Document minimum standalone setup for GraphQL support

Most Spring for GraphQL applications use Spring Boot as a way to
auto-configure the required infrastructure for running GraphQL
applications.

This commit documents a minimal setup for Spring applications not
relying on Spring Boot. This assumes an existing infrastructure for a
Spring MVC application.

Closes gh-606

spring-graphql-docs/modules/ROOT/nav.adoc

@@ -12,4 +12,5 @@
 * xref:graphiql.adoc[]
 * xref:testing.adoc[]
 * xref:boot-starter.adoc[]
+* xref:standalone-setup.adoc[]
 * xref:samples.adoc[]

spring-graphql-docs/modules/ROOT/pages/standalone-setup.adoc

@@ -0,0 +1,15 @@
+[[standalone-setup]]
+= Standalone Setup
+
+If your application is not using Spring Boot, you are responsible for setting up the relevant Spring for GraphQL components.
+Assuming that your application is already configured for Spring MVC controllers, the minimum setup will require several beans.
+
+include-code::GraphQlConfiguration[]
+<1> The `AnnotatedControllerConfigurer` bean is responsible for detecting GraphQL `@Controller` handlers.
+<2> The `ExecutionGraphQlService` processes GraphQL requests in a transport-agnostic fashion.
+<3> The `GraphQlSource` builder is the main configuration point. Explore its API for more options.
+<4> The `RouterFunction` exposes the GraphQL routes as {spring-framework-ref-docs}/web/webmvc-functional.html[functional endpoints].
+<5> You can then expose various transports (WebSocket, SSE, HTTP) over different routes.
+
+Spring for GraphQL offers many other options and integrations with Spring projects.
+For more on this, you can xref:boot-starter.adoc[explore the Spring Boot auto-configurations].

spring-graphql-docs/src/main/java/org/springframework/graphql/docs/standalonesetup/GraphQlConfiguration.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2020-2024 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.standalonesetup;
+
+import java.util.List;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.graphql.ExecutionGraphQlService;
+import org.springframework.graphql.data.method.annotation.support.AnnotatedControllerConfigurer;
+import org.springframework.graphql.execution.ConnectionTypeDefinitionConfigurer;
+import org.springframework.graphql.execution.DefaultBatchLoaderRegistry;
+import org.springframework.graphql.execution.DefaultExecutionGraphQlService;
+import org.springframework.graphql.execution.GraphQlSource;
+import org.springframework.graphql.server.WebGraphQlHandler;
+import org.springframework.graphql.server.webmvc.GraphQlHttpHandler;
+import org.springframework.graphql.server.webmvc.GraphQlRequestPredicates;
+import org.springframework.graphql.server.webmvc.GraphiQlHandler;
+import org.springframework.web.servlet.function.RequestPredicate;
+import org.springframework.web.servlet.function.RouterFunction;
+import org.springframework.web.servlet.function.RouterFunctions;
+import org.springframework.web.servlet.function.ServerResponse;
+
+@Configuration(proxyBeanMethods = false)
+public class GraphQlConfiguration {
+
+	@Bean // <1>
+	public AnnotatedControllerConfigurer controllerConfigurer() {
+		return new AnnotatedControllerConfigurer();
+	}
+
+	@Bean // <2>
+	public ExecutionGraphQlService executionGraphQlService(AnnotatedControllerConfigurer controllerConfigurer) {
+		GraphQlSource graphQlSource = GraphQlSource.schemaResourceBuilder() // <3>
+				.schemaResources(new ClassPathResource("graphql/schema.graphqls"))
+				.configureTypeDefinitions(new ConnectionTypeDefinitionConfigurer())
+				.configureRuntimeWiring(controllerConfigurer)
+				.exceptionResolvers(List.of(controllerConfigurer.getExceptionResolver()))
+				.build();
+		DefaultBatchLoaderRegistry batchLoaderRegistry = new DefaultBatchLoaderRegistry();
+		DefaultExecutionGraphQlService service = new DefaultExecutionGraphQlService(graphQlSource);
+		service.addDataLoaderRegistrar(batchLoaderRegistry);
+		return service;
+	}
+
+
+	@Bean // <4>
+	public RouterFunction<ServerResponse> graphQlRouterFunction(ExecutionGraphQlService graphQlService) {
+		WebGraphQlHandler webGraphQlHandler = WebGraphQlHandler.builder(graphQlService).build();
+		GraphQlHttpHandler graphQlHttpHandler = new GraphQlHttpHandler(webGraphQlHandler);
+		RequestPredicate graphQlPredicate = GraphQlRequestPredicates.graphQlHttp("/graphql");
+		GraphiQlHandler graphiQlHandler = new GraphiQlHandler("/graphql", "");
+		return RouterFunctions.route() // <5>
+				.route(graphQlPredicate, graphQlHttpHandler::handleRequest)
+				.GET("/graphiql", graphiQlHandler::handleRequest)
+				.build();
+	}
+}