Leniently accept custom DeferredResult etc subclasses for null values

Issue: SPR-14423

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/InvocableHandlerMethod.java

@@ -283,10 +283,10 @@ public class InvocableHandlerMethod extends HandlerMethod {
 			if (this.returnValue != null) {
 				return this.returnValue.getClass();
 			}
-			if (ResolvableType.NONE.equals(this.returnType)) {
+			if (!ResolvableType.NONE.equals(this.returnType)) {
-				throw new IllegalArgumentException("Expected Future-like type with generic parameter");
+				return this.returnType.getRawClass();
 			}
-			return this.returnType.getRawClass();
+			return super.getParameterType();
 		}
 
 		@Override

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

@@ -281,16 +281,10 @@ public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
 			if (this.returnValue != null) {
 				return this.returnValue.getClass();
 			}
-			Class<?> parameterType = super.getParameterType();
+			if (!ResolvableType.NONE.equals(this.returnType)) {
-			if (ResponseBodyEmitter.class.isAssignableFrom(parameterType) ||
+				return this.returnType.getRawClass();
-					StreamingResponseBody.class.isAssignableFrom(parameterType)) {
-				return parameterType;
 			}
-			if (ResolvableType.NONE.equals(this.returnType)) {
+			return super.getParameterType();
-				throw new IllegalArgumentException("Expected one of Callable, DeferredResult, or ListenableFuture: " +
-						super.getParameterType());
-			}
-			return this.returnType.getRawClass();
 		}
 
 		@Override

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletInvocableHandlerMethodTests.java

@@ -56,6 +56,7 @@ import static org.mockito.Mockito.*;
  *
  * @author Rossen Stoyanchev
  * @author Sam Brannen
+ * @author Juergen Hoeller
  */
 public class ServletInvocableHandlerMethodTests {
 
@@ -126,9 +127,7 @@ public class ServletInvocableHandlerMethodTests {
 				this.mavContainer.isRequestHandled());
 	}
 
-	// SPR-9159
+	@Test  // SPR-9159
-
-	@Test
 	public void invokeAndHandle_NotVoidWithResponseStatusAndReason() throws Exception {
 		ServletInvocableHandlerMethod handlerMethod = getHandlerMethod(new Handler(), "responseStatusWithReason");
 		handlerMethod.invokeAndHandle(this.webRequest, this.mavContainer);
@@ -138,7 +137,7 @@ public class ServletInvocableHandlerMethodTests {
 		assertEquals("400 Bad Request", this.response.getErrorMessage());
 	}
 
-	@Test(expected=HttpMessageNotWritableException.class)
+	@Test(expected = HttpMessageNotWritableException.class)
 	public void invokeAndHandle_Exception() throws Exception {
 		this.returnValueHandlers.addHandler(new ExceptionRaisingReturnValueHandler());
 
@@ -169,23 +168,47 @@ public class ServletInvocableHandlerMethodTests {
 
 	@Test
 	public void wrapConcurrentResult_MethodLevelResponseBody() throws Exception {
-		wrapConcurrentResult_ResponseBody(new MethodLevelResponseBodyHandler());
+		wrapConcurrentResult_ResponseBody(new MethodLevelResponseBodyHandler(), "bar", String.class);
+	}
+
+	@Test
+	public void wrapConcurrentResult_MethodLevelResponseBodyEmpty() throws Exception {
+		wrapConcurrentResult_ResponseBody(new MethodLevelResponseBodyHandler(), null, String.class);
 	}
 
 	@Test
 	public void wrapConcurrentResult_TypeLevelResponseBody() throws Exception {
-		wrapConcurrentResult_ResponseBody(new TypeLevelResponseBodyHandler());
+		wrapConcurrentResult_ResponseBody(new TypeLevelResponseBodyHandler(), "bar", String.class);
+	}
+
+	@Test
+	public void wrapConcurrentResult_TypeLevelResponseBodyEmpty() throws Exception {
+		wrapConcurrentResult_ResponseBody(new TypeLevelResponseBodyHandler(), null, String.class);
+	}
+
+	@Test
+	public void wrapConcurrentResult_DeferredResultSubclass() throws Exception {
+		wrapConcurrentResult_ResponseBody(new DeferredResultSubclassHandler(), "bar", String.class);
 	}
 
-	private void wrapConcurrentResult_ResponseBody(Object handler) throws Exception {
+	@Test
+	public void wrapConcurrentResult_DeferredResultSubclassEmpty() throws Exception {
+		wrapConcurrentResult_ResponseBody(new DeferredResultSubclassHandler(), null, CustomDeferredResult.class);
+	}
+
+	private void wrapConcurrentResult_ResponseBody(Object handler, Object result, Class<?> expectedReturnType)
+			throws Exception {
+
 		List<HttpMessageConverter<?>> converters = new ArrayList<>();
 		converters.add(new StringHttpMessageConverter());
+		this.returnValueHandlers.addHandler(new ModelAndViewMethodReturnValueHandler());
 		this.returnValueHandlers.addHandler(new RequestResponseBodyMethodProcessor(converters));
 		ServletInvocableHandlerMethod handlerMethod = getHandlerMethod(handler, "handle");
-		handlerMethod = handlerMethod.wrapConcurrentResult("bar");
-		handlerMethod.invokeAndHandle(this.webRequest, this.mavContainer);
 
-		assertEquals("bar", this.response.getContentAsString());
+		handlerMethod = handlerMethod.wrapConcurrentResult(result);
+		handlerMethod.invokeAndHandle(this.webRequest, this.mavContainer);
+		assertEquals((result != null ? result.toString() : ""), this.response.getContentAsString());
+		assertEquals(expectedReturnType, handlerMethod.getReturnValueType(result).getParameterType());
 	}
 
 	@Test
@@ -200,9 +223,7 @@ public class ServletInvocableHandlerMethodTests {
 		assertEquals("bar", this.response.getContentAsString());
 	}
 
-	// SPR-12287
+	@Test  // SPR-12287
-
-	@Test
 	public void wrapConcurrentResult_ResponseEntityNullBody() throws Exception {
 		List<HttpMessageConverter<?>> converters = new ArrayList<>();
 		converters.add(new StringHttpMessageConverter());
@@ -256,9 +277,7 @@ public class ServletInvocableHandlerMethodTests {
 		assertEquals("", this.response.getContentAsString());
 	}
 
-	// SPR-12287 (16/Oct/14 comments)
+	@Test  // SPR-12287 (16/Oct/14 comments)
-
-	@Test
 	public void responseEntityRawTypeWithNullBody() throws Exception {
 		List<HttpMessageConverter<?>> converters = Collections.singletonList(new StringHttpMessageConverter());
 		List<Object> advice = Collections.singletonList(mock(ResponseBodyAdvice.class));
@@ -281,6 +300,7 @@ public class ServletInvocableHandlerMethodTests {
 		return handlerMethod;
 	}
 
+
 	@SuppressWarnings("unused")
 	@ResponseStatus
 	@Retention(RetentionPolicy.RUNTIME)
@@ -290,6 +310,7 @@ public class ServletInvocableHandlerMethodTests {
 		HttpStatus responseStatus() default HttpStatus.INTERNAL_SERVER_ERROR;
 	}
 
+
 	@SuppressWarnings("unused")
 	private static class Handler {
 
@@ -321,6 +342,7 @@ public class ServletInvocableHandlerMethodTests {
 		}
 	}
 
+
 	@SuppressWarnings("unused")
 	@ResponseStatus(HttpStatus.BAD_REQUEST)
 	private static class ResponseStatusHandler {
@@ -329,6 +351,7 @@ public class ServletInvocableHandlerMethodTests {
 		}
 	}
 
+
 	private static class MethodLevelResponseBodyHandler {
 
 		@ResponseBody
@@ -337,6 +360,7 @@ public class ServletInvocableHandlerMethodTests {
 		}
 	}
 
+
 	@SuppressWarnings("unused")
 	@ResponseBody
 	private static class TypeLevelResponseBodyHandler {
@@ -346,6 +370,20 @@ public class ServletInvocableHandlerMethodTests {
 		}
 	}
 
+
+	private static class DeferredResultSubclassHandler {
+
+		@ResponseBody
+		public CustomDeferredResult handle() {
+			return new CustomDeferredResult();
+		}
+	}
+
+
+	private static class CustomDeferredResult extends DeferredResult<String> {
+	}
+
+
 	@SuppressWarnings("unused")
 	private static class ResponseEntityHandler {
 
@@ -358,6 +396,7 @@ public class ServletInvocableHandlerMethodTests {
 		}
 	}
 
+
 	private static class ExceptionRaisingReturnValueHandler implements HandlerMethodReturnValueHandler {
 
 		@Override
@@ -372,6 +411,7 @@ public class ServletInvocableHandlerMethodTests {
 		}
 	}
 
+
 	@SuppressWarnings("unused")
 	private static class AsyncHandler {