Fix Reactive Server auto-configuration ordering

This commit ensures that Tomcat is the first reactive server configured
if the Tomcat dependency is on classpath.

Spring Boot chose Reactor Netty as the default for the reactive web
starter, but the Reactor Netty dependency can be used also for its HTTP
client. In case developers are adding Tomcat, Undertow or Jetty on their
classpath, we must configure those and consider Reactor Netty for the
client only.

Fixes spring-projectsgh-12176

Author: bclozel

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/ReactiveWebServerFactoryAutoConfiguration.java

@@ -50,10 +50,10 @@
 @ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
 @EnableConfigurationProperties(ServerProperties.class)
 @Import({ ReactiveWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
-		ReactiveWebServerFactoryConfiguration.EmbeddedNetty.class,
 		ReactiveWebServerFactoryConfiguration.EmbeddedTomcat.class,
 		ReactiveWebServerFactoryConfiguration.EmbeddedJetty.class,
-		ReactiveWebServerFactoryConfiguration.EmbeddedUndertow.class })
+		ReactiveWebServerFactoryConfiguration.EmbeddedUndertow.class,
+		ReactiveWebServerFactoryConfiguration.EmbeddedNetty.class })
 public class ReactiveWebServerFactoryAutoConfiguration {
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/ReactiveWebServerFactoryAutoConfigurationTests.java

@@ -16,20 +16,19 @@
 
 package org.springframework.boot.autoconfigure.web.reactive;
 
-import org.hamcrest.Matchers;
-import org.junit.Rule;
 import org.junit.Test;
-import org.junit.rules.ExpectedException;
 import org.mockito.Mockito;
 
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
+import org.springframework.boot.web.embedded.tomcat.TomcatReactiveWebServerFactory;
 import org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext;
 import org.springframework.boot.web.reactive.server.ConfigurableReactiveWebServerFactory;
 import org.springframework.boot.web.reactive.server.ReactiveWebServerFactory;
 import org.springframework.boot.web.server.WebServerFactoryCustomizer;
 import org.springframework.context.ApplicationContextException;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
-import org.springframework.context.annotation.Import;
 import org.springframework.http.server.reactive.HttpHandler;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -41,54 +40,73 @@
  */
 public class ReactiveWebServerFactoryAutoConfigurationTests {
 
-	private AnnotationConfigReactiveWebServerApplicationContext context;
-
-	@Rule
-	public ExpectedException thrown = ExpectedException.none();
+	private ReactiveWebApplicationContextRunner contextRunner =
+			new ReactiveWebApplicationContextRunner(AnnotationConfigReactiveWebServerApplicationContext::new)
+					.withConfiguration(AutoConfigurations.of(ReactiveWebServerFactoryAutoConfiguration.class));
 
 	@Test
 	public void createFromConfigClass() {
-		this.context = new AnnotationConfigReactiveWebServerApplicationContext(
-				BaseConfiguration.class);
-		assertThat(this.context.getBeansOfType(ReactiveWebServerFactory.class))
-				.hasSize(1);
-		assertThat(this.context.getBeansOfType(WebServerFactoryCustomizer.class))
-				.hasSize(1);
-		assertThat(this.context
-				.getBeansOfType(ReactiveWebServerFactoryCustomizer.class))
-						.hasSize(1);
+		this.contextRunner
+				.withUserConfiguration(MockWebServerAutoConfiguration.class,
+						HttpHandlerConfiguration.class)
+				.run(context -> {
+					assertThat(context.getBeansOfType(ReactiveWebServerFactory.class))
+							.hasSize(1);
+					assertThat(context.getBeansOfType(WebServerFactoryCustomizer.class))
+							.hasSize(1);
+					assertThat(context.getBeansOfType(ReactiveWebServerFactoryCustomizer.class))
+							.hasSize(1);
+				});
 	}
 
 	@Test
 	public void missingHttpHandler() {
-		this.thrown.expect(ApplicationContextException.class);
-		this.thrown.expectMessage(Matchers.containsString("missing HttpHandler bean"));
-		this.context = new AnnotationConfigReactiveWebServerApplicationContext(
-				MissingHttpHandlerConfiguration.class);
+		this.contextRunner
+				.withUserConfiguration(MockWebServerAutoConfiguration.class)
+				.run(context -> {
+					assertThat(context.getStartupFailure())
+							.isInstanceOf(ApplicationContextException.class)
+							.hasMessageContaining("missing HttpHandler bean");
+				});
 	}
 
 	@Test
 	public void multipleHttpHandler() {
-		this.thrown.expect(ApplicationContextException.class);
-		this.thrown.expectMessage(Matchers.containsString(
-				"multiple HttpHandler beans : httpHandler,additionalHttpHandler"));
-		this.context = new AnnotationConfigReactiveWebServerApplicationContext(
-				BaseConfiguration.class, TooManyHttpHandlers.class);
+		this.contextRunner
+				.withUserConfiguration(MockWebServerAutoConfiguration.class,
+						HttpHandlerConfiguration.class, TooManyHttpHandlers.class)
+				.run(context -> {
+					assertThat(context.getStartupFailure())
+							.isInstanceOf(ApplicationContextException.class)
+							.hasMessageContaining("multiple HttpHandler beans : " +
+									"httpHandler,additionalHttpHandler");
+				});
 	}
 
 	@Test
 	public void customizeReactiveWebServer() {
-		this.context = new AnnotationConfigReactiveWebServerApplicationContext(
-				BaseConfiguration.class, ReactiveWebServerCustomization.class);
-		MockReactiveWebServerFactory factory = this.context
-				.getBean(MockReactiveWebServerFactory.class);
-		assertThat(factory.getPort()).isEqualTo(9000);
+		this.contextRunner
+				.withUserConfiguration(MockWebServerAutoConfiguration.class,
+						HttpHandlerConfiguration.class, ReactiveWebServerCustomization.class)
+				.run(context -> {
+					assertThat(context.getBean(MockReactiveWebServerFactory.class).getPort())
+							.isEqualTo(9000);
+				});
+	}
+
+	@Test
+	public void defaultWebServerIsTomcat() {
+		// Tomcat should be chosen over Netty if the Tomcat library is present.
+		this.contextRunner
+				.withUserConfiguration(HttpHandlerConfiguration.class)
+				.run(context -> {
+					assertThat(context.getBean(ReactiveWebServerFactory.class))
+							.isInstanceOf(TomcatReactiveWebServerFactory.class);
+				});
 	}
 
 	@Configuration
-	@Import({ MockWebServerAutoConfiguration.class,
-			ReactiveWebServerFactoryAutoConfiguration.class })
-	protected static class BaseConfiguration {
+	protected static class HttpHandlerConfiguration {
 
 		@Bean
 		public HttpHandler httpHandler() {
@@ -97,13 +115,6 @@ public HttpHandler httpHandler() {
 
 	}
 
-	@Configuration
-	@Import({ MockWebServerAutoConfiguration.class,
-			ReactiveWebServerFactoryAutoConfiguration.class })
-	protected static class MissingHttpHandlerConfiguration {
-
-	}
-
 	@Configuration
 	protected static class TooManyHttpHandlers {