Fix detection of ScheduledExecutorService

`@EnableScheduling` detects primarily a suitable `TaskScheduler` in the
context and fallbacks to the presence of a `ScheduledExecutorService` if
that is not the case.

This commit improves the auto-configuration to back off when such
scheduled executor service is present, so that the framework
initialization code can pick it up as usual.

Closes gh-15032

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/task/TaskSchedulingAutoConfiguration.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.autoconfigure.task;
 
+import java.util.concurrent.ScheduledExecutorService;
+
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
@@ -44,7 +46,8 @@ public class TaskSchedulingAutoConfiguration {
 
 	@Bean
 	@ConditionalOnBean(name = TaskManagementConfigUtils.SCHEDULED_ANNOTATION_PROCESSOR_BEAN_NAME)
-	@ConditionalOnMissingBean({ SchedulingConfigurer.class, TaskScheduler.class })
+	@ConditionalOnMissingBean({ SchedulingConfigurer.class, TaskScheduler.class,
+			ScheduledExecutorService.class })
 	public ThreadPoolTaskScheduler taskScheduler(TaskSchedulerBuilder builder) {
 		return builder.build();
 	}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/task/TaskSchedulingAutoConfigurationTests.java

@@ -18,6 +18,8 @@ package org.springframework.boot.autoconfigure.task;
 
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
 
 import org.junit.Test;
 
@@ -96,6 +98,19 @@ public class TaskSchedulingAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	public void enableSchedulingWithExistingScheduledExecutorServiceBacksOff() {
+		this.contextRunner.withUserConfiguration(SchedulingConfiguration.class,
+				ScheduledExecutorServiceConfiguration.class).run((context) -> {
+					assertThat(context).doesNotHaveBean(TaskScheduler.class);
+					assertThat(context).hasSingleBean(ScheduledExecutorService.class);
+					TestBean bean = context.getBean(TestBean.class);
+					Thread.sleep(15);
+					assertThat(bean.threadNames)
+							.allMatch((name) -> name.contains("pool-"));
+				});
+	}
+
 	@Test
 	public void enableSchedulingWithConfigurerBacksOff() {
 		this.contextRunner.withUserConfiguration(SchedulingConfiguration.class,
@@ -123,6 +138,16 @@ public class TaskSchedulingAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	static class ScheduledExecutorServiceConfiguration {
+
+		@Bean
+		public ScheduledExecutorService customScheduledExecutorService() {
+			return Executors.newScheduledThreadPool(2);
+		}
+
+	}
+
 	@Configuration
 	static class TaskSchedulerCustomizerConfiguration {
 

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -6382,8 +6382,8 @@ A `ThreadPoolTaskScheduler` can also be auto-configured if need to be associated
 scheduled task execution (`@EnableScheduling`). The thread pool uses one thread by default
 and those settings can be fine-tuned using the `spring.task.scheduling` namespace.
 
-Both a `TaskExecutorBuilder` bean and a `TaskSchedulerBuilder` bean are made available in the
-context if a custom executor or scheduler needs to be created.
+Both a `TaskExecutorBuilder` bean and a `TaskSchedulerBuilder` bean are made available in
+the context if a custom executor or scheduler needs to be created.