Support Optional for ProjectPayload arguments

Closes gh-506

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -16,6 +16,8 @@
 package org.springframework.graphql.data.method.annotation.support;
 
 
+import java.util.Optional;
+
 import graphql.schema.DataFetchingEnvironment;
 
 import org.springframework.context.ApplicationContext;
@@ -77,24 +79,33 @@ public class ProjectedPayloadMethodArgumentResolver implements HandlerMethodArgu
 
 	@Override
 	public boolean supportsParameter(MethodParameter parameter) {
-		Class<?> type = parameter.getParameterType();
-
-		if (!type.isInterface()) {
-			return false;
-		}
-
-		return AnnotatedElementUtils.findMergedAnnotation(type, ProjectedPayload.class) != null;
+		Class<?> type = parameter.nestedIfOptional().getNestedParameterType();
+		return (type.isInterface() &&
+				AnnotatedElementUtils.findMergedAnnotation(type, ProjectedPayload.class) != null);
 	}
 
 	@Override
 	public Object resolveArgument(MethodParameter parameter, DataFetchingEnvironment environment) throws Exception {
+
 		String name = (parameter.hasParameterAnnotation(Argument.class) ?
 				ArgumentMethodArgumentResolver.getArgumentName(parameter) : null);
 
+		Class<?> projectionType = parameter.getParameterType();
+
+		boolean isOptional = parameter.isOptional();
+		if (isOptional) {
+			projectionType = parameter.nestedIfOptional().getNestedParameterType();
+		}
+
 		Object projectionSource = (name != null ?
 				environment.getArgument(name) : environment.getArguments());
 
-		return project(parameter.getParameterType(), projectionSource);
+		Object value = null;
+		if (!isOptional || projectionSource != null) {
+			value = project(projectionType, projectionSource);
+		}
+
+		return (isOptional ? Optional.ofNullable(value) : value);
 	}
 
 	protected Object project(Class<?> projectionType, Object projectionSource){

spring-graphql/src/test/java/org/springframework/graphql/data/method/annotation/support/ProjectedPayloadMethodArgumentResolverTests.java

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2020-2022 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.method.annotation.support;
+
+import java.util.List;
+import java.util.Optional;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.context.support.StaticApplicationContext;
+import org.springframework.core.MethodParameter;
+import org.springframework.data.web.ProjectedPayload;
+import org.springframework.graphql.Book;
+import org.springframework.graphql.data.method.annotation.Argument;
+import org.springframework.graphql.data.method.annotation.QueryMapping;
+import org.springframework.stereotype.Controller;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ *
+ */
+public class ProjectedPayloadMethodArgumentResolverTests extends ArgumentResolverTestSupport {
+
+	private ProjectedPayloadMethodArgumentResolver resolver;
+
+
+	@BeforeEach
+	void setUp() {
+		StaticApplicationContext context = new StaticApplicationContext();
+		this.resolver = new ProjectedPayloadMethodArgumentResolver(context);
+	}
+
+
+	@Test
+	void supports() {
+		MethodParameter param = methodParam(BookController.class, "optionalProjection", Optional.class);
+		assertThat(this.resolver.supportsParameter(param)).isTrue();
+
+		param = methodParam(BookController.class, "optionalString", Optional.class);
+		assertThat(this.resolver.supportsParameter(param)).isFalse();
+	}
+
+	@Test
+	void optionalWrapper() throws Exception {
+
+		Object result = this.resolver.resolveArgument(
+				methodParam(BookController.class, "optionalProjection", Optional.class),
+				environment("{}"));
+
+		assertThat(result).isNotNull().isInstanceOf(Optional.class);
+		assertThat((Optional<?>) result).isNotPresent();
+	}
+
+
+	@SuppressWarnings({"ConstantConditions", "unused"})
+	@Controller
+	static class BookController {
+
+		@QueryMapping
+		public List<Book> optionalProjection(@Argument(name = "where") Optional<BookProjection> projection) {
+			return null;
+		}
+
+		@QueryMapping
+		public void optionalString(@Argument Optional<String> projection) {
+		}
+
+	}
+
+
+	@ProjectedPayload
+	interface BookProjection {
+
+		String getAuthor();
+
+	}
+
+}