DATAREST-511 - Support for executing repository methods returning Optionals.

Added an UnwrappingRepositoryInvokerFactory that transparently unwraps JDK 8 and Guava Optionals to make sure the consuming code works with values or plain nulls correctly.

spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/config/RepositoryRestMvcConfiguration.java

@@ -65,6 +65,7 @@ import org.springframework.data.rest.core.mapping.ResourceDescription;
 import org.springframework.data.rest.core.mapping.ResourceMappings;
 import org.springframework.data.rest.core.support.DomainObjectMerger;
 import org.springframework.data.rest.core.support.RepositoryRelProvider;
+import org.springframework.data.rest.core.support.UnwrappingRepositoryInvokerFactory;
 import org.springframework.data.rest.webmvc.BasePathAwareController;
 import org.springframework.data.rest.webmvc.BasePathAwareHandlerMapping;
 import org.springframework.data.rest.webmvc.BaseUri;
@@ -575,7 +576,8 @@ public class RepositoryRestMvcConfiguration extends HateoasAwareSpringDataWebCon
 
 	@Bean
 	public RepositoryInvokerFactory repositoryInvokerFactory() {
-		return new DefaultRepositoryInvokerFactory(repositories(), defaultConversionService());
+		return new UnwrappingRepositoryInvokerFactory(new DefaultRepositoryInvokerFactory(repositories(),
+				defaultConversionService()));
 	}
 
 	@Bean

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/TestMvcClient.java

@@ -20,6 +20,8 @@ import static org.junit.Assert.*;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
 
+import java.util.Arrays;
+import java.util.Iterator;
 import java.util.List;
 
 import org.springframework.hateoas.Link;
@@ -184,6 +186,27 @@ public class TestMvcClient {
 		return discover.get(0);
 	}
 
+	/**
+	 * Traverses the given link relations from the root.
+	 * 
+	 * @param rels
+	 * @return
+	 * @throws Exception
+	 */
+	public Link discoverUnique(String... rels) throws Exception {
+
+		Iterator<String> toTraverse = Arrays.asList(rels).iterator();
+		Link lastLink = null;
+
+		while (toTraverse.hasNext()) {
+
+			String rel = toTraverse.next();
+			lastLink = lastLink == null ? discoverUnique(rel) : discoverUnique(lastLink, rel);
+		}
+
+		return lastLink;
+	}
+
 	/**
 	 * Given a URI (root), discover the URIs for a given rel.
 	 *

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/mongodb/MongoWebTests.java

@@ -301,4 +301,18 @@ public class MongoWebTests extends CommonWebTests {
 				andExpect(status().isNotModified()).//
 				andExpect(header().string(ETAG, is(notNullValue())));
 	}
+
+	/**
+	 * @see DATAREST-511
+	 */
+	@Test
+	public void invokesQueryResourceReturningAnOptional() throws Exception {
+
+		Profile profile = repository.findAll().iterator().next();
+
+		Link link = client.discoverUnique("profiles", "search", "findById");
+
+		mvc.perform(get(link.expand(profile.getId()).getHref())).//
+				andExpect(status().isOk());
+	}
 }

spring-data-rest-webmvc/src/test/java/org/springframework/data/rest/webmvc/mongodb/ProfileRepository.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -16,6 +16,7 @@
 package org.springframework.data.rest.webmvc.mongodb;
 
 import java.util.List;
+import java.util.Optional;
 
 import org.springframework.data.repository.PagingAndSortingRepository;
 import org.springframework.data.repository.query.Param;
@@ -32,4 +33,9 @@ public interface ProfileRepository extends PagingAndSortingRepository<Profile, S
 	 * @see DATAREST-247
 	 */
 	long countByType(@Param("type") String type);
+
+	/**
+	 * @see DATAREST-511
+	 */
+	Optional<Profile> findById(@Param("id") String id);
 }