Merge branch '3.4.x' into 3.5.x

Closes gh-46180

Author: wilkinsona

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/EmbeddedWebServerFactoryCustomizerAutoConfiguration.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure.web.embedded;
 
 import io.undertow.Undertow;
+import io.undertow.servlet.api.DeploymentInfo;
 import org.apache.catalina.startup.Tomcat;
 import org.apache.coyote.UpgradeProtocol;
 import org.eclipse.jetty.ee10.webapp.WebAppContext;
@@ -110,10 +111,16 @@ public UndertowWebServerFactoryCustomizer undertowWebServerFactoryCustomizer(Env
 			return new UndertowWebServerFactoryCustomizer(environment, serverProperties);
 		}
 
-		@Bean
-		@ConditionalOnThreading(Threading.VIRTUAL)
-		UndertowDeploymentInfoCustomizer virtualThreadsUndertowDeploymentInfoCustomizer() {
-			return (deploymentInfo) -> deploymentInfo.setExecutor(new VirtualThreadTaskExecutor("undertow-"));
+		@Configuration(proxyBeanMethods = false)
+		@ConditionalOnClass(DeploymentInfo.class)
+		static class UndertowServletWebServerFactoryCustomizerConfiguration {
+
+			@Bean
+			@ConditionalOnThreading(Threading.VIRTUAL)
+			UndertowDeploymentInfoCustomizer virtualThreadsUndertowDeploymentInfoCustomizer() {
+				return (deploymentInfo) -> deploymentInfo.setExecutor(new VirtualThreadTaskExecutor("undertow-"));
+			}
+
 		}
 
 	}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/UndertowWebServerFactoryCustomizerConfigurationTests.java

@@ -23,6 +23,7 @@
 
 import org.springframework.boot.autoconfigure.AutoConfigurations;
 import org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration.UndertowWebServerFactoryCustomizerConfiguration;
+import org.springframework.boot.test.context.FilteredClassLoader;
 import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
 import org.springframework.boot.web.embedded.undertow.UndertowDeploymentInfoCustomizer;
 import org.springframework.boot.web.servlet.context.AnnotationConfigServletWebApplicationContext;
@@ -54,4 +55,12 @@ void shouldUseVirtualThreadsIfEnabled() {
 		});
 	}
 
+	@Test
+	@EnabledForJreRange(min = JRE.JAVA_21)
+	void virtualThreadCustomizationBacksOffWithoutUndertowServlet() {
+		this.contextRunner.withPropertyValues("spring.threads.virtual.enabled=true")
+			.withClassLoader(new FilteredClassLoader("io.undertow.servlet"))
+			.run((context) -> assertThat(context).doesNotHaveBean(UndertowDeploymentInfoCustomizer.class));
+	}
+
 }