Consistent lazy resolution of default executor/scheduler for Async/ScheduledAnnotationBeanPostProcessor

Issue: SPR-14030

spring-aop/src/main/java/org/springframework/aop/interceptor/AsyncExecutionAspectSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -31,9 +31,12 @@ import org.apache.commons.logging.LogFactory;
 
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
+import org.springframework.beans.factory.NoSuchBeanDefinitionException;
+import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
 import org.springframework.beans.factory.annotation.BeanFactoryAnnotationUtils;
 import org.springframework.core.task.AsyncListenableTaskExecutor;
 import org.springframework.core.task.AsyncTaskExecutor;
+import org.springframework.core.task.TaskExecutor;
 import org.springframework.core.task.support.TaskExecutorAdapter;
 import org.springframework.lang.UsesJava8;
 import org.springframework.util.ClassUtils;
@@ -58,6 +61,15 @@ import org.springframework.util.concurrent.ListenableFuture;
  */
 public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 
+	/**
+	 * The default name of the {@link TaskExecutor} bean to pick up: "taskExecutor".
+	 * <p>Note that the initial lookup happens by type; this is just the fallback
+	 * in case of multiple executor beans found in the context.
+	 * @since 4.2.6
+	 */
+	public static final String DEFAULT_TASK_EXECUTOR_BEAN_NAME = "taskExecutor";
+
+
 	// Java 8's CompletableFuture type present?
 	private static final boolean completableFuturePresent = ClassUtils.isPresent(
 			"java.util.concurrent.CompletableFuture", AsyncExecutionInterceptor.class.getClassLoader());
@@ -67,7 +79,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 
 	private final Map<Method, AsyncTaskExecutor> executors = new ConcurrentHashMap<Method, AsyncTaskExecutor>(16);
 
-	private Executor defaultExecutor;
+	private volatile Executor defaultExecutor;
 
 	private AsyncUncaughtExceptionHandler exceptionHandler;
 
@@ -75,10 +87,22 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 
 
 	/**
-	 * Create a new {@link AsyncExecutionAspectSupport}, using the provided default
+	 * Create a new instance with a default {@link AsyncUncaughtExceptionHandler}.
-	 * executor unless individual async methods indicate via qualifier that a more
+	 * @param defaultExecutor the {@code Executor} (typically a Spring {@code AsyncTaskExecutor}
-	 * specific executor should be used.
+	 * or {@link java.util.concurrent.ExecutorService}) to delegate to, unless a more specific
-	 * @param defaultExecutor the executor to use when executing asynchronous methods
+	 * executor has been requested via a qualifier on the async method, in which case the
+	 * executor will be looked up at invocation time against the enclosing bean factory
+	 */
+	public AsyncExecutionAspectSupport(Executor defaultExecutor) {
+		this(defaultExecutor, new SimpleAsyncUncaughtExceptionHandler());
+	}
+
+	/**
+	 * Create a new {@link AsyncExecutionAspectSupport} with the given exception handler.
+	 * @param defaultExecutor the {@code Executor} (typically a Spring {@code AsyncTaskExecutor}
+	 * or {@link java.util.concurrent.ExecutorService}) to delegate to, unless a more specific
+	 * executor has been requested via a qualifier on the async method, in which case the
+	 * executor will be looked up at invocation time against the enclosing bean factory
 	 * @param exceptionHandler the {@link AsyncUncaughtExceptionHandler} to use
 	 */
 	public AsyncExecutionAspectSupport(Executor defaultExecutor, AsyncUncaughtExceptionHandler exceptionHandler) {
@@ -86,22 +110,14 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 		this.exceptionHandler = exceptionHandler;
 	}
 
-	/**
-	 * Create a new instance with a default {@link AsyncUncaughtExceptionHandler}.
-	 */
-	public AsyncExecutionAspectSupport(Executor defaultExecutor) {
-		this(defaultExecutor, new SimpleAsyncUncaughtExceptionHandler());
-	}
-
 
 	/**
 	 * Supply the executor to be used when executing async methods.
-	 * @param defaultExecutor the {@code Executor} (typically a Spring {@code
+	 * @param defaultExecutor the {@code Executor} (typically a Spring {@code AsyncTaskExecutor}
-	 * AsyncTaskExecutor} or {@link java.util.concurrent.ExecutorService}) to delegate to
+	 * or {@link java.util.concurrent.ExecutorService}) to delegate to, unless a more specific
-	 * unless a more specific executor has been requested via a qualifier on the async
+	 * executor has been requested via a qualifier on the async method, in which case the
-	 * method, in which case the executor will be looked up at invocation time against the
+	 * executor will be looked up at invocation time against the enclosing bean factory
-	 * enclosing bean factory.
+	 * @see #getExecutorQualifier(Method)
-	 * @see #getExecutorQualifier
 	 * @see #setBeanFactory(BeanFactory)
 	 */
 	public void setExecutor(Executor defaultExecutor) {
@@ -128,26 +144,32 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	/**
 	 * Determine the specific executor to use when executing the given method.
 	 * Should preferably return an {@link AsyncListenableTaskExecutor} implementation.
-	 * @return the executor to use (or {@code null}, but just if no default executor has been set)
+	 * @return the executor to use (or {@code null}, but just if no default executor is available)
 	 */
 	protected AsyncTaskExecutor determineAsyncExecutor(Method method) {
 		AsyncTaskExecutor executor = this.executors.get(method);
 		if (executor == null) {
-			Executor executorToUse = this.defaultExecutor;
+			Executor targetExecutor;
 			String qualifier = getExecutorQualifier(method);
 			if (StringUtils.hasLength(qualifier)) {
-				if (this.beanFactory == null) {
+				targetExecutor = findQualifiedExecutor(this.beanFactory, qualifier);
-					throw new IllegalStateException("BeanFactory must be set on " + getClass().getSimpleName() +
-							" to access qualified executor '" + qualifier + "'");
-				}
-				executorToUse = BeanFactoryAnnotationUtils.qualifiedBeanOfType(
-						this.beanFactory, Executor.class, qualifier);
 			}
-			else if (executorToUse == null) {
+			else {
+				targetExecutor = this.defaultExecutor;
+				if (targetExecutor == null) {
+					synchronized (this.executors) {
+						if (this.defaultExecutor == null) {
+							this.defaultExecutor = getDefaultExecutor(this.beanFactory);
+						}
+						targetExecutor = this.defaultExecutor;
+					}
+				}
+			}
+			if (targetExecutor == null) {
 				return null;
 			}
-			executor = (executorToUse instanceof AsyncListenableTaskExecutor ?
+			executor = (targetExecutor instanceof AsyncListenableTaskExecutor ?
-					(AsyncListenableTaskExecutor) executorToUse : new TaskExecutorAdapter(executorToUse));
+					(AsyncListenableTaskExecutor) targetExecutor : new TaskExecutorAdapter(targetExecutor));
 			this.executors.put(method, executor);
 		}
 		return executor;
@@ -160,11 +182,69 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	 * been specified and that the {@linkplain #setExecutor(Executor) default executor}
 	 * should be used.
 	 * @param method the method to inspect for executor qualifier metadata
-	 * @return the qualifier if specified, otherwise empty string or {@code null}
+	 * @return the qualifier if specified, otherwise empty String or {@code null}
 	 * @see #determineAsyncExecutor(Method)
+	 * @see #findQualifiedExecutor(BeanFactory, String)
 	 */
 	protected abstract String getExecutorQualifier(Method method);
 
+	/**
+	 * Retrieve a target executor for the given qualifier.
+	 * @param qualifier the qualifier to resolve
+	 * @return the target executor, or {@code null} if none available
+	 * @since 4.2.6
+	 * @see #getExecutorQualifier(Method)
+	 */
+	protected Executor findQualifiedExecutor(BeanFactory beanFactory, String qualifier) {
+		if (beanFactory == null) {
+			throw new IllegalStateException("BeanFactory must be set on " + getClass().getSimpleName() +
+					" to access qualified executor '" + qualifier + "'");
+		}
+		return BeanFactoryAnnotationUtils.qualifiedBeanOfType(beanFactory, Executor.class, qualifier);
+	}
+
+	/**
+	 * Retrieve or build a default executor for this advice instance.
+	 * An executor returned from here will be cached for further use.
+	 * <p>The default implementation searches for a unique {@link TaskExecutor} bean
+	 * in the context, or for an {@link Executor} bean named "taskExecutor" otherwise.
+	 * If neither of the two is resolvable, this implementation will return {@code null}.
+	 * @param beanFactory the BeanFactory to use for a default executor lookup
+	 * @return the default executor, or {@code null} if none available
+	 * @since 4.2.6
+	 * @see #findQualifiedExecutor(BeanFactory, String)
+	 * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
+	 */
+	protected Executor getDefaultExecutor(BeanFactory beanFactory) {
+		if (beanFactory != null) {
+			try {
+				// Search for TaskExecutor bean... not plain Executor since that would
+				// match with ScheduledExecutorService as well, which is unusable for
+				// our purposes here. TaskExecutor is more clearly designed for it.
+				return beanFactory.getBean(TaskExecutor.class);
+			}
+			catch (NoUniqueBeanDefinitionException ex) {
+				try {
+					return beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
+				}
+				catch (NoSuchBeanDefinitionException ex2) {
+					if (logger.isInfoEnabled()) {
+						logger.info("More than one TaskExecutor bean found within the context, and none is named " +
+								"'taskExecutor'. Mark one of them as primary or name it 'taskExecutor' (possibly " +
+								"as an alias) in order to use it for async processing: " + ex.getBeanNamesFound());
+					}
+				}
+			}
+			catch (NoSuchBeanDefinitionException ex) {
+				logger.debug("Could not find default TaskExecutor bean", ex);
+				// Giving up -> either using local default executor or none at all...
+				logger.info("No TaskExecutor bean found for async processing");
+			}
+		}
+		return null;
+	}
+
+
 	/**
 	 * Delegate for actually executing the given task with the chosen executor.
 	 * @param task the task to execute

spring-aop/src/main/java/org/springframework/aop/interceptor/AsyncExecutionInterceptor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -26,9 +26,11 @@ import org.aopalliance.intercept.MethodInterceptor;
 import org.aopalliance.intercept.MethodInvocation;
 
 import org.springframework.aop.support.AopUtils;
+import org.springframework.beans.factory.BeanFactory;
 import org.springframework.core.BridgeMethodResolver;
 import org.springframework.core.Ordered;
 import org.springframework.core.task.AsyncTaskExecutor;
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -65,22 +67,27 @@ import org.springframework.util.ClassUtils;
  */
 public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport implements MethodInterceptor, Ordered {
 
+	/**
+	 * Create a new instance with a default {@link AsyncUncaughtExceptionHandler}.
+	 * @param defaultExecutor the {@link Executor} (typically a Spring {@link AsyncTaskExecutor}
+	 * or {@link java.util.concurrent.ExecutorService}) to delegate to;
+	 * as of 4.2.6, a local executor for this interceptor will be built otherwise
+	 */
+	public AsyncExecutionInterceptor(Executor defaultExecutor) {
+		super(defaultExecutor);
+	}
+
 	/**
 	 * Create a new {@code AsyncExecutionInterceptor}.
 	 * @param defaultExecutor the {@link Executor} (typically a Spring {@link AsyncTaskExecutor}
-	 * or {@link java.util.concurrent.ExecutorService}) to delegate to
+	 * or {@link java.util.concurrent.ExecutorService}) to delegate to;
+	 * as of 4.2.6, a local executor for this interceptor will be built otherwise
 	 * @param exceptionHandler the {@link AsyncUncaughtExceptionHandler} to use
 	 */
 	public AsyncExecutionInterceptor(Executor defaultExecutor, AsyncUncaughtExceptionHandler exceptionHandler) {
 		super(defaultExecutor, exceptionHandler);
 	}
 
-	/**
-	 * Create a new instance with a default {@link AsyncUncaughtExceptionHandler}.
-	 */
-	public AsyncExecutionInterceptor(Executor defaultExecutor) {
-		super(defaultExecutor);
-	}
 
 	/**
 	 * Intercept the given method invocation, submit the actual calling of the method to
@@ -136,6 +143,20 @@ public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport imple
 		return null;
 	}
 
+	/**
+	 * This implementation searches for a unique {@link org.springframework.core.task.TaskExecutor}
+	 * bean in the context, or for an {@link Executor} bean named "taskExecutor" otherwise.
+	 * If neither of the two is resolvable (e.g. if no {@code BeanFactory} was configured at all),
+	 * this implementation falls back to a newly created {@link SimpleAsyncTaskExecutor} instance
+	 * for local use if no default could be found.
+	 * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
+	 */
+	@Override
+	protected Executor getDefaultExecutor(BeanFactory beanFactory) {
+		Executor defaultExecutor = super.getDefaultExecutor(beanFactory);
+		return (defaultExecutor != null ? defaultExecutor : new SimpleAsyncTaskExecutor());
+	}
+
 	@Override
 	public int getOrder() {
 		return Ordered.HIGHEST_PRECEDENCE;

spring-aspects/src/main/java/org/springframework/scheduling/aspectj/AbstractAsyncExecutionAspect.aj

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -54,8 +54,8 @@ public abstract aspect AbstractAsyncExecutionAspect extends AsyncExecutionAspect
 	 * Apply around advice to methods matching the {@link #asyncMethod()} pointcut,
 	 * submit the actual calling of the method to the correct task executor and return
 	 * immediately to the caller.
-	 * @return {@link Future} if the original method returns {@code Future}; {@code null}
+	 * @return {@link Future} if the original method returns {@code Future};
-	 * otherwise.
+	 * {@code null} otherwise
 	 */
 	@SuppressAjWarnings("adviceDidNotMatch")
 	Object around() : asyncMethod() {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -42,7 +42,8 @@ public class AnnotationAsyncExecutionInterceptor extends AsyncExecutionIntercept
 	 * Create a new {@code AnnotationAsyncExecutionInterceptor} with the given executor
 	 * and a simple {@link AsyncUncaughtExceptionHandler}.
 	 * @param defaultExecutor the executor to be used by default if no more specific
-	 * executor has been qualified at the method level using {@link Async#value()}
+	 * executor has been qualified at the method level using {@link Async#value()};
+	 * as of 4.2.6, a local executor for this interceptor will be built otherwise
 	 */
 	public AnnotationAsyncExecutionInterceptor(Executor defaultExecutor) {
 		super(defaultExecutor);
@@ -51,7 +52,8 @@ public class AnnotationAsyncExecutionInterceptor extends AsyncExecutionIntercept
 	/**
 	 * Create a new {@code AnnotationAsyncExecutionInterceptor} with the given executor.
 	 * @param defaultExecutor the executor to be used by default if no more specific
-	 * executor has been qualified at the method level using {@link Async#value()}
+	 * executor has been qualified at the method level using {@link Async#value()};
+	 * as of 4.2.6, a local executor for this interceptor will be built otherwise
 	 * @param exceptionHandler the {@link AsyncUncaughtExceptionHandler} to use to
 	 * handle exceptions thrown by asynchronous method executions with {@code void}
 	 * return type
@@ -74,7 +76,7 @@ public class AnnotationAsyncExecutionInterceptor extends AsyncExecutionIntercept
 	 */
 	@Override
 	protected String getExecutorQualifier(Method method) {
-		// maintainer's note: changes made here should also be made in
+		// Maintainer's note: changes made here should also be made in
 		// AnnotationAsyncExecutionAspect#getExecutorQualifier
 		Async async = AnnotationUtils.findAnnotation(method, Async.class);
 		if (async == null) {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -32,7 +32,6 @@ import org.springframework.aop.support.ComposablePointcut;
 import org.springframework.aop.support.annotation.AnnotationMatchingPointcut;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
-import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
@@ -73,8 +72,10 @@ public class AsyncAnnotationAdvisor extends AbstractPointcutAdvisor implements B
 	/**
 	 * Create a new {@code AsyncAnnotationAdvisor} for the given task executor.
 	 * @param executor the task executor to use for asynchronous methods
-	 * @param exceptionHandler the {@link org.springframework.aop.interceptor.AsyncUncaughtExceptionHandler} to use to
+	 * (can be {@code null} to trigger default executor resolution)
+	 * @param exceptionHandler the {@link AsyncUncaughtExceptionHandler} to use to
 	 * handle unexpected exception thrown by asynchronous method executions
+	 * @see AnnotationAsyncExecutionInterceptor#getDefaultExecutor(BeanFactory)
 	 */
 	@SuppressWarnings("unchecked")
 	public AsyncAnnotationAdvisor(Executor executor, AsyncUncaughtExceptionHandler exceptionHandler) {
@@ -87,9 +88,6 @@ public class AsyncAnnotationAdvisor extends AbstractPointcutAdvisor implements B
 		catch (ClassNotFoundException ex) {
 			// If EJB 3.1 API not present, simply ignore.
 		}
-		if (executor == null) {
-			executor = new SimpleAsyncTaskExecutor();
-		}
 		if (exceptionHandler != null) {
 			this.exceptionHandler = exceptionHandler;
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -25,8 +25,6 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.aop.framework.autoproxy.AbstractBeanFactoryAwareAdvisingPostProcessor;
 import org.springframework.aop.interceptor.AsyncUncaughtExceptionHandler;
 import org.springframework.beans.factory.BeanFactory;
-import org.springframework.beans.factory.NoSuchBeanDefinitionException;
-import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
 import org.springframework.core.task.TaskExecutor;
 import org.springframework.util.Assert;
 
@@ -68,8 +66,10 @@ public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAd
 	 * <p>Note that the initial lookup happens by type; this is just the fallback
 	 * in case of multiple executor beans found in the context.
 	 * @since 4.2
+	 * @see AnnotationAsyncExecutionInterceptor#DEFAULT_TASK_EXECUTOR_BEAN_NAME
 	 */
-	public static final String DEFAULT_TASK_EXECUTOR_BEAN_NAME = "taskExecutor";
+	public static final String DEFAULT_TASK_EXECUTOR_BEAN_NAME =
+			AnnotationAsyncExecutionInterceptor.DEFAULT_TASK_EXECUTOR_BEAN_NAME;
 
 
 	protected final Log logger = LogFactory.getLog(getClass());
@@ -102,6 +102,13 @@ public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAd
 
 	/**
 	 * Set the {@link Executor} to use when invoking methods asynchronously.
+	 * <p>If not specified, default executor resolution will apply: searching for a
+	 * unique {@link TaskExecutor} bean in the context, or for an {@link Executor}
+	 * bean named "taskExecutor" otherwise. If neither of the two is resolvable,
+	 * a local default executor will be created within the interceptor.
+	 * @see AsyncAnnotationAdvisor#AsyncAnnotationAdvisor(Executor, AsyncUncaughtExceptionHandler)
+	 * @see AnnotationAsyncExecutionInterceptor#getDefaultExecutor(BeanFactory)
+	 * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
 	 */
 	public void setExecutor(Executor executor) {
 		this.executor = executor;
@@ -121,31 +128,7 @@ public class AsyncAnnotationBeanPostProcessor extends AbstractBeanFactoryAwareAd
 	public void setBeanFactory(BeanFactory beanFactory) {
 		super.setBeanFactory(beanFactory);
 
-		Executor executorToUse = this.executor;
+		AsyncAnnotationAdvisor advisor = new AsyncAnnotationAdvisor(this.executor, this.exceptionHandler);
-		if (executorToUse == null) {
-			try {
-				// Search for TaskExecutor bean... not plain Executor since that would
-				// match with ScheduledExecutorService as well, which is unusable for
-				// our purposes here. TaskExecutor is more clearly designed for it.
-				executorToUse = beanFactory.getBean(TaskExecutor.class);
-			}
-			catch (NoUniqueBeanDefinitionException ex) {
-				try {
-					executorToUse = beanFactory.getBean(DEFAULT_TASK_EXECUTOR_BEAN_NAME, Executor.class);
-				}
-				catch (NoSuchBeanDefinitionException ex2) {
-					logger.info("More than one TaskExecutor bean found within the context, and none is " +
-							"named 'taskExecutor'. Mark one of them as primary or name it 'taskExecutor' " +
-							"(possibly as an alias) in order to use it for async annotation processing.");
-				}
-			}
-			catch (NoSuchBeanDefinitionException ex) {
-				logger.info("No TaskExecutor bean found for async annotation processing.");
-				// Giving up -> falling back to default executor within the advisor...
-			}
-		}
-
-		AsyncAnnotationAdvisor advisor =  new AsyncAnnotationAdvisor(executorToUse, this.exceptionHandler);
 		if (this.asyncAnnotationType != null) {
 			advisor.setAsyncAnnotationType(this.asyncAnnotationType);
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -41,26 +41,28 @@ import org.springframework.core.Ordered;
  * @Configuration
  * @EnableAsync
  * public class AppConfig {
+ *
  *     @Bean
  *     public MyAsyncBean asyncBean() {
  *         return new MyAsyncBean();
  *     }
  * }</pre>
  *
- *
  * <p>The {@link #mode} attribute controls how advice is applied; if the mode is
  * {@link AdviceMode#PROXY} (the default), then the other attributes control the behavior
  * of the proxying.
  *
- * <p>Note that if the {@linkplain #mode} is set to {@link AdviceMode#ASPECTJ}, then
+ * <p>Note that if the {@linkplain #mode} is set to {@link AdviceMode#ASPECTJ}, then the
- * the value of the {@link #proxyTargetClass} attribute will be ignored. Note also
+ * value of the {@link #proxyTargetClass} attribute will be ignored. Note also that in
- * that in this case the {@code spring-aspects} module JAR must be present on the
+ * this case the {@code spring-aspects} module JAR must be present on the classpath.
- * classpath.
  *
- * <p>By default, a {@link org.springframework.core.task.SimpleAsyncTaskExecutor
+ * <p>By default, Spring will be searching for an associated thread pool definition:
- * SimpleAsyncTaskExecutor} will be used to process async method invocations. Besides,
+ * either a unique {@link org.springframework.core.task.TaskExecutor} bean in the context,
- * annotated methods having a {@code void} return type cannot transmit any exception
+ * or an {@link java.util.concurrent.Executor} bean named "taskExecutor" otherwise. If
- * back to the caller. By default, such uncaught exceptions are only logged.
+ * neither of the two is resolvable, a {@link org.springframework.core.task.SimpleAsyncTaskExecutor}
+ * will be used to process async method invocations. Besides, annotated methods having a
+ * {@code void} return type cannot transmit any exception back to the caller. By default,
+ * such uncaught exceptions are only logged.
  *
  * <p>To customize all this, implement {@link AsyncConfigurer} and
  * provide:
@@ -129,6 +131,7 @@ import org.springframework.core.Ordered;
  * through direct access to actual componentry.
  *
  * @author Chris Beams
+ * @author Juergen Hoeller
  * @author Stephane Nicoll
  * @author Sam Brannen
  * @since 3.1
@@ -175,9 +178,8 @@ public @interface EnableAsync {
 	AdviceMode mode() default AdviceMode.PROXY;
 
 	/**
-	 * Indicate the order in which the
+	 * Indicate the order in which the {@link AsyncAnnotationBeanPostProcessor}
-	 * {@link org.springframework.scheduling.annotation.AsyncAnnotationBeanPostProcessor
+	 * should be applied.
-	 * AsyncAnnotationBeanPostProcessor} should be applied.
 	 * <p>The default is {@link Ordered#LOWEST_PRECEDENCE} in order to run
 	 * after all other post-processors, so that it can add an advisor to
 	 * existing proxies rather than double-proxy.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -37,16 +37,18 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  * @Configuration
  * @EnableScheduling
  * public class AppConfig {
+ *
  *     // various @Bean definitions
  * }</pre>
  *
  * This enables detection of @{@link Scheduled} annotations on any Spring-managed
- * bean in the container.  For example, given a class {@code MyTask}
+ * bean in the container. For example, given a class {@code MyTask}
  *
  * <pre class="code">
  * package com.myco.tasks;
  *
  * public class MyTask {
+ *
  *     &#064;Scheduled(fixedRate=1000)
  *     public void work() {
  *         // task execution logic
@@ -60,6 +62,7 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  * &#064;Configuration
  * &#064;EnableScheduling
  * public class AppConfig {
+ *
  *     &#064;Bean
  *     public MyTask task() {
  *         return new MyTask();
@@ -84,14 +87,21 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  * &#064;Configuration
  * &#064;EnableScheduling
  * public class AppConfig {
+ *
  *     &#064;Scheduled(fixedRate=1000)
  *     public void work() {
  *         // task execution logic
  *     }
  * }</pre>
  *
- * In all of the above scenarios, a default single-threaded task executor is used.
+ * <p>By default, will be searching for an associated scheduler definition: either
- * When more control is desired, a {@code @Configuration} class may implement
+ * a unique {@link org.springframework.scheduling.TaskScheduler} bean in the context,
+ * or a {@code TaskScheduler} bean named "taskScheduler" otherwise; the same lookup
+ * will also be performed for a {@link java.util.concurrent.ScheduledExecutorService}
+ * bean. If neither of the two is resolvable, a local single-threaded default
+ * scheduler will be created and used within the registrar.
+ *
+ * <p>When more control is desired, a {@code @Configuration} class may implement
  * {@link SchedulingConfigurer}. This allows access to the underlying
  * {@link ScheduledTaskRegistrar} instance. For example, the following example
  * demonstrates how to customize the {@link Executor} used to execute scheduled
@@ -101,6 +111,7 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  * &#064;Configuration
  * &#064;EnableScheduling
  * public class AppConfig implements SchedulingConfigurer {
+ *
  *     &#064;Override
  *     public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
  *         taskRegistrar.setScheduler(taskExecutor());
@@ -112,11 +123,11 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  *     }
  * }</pre>
  *
- * Note in the example above the use of {@code @Bean(destroyMethod="shutdown")}. This
+ * <p>Note in the example above the use of {@code @Bean(destroyMethod="shutdown")}.
- * ensures that the task executor is properly shut down when the Spring application
+ * This ensures that the task executor is properly shut down when the Spring
- * context itself is closed.
+ * application context itself is closed.
  *
- * Implementing {@code SchedulingConfigurer} also allows for fine-grained
+ * <p>Implementing {@code SchedulingConfigurer} also allows for fine-grained
  * control over task registration via the {@code ScheduledTaskRegistrar}.
  * For example, the following configures the execution of a particular bean
  * method per a custom {@code Trigger} implementation:
@@ -125,6 +136,7 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  * &#064;Configuration
  * &#064;EnableScheduling
  * public class AppConfig implements SchedulingConfigurer {
+ *
  *     &#064;Override
  *     public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
  *         taskRegistrar.setScheduler(taskScheduler());
@@ -167,6 +179,7 @@ import org.springframework.scheduling.config.ScheduledTaskRegistrar;
  * through direct access to actual componentry.<p>
  *
  * @author Chris Beams
+ * @author Juergen Hoeller
  * @since 3.1
  * @see Scheduled
  * @see SchedulingConfiguration

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -89,6 +89,7 @@ public class ScheduledAnnotationBeanPostProcessor implements BeanPostProcessor,
 	 * The default name of the {@link TaskScheduler} bean to pick up: "taskScheduler".
 	 * <p>Note that the initial lookup happens by type; this is just the fallback
 	 * in case of multiple scheduler beans found in the context.
+	 * @since 4.2
 	 */
 	public static final String DEFAULT_TASK_SCHEDULER_BEAN_NAME = "taskScheduler";
 
@@ -118,6 +119,12 @@ public class ScheduledAnnotationBeanPostProcessor implements BeanPostProcessor,
 	 * Set the {@link org.springframework.scheduling.TaskScheduler} that will invoke
 	 * the scheduled methods, or a {@link java.util.concurrent.ScheduledExecutorService}
 	 * to be wrapped as a TaskScheduler.
+	 * <p>If not specified, default scheduler resolution will apply: searching for a
+	 * unique {@link TaskScheduler} bean in the context, or for a {@link TaskScheduler}
+	 * bean named "taskScheduler" otherwise; the same lookup will also be performed for
+	 * a {@link ScheduledExecutorService} bean. If neither of the two is resolvable,
+	 * a local single-threaded default scheduler will be created within the registrar.
+	 * @see #DEFAULT_TASK_SCHEDULER_BEAN_NAME
 	 */
 	public void setScheduler(Object scheduler) {
 		this.scheduler = scheduler;
@@ -195,10 +202,13 @@ public class ScheduledAnnotationBeanPostProcessor implements BeanPostProcessor,
 							this.beanFactory.getBean(DEFAULT_TASK_SCHEDULER_BEAN_NAME, TaskScheduler.class));
 				}
 				catch (NoSuchBeanDefinitionException ex2) {
-					throw new IllegalStateException("More than one TaskScheduler bean exists within the context, and " +
+					if (logger.isInfoEnabled()) {
-							"none is named 'taskScheduler'. Mark one of them as primary or name it 'taskScheduler' "+
+						logger.info("More than one TaskScheduler bean exists within the context, and " +
-							"(possibly as an alias); or implement the SchedulingConfigurer interface and call " +
+								"none is named 'taskScheduler'. Mark one of them as primary or name it 'taskScheduler' " +
-							"ScheduledTaskRegistrar#setScheduler explicitly within the configureTasks() callback.", ex);
+								"(possibly as an alias); or implement the SchedulingConfigurer interface and call " +
+								"ScheduledTaskRegistrar#setScheduler explicitly within the configureTasks() callback: " +
+								ex.getBeanNamesFound());
+					}
 				}
 			}
 			catch (NoSuchBeanDefinitionException ex) {
@@ -208,14 +218,24 @@ public class ScheduledAnnotationBeanPostProcessor implements BeanPostProcessor,
 					this.registrar.setScheduler(this.beanFactory.getBean(ScheduledExecutorService.class));
 				}
 				catch (NoUniqueBeanDefinitionException ex2) {
-					throw new IllegalStateException("More than one ScheduledExecutorService bean exists within " +
+					try {
-							"the context. Mark one of them as primary; or implement the SchedulingConfigurer " +
+						this.registrar.setScheduler(
-							"interface and call ScheduledTaskRegistrar#setScheduler explicitly within the " +
+								this.beanFactory.getBean(DEFAULT_TASK_SCHEDULER_BEAN_NAME, ScheduledExecutorService.class));
-							"configureTasks() callback.", ex);
+					}
+					catch (NoSuchBeanDefinitionException ex3) {
+						if (logger.isInfoEnabled()) {
+							logger.info("More than one ScheduledExecutorService bean exists within the context, and " +
+									"none is named 'taskScheduler'. Mark one of them as primary or name it 'taskScheduler' " +
+									"(possibly as an alias); or implement the SchedulingConfigurer interface and call " +
+									"ScheduledTaskRegistrar#setScheduler explicitly within the configureTasks() callback: " +
+									ex2.getBeanNamesFound());
+						}
+					}
 				}
 				catch (NoSuchBeanDefinitionException ex2) {
-					logger.debug("Could not find default ScheduledExecutorService bean", ex);
+					logger.debug("Could not find default ScheduledExecutorService bean", ex2);
 					// Giving up -> falling back to default scheduler within the registrar...
+					logger.info("No TaskScheduler/ScheduledExecutorService bean found for scheduled processing");
 				}
 			}
 		}