Stop resolving AsyncConfigurer instances eagerly

Closes gh-27808

Author: snicoll

spring-context/src/main/java/org/springframework/scheduling/annotation/AbstractAsyncConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -16,18 +16,22 @@
 
 package org.springframework.scheduling.annotation;
 
-import java.util.Collection;
+import java.util.List;
 import java.util.concurrent.Executor;
+import java.util.function.Function;
 import java.util.function.Supplier;
+import java.util.stream.Collectors;
 
 import org.springframework.aop.interceptor.AsyncUncaughtExceptionHandler;
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.ImportAware;
 import org.springframework.core.annotation.AnnotationAttributes;
 import org.springframework.core.type.AnnotationMetadata;
 import org.springframework.lang.Nullable;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.function.SingletonSupplier;
 
 /**
  * Abstract base {@code Configuration} class providing common structure for enabling
@@ -65,17 +69,27 @@ public void setImportMetadata(AnnotationMetadata importMetadata) {
 	/**
 	 * Collect any {@link AsyncConfigurer} beans through autowiring.
 	 */
-	@Autowired(required = false)
-	void setConfigurers(Collection<AsyncConfigurer> configurers) {
-		if (CollectionUtils.isEmpty(configurers)) {
-			return;
-		}
-		if (configurers.size() > 1) {
-			throw new IllegalStateException("Only one AsyncConfigurer may exist");
-		}
-		AsyncConfigurer configurer = configurers.iterator().next();
-		this.executor = configurer::getAsyncExecutor;
-		this.exceptionHandler = configurer::getAsyncUncaughtExceptionHandler;
+	@Autowired
+	void setConfigurers(ObjectProvider<AsyncConfigurer> configurers) {
+		Supplier<AsyncConfigurer> asyncConfigurer = SingletonSupplier.of(() -> {
+			List<AsyncConfigurer> candidates = configurers.stream().collect(Collectors.toList());
+			if (CollectionUtils.isEmpty(candidates)) {
+				return null;
+			}
+			if (candidates.size() > 1) {
+				throw new IllegalStateException("Only one AsyncConfigurer may exist");
+			}
+			return candidates.get(0);
+		});
+		this.executor = adapt(asyncConfigurer, AsyncConfigurer::getAsyncExecutor);
+		this.exceptionHandler = adapt(asyncConfigurer, AsyncConfigurer::getAsyncUncaughtExceptionHandler);
+	}
+
+	private <T> Supplier<T> adapt(Supplier<AsyncConfigurer> supplier, Function<AsyncConfigurer, T> provider) {
+		return () -> {
+			AsyncConfigurer asyncConfigurer = supplier.get();
+			return (asyncConfigurer != null) ? provider.apply(asyncConfigurer) : null;
+		};
 	}
 
 }