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/main/java/org/springframework/data/repository/core/RepositoryMetadata.java

@@ -20,11 +20,13 @@ import java.util.Collection;
 import java.util.Set;
 
 import org.springframework.data.repository.support.Repositories;
+import org.springframework.data.util.TypeInformation;
 
 /**
  * Metadata for repository interfaces.
  *
  * @author Oliver Gierke
+ * @author Mark Paluch
  */
 public interface RepositoryMetadata {
 
@@ -50,11 +52,22 @@ public interface RepositoryMetadata {
 	Class<?> getRepositoryInterface();
 
 	/**
-	 * Returns the domain class returned by the given {@link Method}. Will extract the type from {@link Collection}s and
-	 * {@link org.springframework.data.domain.Page} as well.
+	 * Returns the type {@link Method} return type as it is declared in the repository. Considers suspended methods and
+	 * does not unwrap component types but leaves those for further inspection.
 	 *
 	 * @param method
 	 * @return
+	 * @since 2.4
+	 */
+	TypeInformation<?> getReturnType(Method method);
+
+	/**
+	 * Returns the domain class returned by the given {@link Method}. In contrast to {@link #getReturnType(Method)}, this
+	 * method extracts the type from {@link Collection}s and {@link org.springframework.data.domain.Page} as well.
+	 *
+	 * @param method
+	 * @return
+	 * @see #getReturnType(Method)
 	 */
 	Class<?> getReturnedDomainClass(Method method);
 

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

@@ -40,6 +40,7 @@ import org.springframework.util.Assert;
  * @author Oliver Gierke
  * @author Thomas Darimont
  * @author Jens Schauder
+ * @author Mark Paluch
  */
 public abstract class AbstractRepositoryMetadata implements RepositoryMetadata {
 
@@ -79,9 +80,10 @@ public abstract class AbstractRepositoryMetadata implements RepositoryMetadata {
 
 	/*
 	 * (non-Javadoc)
-	 * @see org.springframework.data.repository.core.RepositoryMetadata#getReturnedDomainClass(java.lang.reflect.Method)
+	 * @see org.springframework.data.repository.core.RepositoryMetadata#getReturnType(java.lang.reflect.Method)
 	 */
-	public Class<?> getReturnedDomainClass(Method method) {
+	@Override
+	public TypeInformation<?> getReturnType(Method method) {
 
 		TypeInformation<?> returnType = null;
 		if (KotlinDetector.isKotlinType(method.getDeclaringClass()) && KotlinReflectionUtils.isSuspend(method)) {
@@ -95,7 +97,15 @@ public abstract class AbstractRepositoryMetadata implements RepositoryMetadata {
 			returnType = typeInformation.getReturnType(method);
 		}
 
-		return QueryExecutionConverters.unwrapWrapperTypes(returnType).getType();
+		return returnType;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.repository.core.RepositoryMetadata#getReturnedDomainClass(java.lang.reflect.Method)
+	 */
+	public Class<?> getReturnedDomainClass(Method method) {
+		return QueryExecutionConverters.unwrapWrapperTypes(getReturnType(method)).getType();
 	}
 
 	/*

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

@@ -32,6 +32,7 @@ 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;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
@@ -244,6 +245,15 @@ class DefaultRepositoryInformation implements RepositoryInformation {
 		return metadata.getReturnedDomainClass(method);
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.repository.core.RepositoryMetadata#getReturnType(java.lang.reflect.Method)
+	 */
+	@Override
+	public TypeInformation<?> getReturnType(Method method) {
+		return metadata.getReturnType(method);
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.repository.core.RepositoryMetadata#getCrudMethods()

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>
+	}
+}