Support contructor binding for input arguments

See gh-139

spring-graphql/build.gradle

@@ -1,3 +1,6 @@
+plugins {
+	id 'org.jetbrains.kotlin.jvm' version '1.5.31'
+}
 description = "GraphQL Support for Spring Applications"
 
 dependencies {
@@ -34,6 +37,7 @@ dependencies {
 
 	testRuntimeOnly 'org.apache.logging.log4j:log4j-core'
 	testRuntimeOnly 'org.apache.logging.log4j:log4j-slf4j-impl'
+	implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
 }
 
 test {
@@ -42,3 +46,16 @@ test {
 		events "passed", "skipped", "failed"
 	}
 }
+repositories {
+	mavenCentral()
+}
+compileKotlin {
+	kotlinOptions {
+		jvmTarget = "1.8"
+	}
+}
+compileTestKotlin {
+	kotlinOptions {
+		jvmTarget = "1.8"
+	}
+}

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

@@ -18,6 +18,7 @@ package org.springframework.graphql.data.method.annotation.support;
 import java.lang.reflect.Constructor;
 import java.util.Collection;
 import java.util.Iterator;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Stack;
@@ -107,13 +108,32 @@ public class ArgumentMethodArgumentResolver implements HandlerMethodArgumentReso
 		Object target;
 		if (rawValue instanceof Map) {
 			Constructor<?> ctor = BeanUtils.getResolvableConstructor(targetType);
-			target = BeanUtils.instantiateClass(ctor);
-			DataBinder dataBinder = new DataBinder(target);
-			Assert.isTrue(ctor.getParameterCount() == 0,
-					() -> "Argument of type [" + targetType.getName() +
-							"] cannot be instantiated because of missing default constructor.");
-			MutablePropertyValues mpvs = extractPropertyValues((Map) rawValue);
-			dataBinder.bind(mpvs);
+			MutablePropertyValues propertyValues = extractPropertyValues((Map) rawValue);
+
+			if (ctor.getParameterCount() == 0) {
+				target = BeanUtils.instantiateClass(ctor);
+				DataBinder dataBinder = new DataBinder(target);
+				dataBinder.bind(propertyValues);
+			} else {
+				// Data class constructor
+				DataBinder binder = new DataBinder(null);
+				String[] paramNames = BeanUtils.getParameterNames(ctor);
+				Class<?>[] paramTypes = ctor.getParameterTypes();
+				Object[] args = new Object[paramTypes.length];
+				for (int i = 0; i < paramNames.length; i++) {
+					String paramName = paramNames[i];
+					Object value = propertyValues.get(paramName);
+					value = (value instanceof List ? ((List<?>) value).toArray() : value);
+					MethodParameter methodParam = new MethodParameter(ctor, i);
+					if (value == null && methodParam.isOptional()) {
+						args[i] = (methodParam.getParameterType() == Optional.class ? Optional.empty() : null);
+					}
+					else {
+						args[i] = binder.convertIfNecessary(value, paramTypes[i], methodParam);
+					}
+				}
+				target = BeanUtils.instantiateClass(ctor, args);
+			}
 		}
 		else if (targetType.isAssignableFrom(rawValue.getClass())) {
 			return returnValue(rawValue, targetType);

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

@@ -86,6 +86,18 @@ class ArgumentMethodArgumentResolverTests {
 				.hasFieldOrPropertyWithValue("authorId", 42L);
 	}
 
+	@Test
+	void shouldResolveKotlinBeanArgument() throws Exception {
+		Method addBook = ClassUtils.getMethod(BookController.class, "ktAddBook", KotlinBookInput.class);
+		String payload = "{\"bookInput\": { \"name\": \"test name\", \"authorId\": 42} }";
+		DataFetchingEnvironment environment = initEnvironment(payload);
+		MethodParameter methodParameter = getMethodParameter(addBook, 0);
+		Object result = resolver.resolveArgument(methodParameter, environment);
+		assertThat(result).isNotNull().isInstanceOf(KotlinBookInput.class);
+		assertThat((KotlinBookInput) result).hasFieldOrPropertyWithValue("name", "test name")
+				.hasFieldOrPropertyWithValue("authorId", 42L);
+	}
+
 	@Test
 	void shouldResolveListOfJavaBeansArgument() throws Exception {
 		Method addBooks = ClassUtils.getMethod(BookController.class, "addBooks", List.class);
@@ -147,6 +159,11 @@ class ArgumentMethodArgumentResolverTests {
 			return null;
 		}
 
+		@MutationMapping
+		public Book ktAddBook(@Argument KotlinBookInput bookInput) {
+			return null;
+		}
+
 		@MutationMapping
 		public List<Book> addBooks(@Argument List<Book> books) {
 			return null;

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

@@ -0,0 +1,3 @@
+package org.springframework.graphql.data.method.annotation.support;
+
+data class KotlinBookInput(val name: String, val authorId: Long)