DATACMNS-1689 - Resolve suspendable query method result type from Continuation parameter.

Kotlin coroutine methods are compiled returning Object so the original return type gets lost. We resolve the return type from the Continuation parameter that synthetized as last parameter in a coroutine method declaration.

src/main/java/org/springframework/data/repository/core/support/AbstractRepositoryMetadata.java

@@ -18,8 +18,10 @@ package org.springframework.data.repository.core.support;
 import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.List;
 import java.util.Set;
 
+import org.springframework.core.KotlinDetector;
 import org.springframework.data.domain.Pageable;
 import org.springframework.data.repository.Repository;
 import org.springframework.data.repository.core.CrudMethods;
@@ -27,6 +29,7 @@ import org.springframework.data.repository.core.RepositoryMetadata;
 import org.springframework.data.repository.util.QueryExecutionConverters;
 import org.springframework.data.repository.util.ReactiveWrappers;
 import org.springframework.data.util.ClassTypeInformation;
+import org.springframework.data.util.KotlinReflectionUtils;
 import org.springframework.data.util.Lazy;
 import org.springframework.data.util.TypeInformation;
 import org.springframework.util.Assert;
@@ -79,7 +82,20 @@ public abstract class AbstractRepositoryMetadata implements RepositoryMetadata {
 	 * @see org.springframework.data.repository.core.RepositoryMetadata#getReturnedDomainClass(java.lang.reflect.Method)
 	 */
 	public Class<?> getReturnedDomainClass(Method method) {
-		return QueryExecutionConverters.unwrapWrapperTypes(typeInformation.getReturnType(method)).getType();
+
+		TypeInformation<?> returnType = null;
+		if (KotlinDetector.isKotlinType(method.getDeclaringClass()) && KotlinReflectionUtils.isSuspend(method)) {
+
+			// the last parameter is Continuation<? super T> or Continuation<? super Flow<? super T>>
+			List<TypeInformation<?>> types = typeInformation.getParameterTypes(method);
+			returnType = types.get(types.size() - 1).getComponentType();
+		}
+
+		if (returnType == null) {
+			returnType = typeInformation.getReturnType(method);
+		}
+
+		return QueryExecutionConverters.unwrapWrapperTypes(returnType).getType();
 	}
 
 	/*

src/test/kotlin/org/springframework/data/repository/kotlin/CoroutineRepositoryMetadataUnitTests.kt

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2020 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.data.repository.kotlin
+
+import kotlinx.coroutines.flow.Flow
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import org.springframework.data.repository.core.support.DefaultRepositoryMetadata
+import org.springframework.data.repository.sample.User
+import org.springframework.data.util.ReflectionUtils
+import kotlin.coroutines.Continuation
+
+/**
+ * Unit tests for [org.springframework.data.repository.core.RepositoryMetadata].
+ *
+ * @author Mark Paluch
+ */
+class CoroutineRepositoryMetadataUnitTests {
+
+	@Test // DATACMNS-1689
+	fun shouldDetermineCorrectResultType() {
+
+		val metadata = DefaultRepositoryMetadata(MyCoRepository::class.java)
+		val method = ReflectionUtils.findRequiredMethod(MyCoRepository::class.java, "findOne", String::class.java, Continuation::class.java);
+
+		assertThat(metadata.getReturnedDomainClass(method)).isEqualTo(User::class.java)
+	}
+
+	@Test // DATACMNS-1689
+	fun shouldDetermineCorrectOptionalResultType() {
+
+		val metadata = DefaultRepositoryMetadata(MyCoRepository::class.java)
+		val method = ReflectionUtils.findRequiredMethod(MyCoRepository::class.java, "findOneOptional", String::class.java, Continuation::class.java);
+
+		assertThat(metadata.getReturnedDomainClass(method)).isEqualTo(User::class.java)
+	}
+
+	@Test // DATACMNS-1689
+	fun shouldDetermineCorrectFlowResultType() {
+
+		val metadata = DefaultRepositoryMetadata(MyCoRepository::class.java)
+		val method = ReflectionUtils.findRequiredMethod(MyCoRepository::class.java, "findMultiple", String::class.java);
+
+		assertThat(metadata.getReturnedDomainClass(method)).isEqualTo(User::class.java)
+	}
+
+	@Test // DATACMNS-1689
+	fun shouldDetermineCorrectSuspendedFlowResultType() {
+
+		val metadata = DefaultRepositoryMetadata(MyCoRepository::class.java)
+		val method = ReflectionUtils.findRequiredMethod(MyCoRepository::class.java, "findMultipleSuspended", String::class.java, Continuation::class.java);
+
+		assertThat(metadata.getReturnedDomainClass(method)).isEqualTo(User::class.java)
+	}
+
+	interface MyCoRepository : CoroutineCrudRepository<User, String> {
+
+		suspend fun findOne(id: String): User
+
+		suspend fun findOneOptional(id: String): User?
+
+		fun findMultiple(id: String): Flow<User>
+
+		suspend fun findMultipleSuspended(id: String): Flow<User>
+	}
+}