DATACMNS-995 - Create ReactiveQueryByExampleExecutor allowing reactive query by example.

Provide a mix-in interface to be used in store modules providing reactive Query by Example query execution.

    Mono<T> findOne(Example<T> sample);
    Flux<T> findAll(Example<T> sample);
    Mono<Long> count(Example<T> sample);
    //...

Original Pull Request: #197

src/main/java/org/springframework/data/repository/query/ReactiveQueryByExampleExecutor.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2017 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.query;
+
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+import org.springframework.data.domain.Example;
+import org.springframework.data.domain.Sort;
+
+/**
+ * Interface to allow execution of Query by Example {@link Example} instances using a reactive infrastructure.
+ *
+ * @param <T>
+ * @author Mark Paluch
+ * @since 2.0
+ */
+public interface ReactiveQueryByExampleExecutor<T> {
+
+	/**
+	 * Returns a single entity matching the given {@link Example} or {@literal null} if none was found.
+	 *
+	 * @param example can be {@literal null}.
+	 * @return a single entity matching the given {@link Example} or {@literal null} if none was found.
+	 */
+	<S extends T> Mono<S> findOne(Example<S> example);
+
+	/**
+	 * Returns all entities matching the given {@link Example}. In case no match could be found an empty {@link Iterable}
+	 * is returned.
+	 *
+	 * @param example can be {@literal null}.
+	 * @return all entities matching the given {@link Example}.
+	 */
+	<S extends T> Flux<S> findAll(Example<S> example);
+
+	/**
+	 * Returns all entities matching the given {@link Example} applying the given {@link Sort}. In case no match could be
+	 * found an empty {@link Iterable} is returned.
+	 *
+	 * @param example can be {@literal null}.
+	 * @param sort the {@link Sort} specification to sort the results by, must not be {@literal null}.
+	 * @return all entities matching the given {@link Example}.
+	 */
+	<S extends T> Flux<S> findAll(Example<S> example, Sort sort);
+
+	/**
+	 * Returns the number of instances matching the given {@link Example}.
+	 *
+	 * @param example the {@link Example} to count instances for, can be {@literal null}.
+	 * @return the number of instances matching the {@link Example}.
+	 */
+	<S extends T> Mono<Long> count(Example<S> example);
+
+	/**
+	 * Checks whether the data store contains elements that match the given {@link Example}.
+	 *
+	 * @param example the {@link Example} to use for the existence check, can be {@literal null}.
+	 * @return {@literal true} if the data store contains elements that match the given {@link Example}.
+	 */
+	<S extends T> Mono<Boolean> exists(Example<S> example);
+}