Support Optional constructor arg for argument binding

Closes gh-470

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

@@ -188,9 +188,10 @@ public class GraphQlArgumentBinder {
 		return (type.resolve(Object.class).equals(Optional.class) ? Optional.ofNullable(value) : value);
 	}
 
-	private boolean isApproximableCollectionType(Object rawValue) {
-		return (CollectionFactory.isApproximableCollectionType(rawValue.getClass()) ||
-				rawValue instanceof List);  // it may be SingletonList
+	private boolean isApproximableCollectionType(@Nullable Object rawValue) {
+		return (rawValue != null &&
+				(CollectionFactory.isApproximableCollectionType(rawValue.getClass()) ||
+						rawValue instanceof List));  // it may be SingletonList
 	}
 
 	@SuppressWarnings({"ConstantConditions", "unchecked"})
@@ -283,12 +284,15 @@ public class GraphQlArgumentBinder {
 			if (rawValue == null && methodParam.isOptional()) {
 				args[i] = (paramTypes[i] == Optional.class ? Optional.empty() : null);
 			}
-			else if (rawValue != null && isApproximableCollectionType(rawValue)) {
+			else if (isApproximableCollectionType(rawValue)) {
 				ResolvableType elementType = ResolvableType.forMethodParameter(methodParam);
 				args[i] = createCollection((Collection<Object>) rawValue, elementType, bindingResult, segments);
 			}
 			else if (rawValue instanceof Map) {
-				args[i] = createValueOrNull((Map<String, Object>) rawValue, paramTypes[i], bindingResult, segments);
+				boolean isOptional = (paramTypes[i] == Optional.class);
+				Class<?> type = (isOptional ? methodParam.nestedIfOptional().getNestedParameterType() : paramTypes[i]);
+				Object value = createValueOrNull((Map<String, Object>) rawValue, type, bindingResult, segments);
+				args[i] = (isOptional ? Optional.ofNullable(value) : value);
 			}
 			else {
 				args[i] = convertValue(rawValue, paramTypes[i], new TypeDescriptor(methodParam), bindingResult, segments);

spring-graphql/src/test/java/org/springframework/graphql/data/GraphQlArgumentBinderTests.java

@@ -21,6 +21,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
@@ -32,6 +33,7 @@ import graphql.schema.DataFetchingEnvironmentImpl;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.core.ResolvableType;
+import org.springframework.format.support.DefaultFormattingConversionService;
 import org.springframework.graphql.Book;
 import org.springframework.validation.BindException;
 import org.springframework.validation.FieldError;
@@ -182,6 +184,26 @@ class GraphQlArgumentBinderTests {
 		assertThat(((PrimaryConstructorItemBean) result).getAge()).isEqualTo(30);
 	}
 
+	@Test
+	void primaryConstructorWithOptionalBeanArgument() throws Exception {
+
+		GraphQlArgumentBinder argumentBinder =
+				new GraphQlArgumentBinder(new DefaultFormattingConversionService());
+
+		Object result = argumentBinder.bind(
+				environment(
+						"{\"key\":{" +
+								"\"item\":{\"name\":\"Item name\"}," +
+								"\"name\":\"Hello\"," +
+								"\"age\":\"30\"}}"),
+				"key",
+				ResolvableType.forClass(PrimaryConstructorOptionalItemBean.class));
+
+		assertThat(result).isNotNull().isInstanceOf(PrimaryConstructorOptionalItemBean.class);
+		assertThat(((PrimaryConstructorOptionalItemBean) result).getItem().get().getName()).isEqualTo("Item name");
+		assertThat(((PrimaryConstructorOptionalItemBean) result).getName().get()).isEqualTo("Hello");
+	}
+
 	@Test
 	void primaryConstructorWithNestedBeanList() throws Exception {
 
@@ -390,6 +412,28 @@ class GraphQlArgumentBinderTests {
 	}
 
 
+	@SuppressWarnings("OptionalUsedAsFieldOrParameterType")
+	static class PrimaryConstructorOptionalItemBean {
+
+		private final Optional<String> name;
+
+		private final Optional<Item> item;
+
+		public PrimaryConstructorOptionalItemBean(Optional<String> name, Optional<Item> item) {
+			this.name = name;
+			this.item = item;
+		}
+
+		public Optional<String> getName() {
+			return this.name;
+		}
+
+		public Optional<Item> getItem() {
+			return item;
+		}
+	}
+
+
 	static class NoPrimaryConstructorBean {
 
 		NoPrimaryConstructorBean(String name) {