DATACMNS-1738 - Expose method to obtain the repository query method return type.

We now expose RepositoryMetadata.getReturnType(…) to obtain the declared method return type. While pure Java code can rely on Method.getReturnType(), suspended Kotlin methods (Coroutines) require additional lookups. This is because the Kotlin compiler messes around with the return type in the bytecode. It changes the return type to java.lang.Object while synthesizing an additional method argument:

interface MyCoroutineRepository : Repository<Person, String> {
	suspend fun suspendedQueryMethod(): Flow<Person>
}

compiles to

interface MyCoroutineRepository extends Repository<Person, String> {
	Object suspendedQueryMethod(Continuation<Flow<Person>> arg0);
}

Therefore, the triviality of obtaining a return type becomes an inspection of the actual method arguments.

Related ticket: https://jira.spring.io/browse/DATAMONGO-2562

src/test/java/org/springframework/data/repository/core/support/DummyRepositoryInformation.java

@@ -22,6 +22,7 @@ import org.springframework.data.repository.core.CrudMethods;
 import org.springframework.data.repository.core.RepositoryInformation;
 import org.springframework.data.repository.core.RepositoryMetadata;
 import org.springframework.data.util.Streamable;
+import org.springframework.data.util.TypeInformation;
 
 public final class DummyRepositoryInformation implements RepositoryInformation {
 
@@ -47,6 +48,11 @@ public final class DummyRepositoryInformation implements RepositoryInformation {
 		return metadata.getRepositoryInterface();
 	}
 
+	@Override
+	public TypeInformation<?> getReturnType(Method method) {
+		return metadata.getReturnType(method);
+	}
+
 	public Class<?> getReturnedDomainClass(Method method) {
 		return getDomainType();
 	}

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

@@ -0,0 +1,55 @@
+/*
+ * 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.core.support
+
+import kotlinx.coroutines.flow.Flow
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.springframework.data.mapping.Person
+import org.springframework.data.repository.Repository
+import org.springframework.data.repository.core.RepositoryMetadata
+import kotlin.coroutines.Continuation
+
+/**
+ * Coroutine unit tests for [RepositoryMetadata].
+ *
+ * @author Mark Paluch
+ */
+class CoroutineRepositoryMetadataUnitTests {
+
+	var metadata: RepositoryMetadata = DefaultRepositoryMetadata(MyCoroutineRepository::class.java)
+
+	@Test // DATACMNS-1508
+	fun `should consider Flow return type`() {
+
+		val queryMethod = MyCoroutineRepository::class.java.getDeclaredMethod("suspendedQueryMethod", Continuation::class.java)
+		assertThat(metadata.getReturnType(queryMethod).type).isEqualTo(Flow::class.java)
+	}
+
+	@Test // DATACMNS-1508
+	fun `should consider suspended Flow return`() {
+
+		val queryMethod = MyCoroutineRepository::class.java.getDeclaredMethod("queryMethod")
+		assertThat(metadata.getReturnType(queryMethod).type).isEqualTo(Flow::class.java)
+	}
+
+	interface MyCoroutineRepository : Repository<Person, String> {
+
+		suspend fun suspendedQueryMethod(): Flow<Person>
+
+		fun queryMethod(): Flow<Person>
+	}
+}