Moved executor null check to AsyncExecutionInterceptor, allowing AbstractAsyncExecutionAspect to fall back to sync execution (as in 3.2.1)

Issue: SPR-10636
(cherry picked from commit b6c54c3)

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

@@ -87,7 +87,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 
 	/**
 	 * Determine the specific executor to use when executing the given method.
-	 * @return the executor to use (never {@code null})
+	 * @return the executor to use (or {@code null}, but just if no default executor has been set)
 	 */
 	protected AsyncTaskExecutor determineAsyncExecutor(Method method) {
 		AsyncTaskExecutor executor = this.executors.get(method);
@@ -101,8 +101,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 						this.beanFactory, Executor.class, qualifier);
 			}
 			else if (executorToUse == null) {
-				throw new IllegalStateException("No executor qualifier specified and no default executor set on " +
-						getClass().getSimpleName() + " either");
+				return null;
 			}
 			executor = (executorToUse instanceof AsyncTaskExecutor ?
 					(AsyncTaskExecutor) executorToUse : new TaskExecutorAdapter(executorToUse));

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

@@ -82,7 +82,13 @@ public class AsyncExecutionInterceptor extends AsyncExecutionAspectSupport
 		Method specificMethod = ClassUtils.getMostSpecificMethod(invocation.getMethod(), targetClass);
 		specificMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
 
-		Future<?> result = determineAsyncExecutor(specificMethod).submit(
+		AsyncTaskExecutor executor = determineAsyncExecutor(specificMethod);
+		if (executor == null) {
+			throw new IllegalStateException(
+					"No executor specified and no default executor set on AsyncExecutionInterceptor either");
+		}
+
+		Future<?> result = executor.submit(
 				new Callable<Object>() {
 					public Object call() throws Exception {
 						try {