(
+					"The HttpHandler has not yet been initialized");
+		}
+
+		@Override
+		public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
+			return this.handler.handle(request, response);
+		}
+
+		public HttpHandler getHandler() {
+			return this.handler;
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/reactive/context/AnnotationConfigReactiveWebServerApplicationContextTests.java

@@ -18,11 +18,16 @@ package org.springframework.boot.web.reactive.context;
 
 import org.junit.Test;
 
+import org.springframework.boot.web.reactive.context.ReactiveWebServerApplicationContext.DeferredHttpHandler;
 import org.springframework.boot.web.reactive.context.config.ExampleReactiveWebServerApplicationConfiguration;
 import org.springframework.boot.web.reactive.server.MockReactiveWebServerFactory;
 import org.springframework.boot.web.reactive.server.ReactiveWebServerFactory;
+import org.springframework.context.ApplicationListener;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.event.ApplicationEventMulticaster;
+import org.springframework.context.event.ContextRefreshedEvent;
+import org.springframework.context.event.SimpleApplicationEventMulticaster;
 import org.springframework.http.server.reactive.HttpHandler;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -80,11 +85,23 @@ public class AnnotationConfigReactiveWebServerApplicationContextTests {
 		verifyContext();
 	}
 
+	@Test
+	public void httpHandlerInitialization() {
+		// gh-14666
+		this.context = new AnnotationConfigReactiveWebServerApplicationContext(
+				InitializationTestConfig.class);
+		verifyContext();
+	}
+
 	private void verifyContext() {
 		MockReactiveWebServerFactory factory = this.context
 				.getBean(MockReactiveWebServerFactory.class);
-		HttpHandler httpHandler = this.context.getBean(HttpHandler.class);
-		assertThat(factory.getWebServer().getHttpHandler()).isEqualTo(httpHandler);
+		HttpHandler expectedHandler = this.context.getBean(HttpHandler.class);
+		HttpHandler actualHandler = factory.getWebServer().getHttpHandler();
+		if (actualHandler instanceof DeferredHttpHandler) {
+			actualHandler = ((DeferredHttpHandler) actualHandler).getHandler();
+		}
+		assertThat(actualHandler).isEqualTo(expectedHandler);
 	}
 
 	@Configuration
@@ -107,4 +124,54 @@ public class AnnotationConfigReactiveWebServerApplicationContextTests {
 
 	}
 
+	@Configuration
+	public static class InitializationTestConfig {
+
+		private static boolean addedListener;
+
+		@Bean
+		public ReactiveWebServerFactory webServerFactory() {
+			return new MockReactiveWebServerFactory();
+		}
+
+		@Bean
+		public HttpHandler httpHander() {
+			if (!addedListener) {
+				throw new RuntimeException(
+						"Handlers should be added after listeners, we're being initialized too early!");
+			}
+			return mock(HttpHandler.class);
+		}
+
+		@Bean
+		public Listener listener() {
+			return new Listener();
+		}
+
+		@Bean
+		public ApplicationEventMulticaster applicationEventMulticaster() {
+			return new SimpleApplicationEventMulticaster() {
+
+				@Override
+				public void addApplicationListenerBean(String listenerBeanName) {
+					super.addApplicationListenerBean(listenerBeanName);
+					if ("listener".equals(listenerBeanName)) {
+						addedListener = true;
+					}
+				}
+
+			};
+		}
+
+		private static class Listener
+				implements ApplicationListener<ContextRefreshedEvent> {
+
+			@Override
+			public void onApplicationEvent(ContextRefreshedEvent event) {
+			}
+
+		}
+
+	}
+
 }