DATACMNS-1063 - Accept Publisher in ReactiveCrudRepository findById(…) and existsById(…).

We now accept Publisher<T> instead of Mono<T> in findById(…) and existsById(…). Users of a ReactiveStreams-based framework are no longer required to perform Publisher to Mono-adoption themselves but can pass a Publisher directly. Both methods use the first emitted value to issue their queries. Additional values are not consumed from the stream.

Original Pull Request: #226

src/main/java/org/springframework/data/repository/reactive/ReactiveCrudRepository.java

@@ -73,11 +73,11 @@ public interface ReactiveCrudRepository<T, ID> extends Repository<T, ID> {
 	/**
 	 * Retrieves an entity by its id supplied by a {@link Mono}.
 	 *
-	 * @param id must not be {@literal null}.
+	 * @param id must not be {@literal null}. Uses the first emitted element to perform the find-query.
 	 * @return the entity with the given id or {@link Mono#empty()} if none found.
 	 * @throws IllegalArgumentException if {@code id} is {@literal null}.
 	 */
-	Mono<T> findById(Mono<ID> id);
+	Mono<T> findById(Publisher<ID> id);
 
 	/**
 	 * Returns whether an entity with the given id exists.
@@ -89,13 +89,14 @@ public interface ReactiveCrudRepository<T, ID> extends Repository<T, ID> {
 	Mono<Boolean> existsById(ID id);
 
 	/**
-	 * Returns whether an entity with the given id, supplied by a {@link Mono}, exists.
+	 * Returns whether an entity with the given id, supplied by a {@link Mono}, exists. Uses the first emitted element to
+	 * perform the exists-query.
 	 *
 	 * @param id must not be {@literal null}.
 	 * @return {@literal true} if an entity with the given id exists, {@literal false} otherwise
 	 * @throws IllegalArgumentException if {@code id} is {@literal null}
 	 */
-	Mono<Boolean> existsById(Mono<ID> id);
+	Mono<Boolean> existsById(Publisher<ID> id);
 
 	/**
 	 * Returns all instances of the type.

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

@@ -20,6 +20,7 @@ import static org.mockito.Mockito.*;
 
 import io.reactivex.Completable;
 import io.reactivex.Maybe;
+import org.reactivestreams.Publisher;
 import reactor.core.publisher.Mono;
 import rx.Single;
 
@@ -37,7 +38,7 @@ import org.springframework.data.repository.reactive.ReactiveSortingRepository;
 
 /**
  * Unit tests for {@link RepositoryFactorySupport} using reactive wrapper types.
- * 
+ *
  * @author Mark Paluch
  * @author Christoph Strobl
  */
@@ -77,7 +78,7 @@ public class ReactiveWrapperRepositoryFactorySupportUnitTests {
 		verify(backingRepo, times(2)).existsById(id);
 	}
 
-	@Test // DATACMNS-836
+	@Test // DATACMNS-836, DATACMNS-1063
 	@SuppressWarnings("unchecked")
 	public void callsRxJava1MethodOnBaseImplementationWithTypeConversion() {
 
@@ -86,7 +87,7 @@ public class ReactiveWrapperRepositoryFactorySupportUnitTests {
 		RxJava1ConvertingRepository repository = factory.getRepository(RxJava1ConvertingRepository.class);
 		repository.existsById(ids);
 
-		verify(backingRepo, times(1)).existsById(any(Mono.class));
+		verify(backingRepo, times(1)).existsById(any(Publisher.class));
 	}
 
 	@Test // DATACMNS-988