Add schema inspection support on startup

Prior to this commit, a Spring for GraphQL application could be started
with a schema and an incomplete set of data fetchers, as the schema
would describe:
* Queries/Mutations/Subscriptions that are not backed by any
  `@Controller` method, any Spring Data repository nor any custom
  `DataFetcher`
* type fields that are not backed by any Java Type property nor any
  registered `DataFetcher`

This problem can be noticed at runtime when a request is sent to the
API. The response can contain a `null` field where data was expected, or
even a GraphQL error because the field was non nullable.
This often happens during development time while developers are
implementing the schema.

This commit adds a new `SchemaInspector` type that visits the GraphQL
schema during the startup phase and looks into the `RuntimeWiring` for
registered `DataFetcher` instances. Because data fetchers can be simple
lambdas and do not require to expose a concrete return type, this also
introduces a new `TypedDataFetcher` interface that returns a
`ResolvableType`. This type is only declared by the data fetcher
implementation, but does not necessarily reflects the concrete type
of the returned instances.
This inspection is best effort and has known limitations, such as Union
types (those will not be inspected). Because of those, the inspection
will not fail the application startup.

The `SchemaInspector` collects all missing fields into a report and its
output is logged at startup at the INFO level. As a first step, the
inspector is package private and is only used by the
`DefaultSchemaResourceGraphQlSourceBuilder`. The inspection cannot be
disabled nor customized. We can expand this feature in future releases
as the team collects feedback from the community.

Closes gh-386

Author: bclozel

spring-graphql/src/main/java/org/springframework/graphql/data/TypedDataFetcher.java

@@ -0,0 +1,41 @@
+/*
+ * 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.data;
+
+import graphql.schema.DataFetcher;
+import graphql.schema.DataFetchingEnvironment;
+
+import org.springframework.core.ResolvableType;
+
+/**
+ * Specialized {@link DataFetcher} that can provide {@link ResolvableType type information}
+ * about the {@link #get(DataFetchingEnvironment) instances returned}.
+ * <p>Such {@code DataFetchers} are often backed by actual Java methods with declared return types.
+ * Declared types might not reflect the concrete type of the returned instance.
+ * @author Brian Clozel
+ * @since 1.2.0
+ */
+public interface TypedDataFetcher<T> extends DataFetcher<T> {
+
+	/**
+	 * The type declared by this {@link DataFetcher}.
+	 * <p>The concrete type of the returned instance might differ from the declared one.
+	 * @return the declared type for the data to be fetched.
+	 */
+	ResolvableType getDeclaredType();
+	
+}

spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/AnnotatedControllerConfigurer.java

@@ -54,13 +54,15 @@
 import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodIntrospector;
 import org.springframework.core.MethodParameter;
+import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.expression.BeanResolver;
 import org.springframework.format.FormatterRegistrar;
 import org.springframework.format.support.DefaultFormattingConversionService;
 import org.springframework.format.support.FormattingConversionService;
 import org.springframework.graphql.data.GraphQlArgumentBinder;
+import org.springframework.graphql.data.TypedDataFetcher;
 import org.springframework.graphql.data.method.HandlerMethod;
 import org.springframework.graphql.data.method.HandlerMethodArgumentResolver;
 import org.springframework.graphql.data.method.HandlerMethodArgumentResolverComposite;
@@ -538,7 +540,7 @@ public String toString() {
 	/**
 	 * {@link DataFetcher} that wrap and invokes a {@link HandlerMethod}.
 	 */
-	static class SchemaMappingDataFetcher implements DataFetcher<Object> {
+	static class SchemaMappingDataFetcher implements TypedDataFetcher<Object> {
 
 		private final MappingInfo info;
 
@@ -629,6 +631,11 @@ private <T> Publisher<T> handleSubscriptionError(
 					.flatMap(errors -> Mono.error(new SubscriptionPublisherException(errors, ex)));
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forMethodReturnType(this.info.getHandlerMethod().getMethod());
+		}
+		
 	}
 
 

spring-graphql/src/main/java/org/springframework/graphql/data/query/QueryByExampleDataFetcher.java

@@ -41,6 +41,7 @@
 import org.springframework.data.util.TypeInformation;
 import org.springframework.graphql.data.GraphQlArgumentBinder;
 import org.springframework.graphql.data.GraphQlRepository;
+import org.springframework.graphql.data.TypedDataFetcher;
 import org.springframework.graphql.execution.RuntimeWiringConfigurer;
 import org.springframework.util.Assert;
 import org.springframework.validation.BindException;
@@ -441,7 +442,7 @@ public interface ReactiveQueryByExampleBuilderCustomizer<T> {
 	}
 
 
-	private static class SingleEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements DataFetcher<R> {
+	private static class SingleEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements TypedDataFetcher<R> {
 
 		private final QueryByExampleExecutor<T> executor;
 
@@ -480,10 +481,14 @@ public R get(DataFetchingEnvironment env) throws BindException {
 			}).orElse(null);
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClass(this.resultType);
+		}
 	}
 
 
-	private static class ManyEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements DataFetcher<Iterable<R>> {
+	private static class ManyEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements TypedDataFetcher<Iterable<R>> {
 
 		private final QueryByExampleExecutor<T> executor;
 
@@ -522,10 +527,15 @@ public Iterable<R> get(DataFetchingEnvironment env) throws BindException {
 			});
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClassWithGenerics(Iterable.class, this.resultType);
+		}
+
 	}
 
 
-	private static class ReactiveSingleEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements DataFetcher<Mono<R>> {
+	private static class ReactiveSingleEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements TypedDataFetcher<Mono<R>> {
 
 		private final ReactiveQueryByExampleExecutor<T> executor;
 
@@ -564,10 +574,15 @@ public Mono<R> get(DataFetchingEnvironment env) throws BindException {
 			});
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClassWithGenerics(Mono.class, this.resultType);
+		}
+
 	}
 
 
-	private static class ReactiveManyEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements DataFetcher<Flux<R>> {
+	private static class ReactiveManyEntityFetcher<T, R> extends QueryByExampleDataFetcher<T> implements TypedDataFetcher<Flux<R>> {
 
 		private final ReactiveQueryByExampleExecutor<T> executor;
 
@@ -606,6 +621,11 @@ public Flux<R> get(DataFetchingEnvironment env) throws BindException {
 			});
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClassWithGenerics(Flux.class, this.resultType);
+		}
+
 	}
 
 }

spring-graphql/src/main/java/org/springframework/graphql/data/query/QuerydslDataFetcher.java

@@ -34,6 +34,7 @@
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
+import org.springframework.core.ResolvableType;
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.data.domain.Sort;
 import org.springframework.data.querydsl.QuerydslPredicateExecutor;
@@ -46,6 +47,7 @@
 import org.springframework.data.repository.query.FluentQuery.FetchableFluentQuery;
 import org.springframework.data.util.TypeInformation;
 import org.springframework.graphql.data.GraphQlRepository;
+import org.springframework.graphql.data.TypedDataFetcher;
 import org.springframework.graphql.execution.RuntimeWiringConfigurer;
 import org.springframework.util.Assert;
 import org.springframework.util.LinkedMultiValueMap;
@@ -538,7 +540,7 @@ public interface ReactiveQuerydslBuilderCustomizer<T> {
 	}
 
 
-	private static class SingleEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements DataFetcher<R> {
+	private static class SingleEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements TypedDataFetcher<R> {
 
 		private final QuerydslPredicateExecutor<T> executor;
 
@@ -581,10 +583,14 @@ public R get(DataFetchingEnvironment env) {
 			}).orElse(null);
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClass(this.resultType);
+		}
 	}
 
 
-	private static class ManyEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements DataFetcher<Iterable<R>> {
+	private static class ManyEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements TypedDataFetcher<Iterable<R>> {
 
 		private final QuerydslPredicateExecutor<T> executor;
 
@@ -625,10 +631,15 @@ public Iterable<R> get(DataFetchingEnvironment env) {
 			});
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClassWithGenerics(Iterable.class, this.resultType);
+		}
+
 	}
 
 
-	private static class ReactiveSingleEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements DataFetcher<Mono<R>> {
+	private static class ReactiveSingleEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements TypedDataFetcher<Mono<R>> {
 
 		private final ReactiveQuerydslPredicateExecutor<T> executor;
 
@@ -670,10 +681,15 @@ public Mono<R> get(DataFetchingEnvironment env) {
 			});
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClassWithGenerics(Mono.class, this.resultType);
+		}
+
 	}
 
 
-	private static class ReactiveManyEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements DataFetcher<Flux<R>> {
+	private static class ReactiveManyEntityFetcher<T, R> extends QuerydslDataFetcher<T> implements TypedDataFetcher<Flux<R>> {
 
 		private final ReactiveQuerydslPredicateExecutor<T> executor;
 
@@ -715,6 +731,11 @@ public Flux<R> get(DataFetchingEnvironment env) {
 			});
 		}
 
+		@Override
+		public ResolvableType getDeclaredType() {
+			return ResolvableType.forClassWithGenerics(Flux.class, this.resultType);
+		}
+
 	}
 
 }

spring-graphql/src/main/java/org/springframework/graphql/execution/DefaultSchemaResourceGraphQlSourceBuilder.java

@@ -139,6 +139,12 @@ protected GraphQLSchema initGraphQlSchema() {
 			}
 		});
 
+		SchemaInspector.Report schemaInspectionReport = new SchemaInspector().inspectSchema(registry, runtimeWiring);
+		if(!schemaInspectionReport.isEmpty()) {
+			logger.info(schemaInspectionReport.getSummary());
+			logger.info(schemaInspectionReport.getDetailedReport());
+		}
+
 		return (this.schemaFactory != null ?
 				this.schemaFactory.apply(registry, runtimeWiring) :
 				new SchemaGenerator().makeExecutableSchema(registry, runtimeWiring));