Configure Virtual Threads support in Reactor

This commit configures the virtual threads support in Reactor Core for
Schedulers if:
* the current JDK is 21 or higher
* the current environment enables this globally with the
  `"spring.threads.virtual.enabled"` property.

This needs to happen early in the application startup process, as this
feature is detected statically when the first schedulers call is made.
As a result, this is being done with an environment post processor and
not with an auto-configuration class.

Closes gh-36302

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/reactor/ReactorEnvironmentPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,31 +18,34 @@ package org.springframework.boot.reactor;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.env.EnvironmentPostProcessor;
+import org.springframework.boot.system.JavaVersion;
 import org.springframework.core.Ordered;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.util.ClassUtils;
 
 /**
- * {@link EnvironmentPostProcessor} to enable the Reactor Debug Agent if available.
+ * {@link EnvironmentPostProcessor} to enable the Reactor global features as early as
+ * possible in the startup process.
  * <p>
- * The debug agent is enabled by default, unless the
- * {@code "spring.reactor.debug-agent.enabled"} configuration property is set to false. We
- * are using here an {@link EnvironmentPostProcessor} instead of an auto-configuration
- * class to enable the agent as soon as possible during the startup process.
+ * If the "reactor-tools" dependency is available, the debug agent is enabled by default,
+ * unless the {@code "spring.reactor.debug-agent.enabled"} configuration property is set
+ * to false.
+ * <p>
+ * If the {@code "spring.threads.virtual.enabled"} property is enabled and the current JVM
+ * is 21 or later, then the Reactor System property is set to configure the Bounded
+ * Elastic Scheduler to use Virtual Threads globally.
  *
  * @author Brian Clozel
- * @since 2.2.0
+ * @since 3.2.0
  */
-public class DebugAgentEnvironmentPostProcessor implements EnvironmentPostProcessor, Ordered {
+public class ReactorEnvironmentPostProcessor implements EnvironmentPostProcessor, Ordered {
 
 	private static final String REACTOR_DEBUGAGENT_CLASS = "reactor.tools.agent.ReactorDebugAgent";
 
-	private static final String DEBUGAGENT_ENABLED_CONFIG_KEY = "spring.reactor.debug-agent.enabled";
-
 	@Override
 	public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
 		if (ClassUtils.isPresent(REACTOR_DEBUGAGENT_CLASS, null)) {
-			Boolean agentEnabled = environment.getProperty(DEBUGAGENT_ENABLED_CONFIG_KEY, Boolean.class);
+			Boolean agentEnabled = environment.getProperty("spring.reactor.debug-agent.enabled", Boolean.class);
 			if (agentEnabled != Boolean.FALSE) {
 				try {
 					Class<?> debugAgent = Class.forName(REACTOR_DEBUGAGENT_CLASS);
@@ -53,6 +56,10 @@ public class DebugAgentEnvironmentPostProcessor implements EnvironmentPostProces
 				}
 			}
 		}
+		if (environment.getProperty("spring.threads.virtual.enabled", boolean.class, false)
+				&& JavaVersion.getJavaVersion().isEqualOrNewerThan(JavaVersion.TWENTY_ONE)) {
+			System.setProperty("reactor.schedulers.defaultBoundedElasticOnVirtualThreads", "true");
+		}
 	}
 
 	@Override

spring-boot-project/spring-boot/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -491,7 +491,7 @@
     {
       "name": "spring.reactor.debug-agent.enabled",
       "type": "java.lang.Boolean",
-      "sourceType": "org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor",
+      "sourceType": "org.springframework.boot.reactor.ReactorEnvironmentPostProcessor",
       "description": "Whether the Reactor Debug Agent should be enabled when reactor-tools is present.",
       "defaultValue": true
     },

spring-boot-project/spring-boot/src/main/resources/META-INF/spring.factories

@@ -57,7 +57,7 @@ org.springframework.boot.context.config.ConfigDataEnvironmentPostProcessor,\
 org.springframework.boot.env.RandomValuePropertySourceEnvironmentPostProcessor,\
 org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
 org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\
-org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor
+org.springframework.boot.reactor.ReactorEnvironmentPostProcessor
 
 # Failure Analyzers
 org.springframework.boot.diagnostics.FailureAnalyzer=\

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/reactor/InstrumentedFluxProvider.java

@@ -22,7 +22,7 @@ import reactor.core.publisher.Flux;
  * Utility class that should be instrumented by the reactor debug agent.
  *
  * @author Brian Clozel
- * @see DebugAgentEnvironmentPostProcessorTests
+ * @see ReactorEnvironmentPostProcessorTests
  */
 class InstrumentedFluxProvider {
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/reactor/ReactorEnvironmentPostProcessorTests.java

@@ -16,8 +16,10 @@
 
 package org.springframework.boot.reactor;
 
-import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledForJreRange;
+import org.junit.jupiter.api.condition.JRE;
 import reactor.core.Scannable;
 import reactor.core.publisher.Flux;
 
@@ -27,17 +29,18 @@ import org.springframework.mock.env.MockEnvironment;
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
- * Tests for {@link DebugAgentEnvironmentPostProcessor}.
+ * Tests for {@link ReactorEnvironmentPostProcessor}.
  *
  * @author Brian Clozel
  */
-@Disabled("We need the not-yet-released reactor-tools 3.4.11 for JDK 17 compatibility")
-@ClassPathOverrides("io.projectreactor:reactor-tools:3.4.11")
-class DebugAgentEnvironmentPostProcessorTests {
+
+@ClassPathOverrides("io.projectreactor:reactor-tools:3.5.9")
+class ReactorEnvironmentPostProcessorTests {
 
 	static {
 		MockEnvironment environment = new MockEnvironment();
-		DebugAgentEnvironmentPostProcessor postProcessor = new DebugAgentEnvironmentPostProcessor();
+		environment.setProperty("spring.threads.virtual.enabled", "true");
+		ReactorEnvironmentPostProcessor postProcessor = new ReactorEnvironmentPostProcessor();
 		postProcessor.postProcessEnvironment(environment, null);
 	}
 
@@ -49,4 +52,21 @@ class DebugAgentEnvironmentPostProcessorTests {
 			.startsWith("Flux.just ⇢ at org.springframework.boot.reactor.InstrumentedFluxProvider.newFluxJust");
 	}
 
+	@Test
+	@EnabledForJreRange(max = JRE.JAVA_20)
+	void shouldNotEnableVirtualThreads() {
+		assertThat(System.getProperty("reactor.schedulers.defaultBoundedElasticOnVirtualThreads")).isNotEqualTo("true");
+	}
+
+	@Test
+	@EnabledForJreRange(min = JRE.JAVA_21)
+	void shouldEnableVirtualThreads() {
+		assertThat(System.getProperty("reactor.schedulers.defaultBoundedElasticOnVirtualThreads")).isEqualTo("true");
+	}
+
+	@AfterEach
+	void cleanup() {
+		System.setProperty("reactor.schedulers.defaultBoundedElasticOnVirtualThreads", "false");
+	}
+
 }