Add trySet method to DeferredResult

The method absorbs any potential StaleAsyncWebRequestException and
returns false instead.

Issue: SPR-8517

spring-web/src/main/java/org/springframework/web/context/request/async/DeferredResult.java

@@ -103,6 +103,22 @@ public final class DeferredResult {
 		}
 	}
 
+	/**
+	 * A variant of {@link #set(Object)} that absorbs a potential, resulting
+	 * {@link StaleAsyncWebRequestException}.
+	 * @return {@code false} if the outcome was a {@code StaleAsyncWebRequestException}
+	 */
+	public boolean trySet(Object result) throws StaleAsyncWebRequestException {
+		try {
+			set(result);
+			return true;
+		}
+		catch (StaleAsyncWebRequestException ex) {
+			// absorb
+		}
+		return false;
+	}
+
 	/**
 	 * Invoked to complete async processing when a timeout occurs before
 	 * {@link #set(Object)} is called. Or if {@link #set(Object)} is already in

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/RequestMappingHandlerAdapter.java

@@ -388,10 +388,11 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter i
 	}
 
 	/**
-	 * Set an AsyncTaskExecutor to use when a controller method returns a Callable.
-	 * <p>The default is a {@link SimpleAsyncTaskExecutor}
-	 *
-	 * TODO... need a better default
+	 * Set the AsyncTaskExecutor to use when a controller method returns a
+	 * {@code Callable}.
+	 * <p>The default instance type is a {@link SimpleAsyncTaskExecutor}.
+	 * It's recommended to change that default in production as the simple
+	 * executor does not re-use threads.
 	 */
 	public void setAsyncTaskExecutor(AsyncTaskExecutor taskExecutor) {
 		this.taskExecutor = taskExecutor;