DATACMNS-483 - Support for wrapper types as return types for repository methods.

The repository method invocation mechanism now post-processes the result of the actual method invocation in case it's not type-compatible with the return type of the invoked method and invokes a conversion if applicable. This allows us to register custom converters for well-known wrapper types. We currently support Optional from both Google Guava and JDK 8.

Implementation notice

We use some JDK 8 type stubs from the Spring Data Build project to be able to compile using JDKs < 8. This is necessary as we need to remain compatible with Spring 3.2.x for the Dijkstra release train and our test executions still run into some issues with it a Java 8 runtime (mostly ASM related). These issues were reported in [0, 1] and we'll probably get off the hack for Dijkstra GA by upgrading to Spring 3.2.9.

[0] https://jira.spring.io/browse/SPR-11719
[1] https://jira.spring.io/browse/SPR-11718

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

@@ -15,9 +15,11 @@
  */
 package org.springframework.data.repository.core.support;
 
+import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 
 import java.io.Serializable;
+import java.lang.reflect.Method;
 import java.util.Collection;
 
 import org.junit.Test;
@@ -28,6 +30,8 @@ import org.springframework.data.repository.Repository;
 import org.springframework.data.repository.core.RepositoryMetadata;
 import org.springframework.data.repository.util.ClassUtils;
 
+import com.google.common.base.Optional;
+
 /**
  * Unit tests for {@link DefaultRepositoryMetadata}.
  * 
@@ -101,6 +105,18 @@ public class DefaultRepositoryMetadataUnitTests {
 		assertEquals(Long.class, metadata.getIdType());
 	}
 
+	/**
+	 * @see DATACMNS-483
+	 */
+	@Test
+	public void discoversDomainTypeOnReturnTypeWrapper() throws Exception {
+
+		RepositoryMetadata metadata = new DefaultRepositoryMetadata(OptionalRepository.class);
+
+		Method method = OptionalRepository.class.getMethod("findByEmailAddress", String.class);
+		assertThat(metadata.getReturnedDomainClass(method), is(typeCompatibleWith(User.class)));
+	}
+
 	@SuppressWarnings("unused")
 	private class User {
 
@@ -167,4 +183,9 @@ public class DefaultRepositoryMetadataUnitTests {
 	static interface ConcreteRepository extends IdTypeFixingRepository<User> {
 
 	}
+
+	static interface OptionalRepository extends Repository<User, Long> {
+
+		Optional<User> findByEmailAddress(String emailAddress);
+	}
 }

src/test/java/org/springframework/data/repository/util/QueryExecutionConvertersUnitTests.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2014 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
+ *
+ *      http://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.util;
+
+import static org.hamcrest.CoreMatchers.*;
+import static org.junit.Assert.*;
+
+import org.junit.Assume;
+import org.junit.Before;
+import org.junit.Test;
+import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.util.ClassUtils;
+
+import com.google.common.base.Optional;
+
+/**
+ * Unit tests for {@link QueryExecutionConverters}.
+ * 
+ * @author Oliver Gierke
+ */
+public class QueryExecutionConvertersUnitTests {
+
+	DefaultConversionService conversionService;
+
+	@Before
+	public void setUp() {
+
+		this.conversionService = new DefaultConversionService();
+		QueryExecutionConverters.registerConvertersIn(conversionService);
+	}
+
+	/**
+	 * @see DATACMNS-483
+	 */
+	@Test
+	@SuppressWarnings("unchecked")
+	public void turnsNullIntoGuavaOptional() {
+
+		Optional<Object> optional = conversionService.convert(new NullableWrapper(null), Optional.class);
+		assertThat(optional, is(Optional.<Object> absent()));
+	}
+
+	/**
+	 * @see DATACMNS-483
+	 */
+	@Test
+	@SuppressWarnings("unchecked")
+	public void turnsNullIntoJdk8Optional() {
+
+		Assume.assumeThat(ClassUtils.isPresent("java.util.Optional", getClass().getClassLoader()), is(true));
+
+		java.util.Optional<Object> optional = conversionService.convert(new NullableWrapper(null), java.util.Optional.class);
+		assertThat(optional, is(java.util.Optional.<Object> empty()));
+	}
+}