Allow HttpHeaders return values for @MVC methods

Allow HttpHeader instances to be returned directly from MVC controller
methods managed by HandlerMethodReturnValueHandler rather than needing
to be wrapped in a ResponseEntity.

Issue: SPR-11129

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

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2002-2014 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.servlet.mvc.method.annotation;
+
+import javax.servlet.http.HttpServletResponse;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.server.ServletServerHttpResponse;
+import org.springframework.util.Assert;
+import org.springframework.web.context.request.NativeWebRequest;
+import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
+import org.springframework.web.method.support.ModelAndViewContainer;
+
+/**
+ * Handles {@link HttpHeaders} return values.
+ *
+ * @author Stephane Nicoll
+ * @since 4.0.1
+ */
+public class HttpHeadersReturnValueHandler implements HandlerMethodReturnValueHandler {
+
+	@Override
+	public boolean supportsReturnType(MethodParameter returnType) {
+		return HttpHeaders.class.isAssignableFrom(returnType.getParameterType());
+	}
+
+	@Override
+	public void handleReturnValue(Object returnValue, MethodParameter returnType,
+								  ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
+		mavContainer.setRequestHandled(true);
+
+		Assert.isInstanceOf(HttpHeaders.class, returnValue);
+		HttpHeaders headers = (HttpHeaders) returnValue;
+
+		HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
+		ServletServerHttpResponse outputMessage = new ServletServerHttpResponse(response);
+		if (!headers.isEmpty()) {
+			outputMessage.getHeaders().putAll(headers);
+		}
+		outputMessage.getBody(); // flush headers
+	}
+}

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

@@ -585,6 +585,7 @@ public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter i
 		handlers.add(new ModelMethodProcessor());
 		handlers.add(new ViewMethodReturnValueHandler());
 		handlers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.contentNegotiationManager));
+		handlers.add(new HttpHeadersReturnValueHandler());
 		handlers.add(new CallableMethodReturnValueHandler());
 		handlers.add(new DeferredResultMethodReturnValueHandler());
 		handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));