Document GraphiQL integration

Closes gh-403

spring-graphql-docs/src/main/java/org/springframework/graphql/docs/graphiql/configuration/GraphiQlConfiguration.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2020-2023 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
+ *
+ *      https://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.graphql.docs.graphiql.configuration;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.annotation.Order;
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.graphql.server.webmvc.GraphiQlHandler;
+import org.springframework.web.servlet.function.RouterFunction;
+import org.springframework.web.servlet.function.RouterFunctions;
+import org.springframework.web.servlet.function.ServerResponse;
+
+@Configuration
+public class GraphiQlConfiguration {
+
+    @Bean
+    @Order(0)
+    public RouterFunction<ServerResponse> graphiQlRouterFunction() {
+        RouterFunctions.Builder builder = RouterFunctions.route();
+        ClassPathResource graphiQlPage = new ClassPathResource("graphiql/index.html"); // <1>
+        GraphiQlHandler graphiQLHandler = new GraphiQlHandler("/graphql", "", graphiQlPage); // <2>
+        builder = builder.GET("/graphiql", graphiQLHandler::handleRequest); // <3>
+        return builder.build(); // <4>
+    }
+}