Explain that enabling virtual threads disables traditional thread pools

Closes gh-41937

Author: mhalbritter

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

@@ -83,30 +83,32 @@ public static class Pool {
 
 		/**
 		 * Queue capacity. An unbounded capacity does not increase the pool and therefore
-		 * ignores the "max-size" property.
+		 * ignores the "max-size" property. Doesn't have an effect if virtual threads are
+		 * enabled.
 		 */
 		private int queueCapacity = Integer.MAX_VALUE;
 
 		/**
-		 * Core number of threads.
+		 * Core number of threads. Doesn't have an effect if virtual threads are enabled.
 		 */
 		private int coreSize = 8;
 
 		/**
 		 * Maximum allowed number of threads. If tasks are filling up the queue, the pool
 		 * can expand up to that size to accommodate the load. Ignored if the queue is
-		 * unbounded.
+		 * unbounded. Doesn't have an effect if virtual threads are enabled.
 		 */
 		private int maxSize = Integer.MAX_VALUE;
 
 		/**
 		 * Whether core threads are allowed to time out. This enables dynamic growing and
-		 * shrinking of the pool.
+		 * shrinking of the pool. Doesn't have an effect if virtual threads are enabled.
 		 */
 		private boolean allowCoreThreadTimeout = true;
 
 		/**
-		 * Time limit for which threads may remain idle before being terminated.
+		 * Time limit for which threads may remain idle before being terminated. Doesn't
+		 * have an effect if virtual threads are enabled.
 		 */
 		private Duration keepAlive = Duration.ofSeconds(60);
 

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

@@ -63,7 +63,8 @@ public void setThreadNamePrefix(String threadNamePrefix) {
 	public static class Pool {
 
 		/**
-		 * Maximum allowed number of threads.
+		 * Maximum allowed number of threads. Doesn't have an effect if virtual threads
+		 * are enabled.
 		 */
 		private int size = 1;
 

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

@@ -897,12 +897,14 @@ public void setBuffered(boolean buffered) {
 		public static class Threads {
 
 			/**
-			 * Maximum amount of worker threads.
+			 * Maximum amount of worker threads. Doesn't have an effect if virtual threads
+			 * are enabled.
 			 */
 			private int max = 200;
 
 			/**
-			 * Minimum amount of worker threads.
+			 * Minimum amount of worker threads. Doesn't have an effect if virtual threads
+			 * are enabled.
 			 */
 			private int minSpare = 10;
 
@@ -1309,12 +1311,14 @@ public static class Threads {
 			private Integer selectors = -1;
 
 			/**
-			 * Maximum number of threads.
+			 * Maximum number of threads. Doesn't have an effect if virtual threads are
+			 * enabled.
 			 */
 			private Integer max = 200;
 
 			/**
-			 * Minimum number of threads.
+			 * Minimum number of threads. Doesn't have an effect if virtual threads are
+			 * enabled.
 			 */
 			private Integer min = 8;
 

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

@@ -387,6 +387,9 @@ If you're running on Java 21 or up, you can enable virtual threads by setting th
 Before turning on this option for your application, you should consider https://docs.oracle.com/en/java/javase/21/core/virtual-threads.html[reading the official Java virtual threads documentation].
 In some cases, applications can experience lower throughput because of "Pinned Virtual Threads"; this page also explains how to detect such cases with JDK Flight Recorder or the `jcmd` CLI.
 
+NOTE: If virtual threads are enabled, properties which configure thread pools don't have an effect anymore.
+That's because virtual threads are scheduled on a JVM wide platform thread pool and not on dedicated thread pools.
+
 WARNING: One side effect of virtual threads is that they are daemon threads.
 A JVM will exit if all of its threads are daemon threads.
 This behavior can be a problem when you rely on `@Scheduled` beans, for example, to keep your application alive.