Support for response headers through WebOutput

See gh-6

spring-graphql-web/src/main/java/org/springframework/graphql/WebFluxGraphQLHandler.java

@@ -51,7 +51,13 @@ public class WebFluxGraphQLHandler implements HandlerFunction<ServerResponse> {
 					WebInput webInput = new WebInput(request.uri(), request.headers().asHttpHeaders(), body);
 					return this.executionChain.execute(webInput);
 				})
-				.flatMap(output -> ServerResponse.ok().bodyValue(output.toSpecification()));
+				.flatMap(output -> {
+					ServerResponse.BodyBuilder builder = ServerResponse.ok();
+					if (output.getHeaders() != null) {
+						builder.headers(headers -> headers.putAll(output.getHeaders()));
+					}
+					return builder.bodyValue(output.toSpecification());
+				});
 	}
 
 }

spring-graphql-web/src/main/java/org/springframework/graphql/WebInterceptorExecutionChain.java

@@ -64,7 +64,7 @@ class WebInterceptorExecutionChain {
 	}
 
 	private Mono<WebOutput> createOutputChain(Mono<ExecutionResult> resultMono) {
-		Mono<WebOutput> outputMono = resultMono.map(WebOutput::new);
+		Mono<WebOutput> outputMono = resultMono.map((ExecutionResult executionResult) -> new WebOutput(executionResult, null));
 		for (int i = this.interceptors.size() - 1 ; i >= 0; i--) {
 			WebInterceptor interceptor = this.interceptors.get(i);
 			outputMono = outputMono.flatMap(interceptor::postHandle);

spring-graphql-web/src/main/java/org/springframework/graphql/WebMvcGraphQLHandler.java

@@ -21,7 +21,6 @@ import java.util.Map;
 
 import javax.servlet.ServletException;
 
-import graphql.ExecutionResult;
 import graphql.GraphQL;
 import reactor.core.publisher.Mono;
 
@@ -60,8 +59,15 @@ public class WebMvcGraphQLHandler implements HandlerFunction<ServerResponse> {
 	 */
 	public ServerResponse handle(ServerRequest request) throws ServletException {
 		WebInput webInput = new WebInput(request.uri(), request.headers().asHttpHeaders(), readBody(request));
-		Mono<WebOutput> outputMono = this.executionChain.execute(webInput);
-		return ServerResponse.ok().body(outputMono.map(ExecutionResult::toSpecification));
+		Mono<ServerResponse> responseMono = this.executionChain.execute(webInput)
+				.map(output -> {
+					ServerResponse.BodyBuilder builder = ServerResponse.ok();
+					if (output.getHeaders() != null) {
+						builder.headers(headers -> headers.putAll(output.getHeaders()));
+					}
+					return builder.body(output.toSpecification());
+				});
+		return ServerResponse.async(responseMono);
 	}
 
 	private static Map<String, Object> readBody(ServerRequest request) throws ServletException {

spring-graphql-web/src/main/java/org/springframework/graphql/WebOutput.java

@@ -24,6 +24,7 @@ import graphql.ExecutionResult;
 import graphql.ExecutionResultImpl;
 import graphql.GraphQLError;
 
+import org.springframework.http.HttpHeaders;
 import org.springframework.lang.Nullable;
 
 
@@ -35,12 +36,16 @@ public class WebOutput implements ExecutionResult {
 
 	private final ExecutionResult executionResult;
 
+	@Nullable
+	private final HttpHeaders headers;
+
 
 	/**
 	 * Create an instance that wraps the given {@link ExecutionResult}.
 	 */
-	public WebOutput(ExecutionResult executionResult) {
+	public WebOutput(ExecutionResult executionResult, @Nullable HttpHeaders headers) {
 		this.executionResult = executionResult;
+		this.headers = headers;
 	}
 
 
@@ -69,6 +74,14 @@ public class WebOutput implements ExecutionResult {
 		return this.executionResult.toSpecification();
 	}
 
+	/**
+	 * Return headers to be added to the HTTP response.
+	 */
+	@Nullable
+	public HttpHeaders getHeaders() {
+		return this.headers;
+	}
+
 	/**
 	 * Transform this {@code WebOutput} instance through a {@link Builder} and
 	 * return a new instance with the modified values.
@@ -90,12 +103,18 @@ public class WebOutput implements ExecutionResult {
 		@Nullable
 		private Map<Object, Object> extensions;
 
+		@Nullable
+		private HttpHeaders headers;
+
+
 		public Builder(WebOutput output) {
 			this.data = output.getData();
 			this.errors = output.getErrors();
 			this.extensions = output.getExtensions();
+			this.headers = output.getHeaders();
 		}
 
+
 		/**
 		 * Set the execution {@link ExecutionResult#getData() data}.
 		 */
@@ -120,8 +139,26 @@ public class WebOutput implements ExecutionResult {
 			return this;
 		}
 
+		public Builder header(String name, String... values) {
+			initHeaders();
+			for (String value : values) {
+				this.headers.add(name, value);
+			}
+			return this;
+		}
+
+		public Builder headers(Consumer<HttpHeaders> consumer) {
+			initHeaders();
+			consumer.accept(this.headers);
+			return this;
+		}
+
+		private void initHeaders() {
+			this.headers = (this.headers != null ? this.headers : new HttpHeaders());
+		}
+
 		public WebOutput build() {
-			return new WebOutput(new ExecutionResultImpl(this.data, this.errors, this.extensions));
+			return new WebOutput(new ExecutionResultImpl(this.data, this.errors, this.extensions), this.headers);
 		}
 	}
 

spring-graphql-web/src/test/java/org/springframework/graphql/WebInterceptorExecutionChainTests.java

@@ -69,8 +69,10 @@ public class WebInterceptorExecutionChainTests {
 
 		assertThat(sb.toString()).isEqualTo(":pre1:pre2:pre3:post3:post2:post1");
 		assertThat(webOutput.isDataPresent()).isTrue();
+		assertThat(webOutput.getHeaders().get("MyHeader")).containsExactly("MyValue3", "MyValue2", "MyValue1");
 	}
 
+
 	private static GraphQL createGraphQL() throws Exception {
 		RuntimeWiring runtimeWiring = RuntimeWiring.newRuntimeWiring()
 				.type(newTypeWiring("Query").dataFetcher("bookById", GraphQLDataFetchers.getBookByIdDataFetcher()))
@@ -105,7 +107,10 @@ public class WebInterceptorExecutionChainTests {
 		@Override
 		public Mono<WebOutput> postHandle(WebOutput webOutput) {
 			this.output.append(":post").append(this.index);
-			return Mono.delay(Duration.ofMillis(50)).map(aLong -> webOutput);
+			return Mono.delay(Duration.ofMillis(50))
+					.map(aLong -> webOutput.transform(builder -> {
+						builder.header("myHeader", "MyValue" + this.index);
+					}));
 		}
 	}