DATACMNS-1231 - Add reactive auditing infrastructure.

We now provide a reactive variant for auditing with ReactiveAuditingHandler and ReactiveIsNewAwareAuditingHandler.

Extracted common auditing functionality into AuditingHandlerSupport which serves as base class for AuditingHandler and ReactiveAuditingHandler.

Original Pull Request: #458

src/main/java/org/springframework/data/domain/AuditorAware.java

@@ -20,7 +20,7 @@ import java.util.Optional;
 /**
  * Interface for components that are aware of the application's current auditor. This will be some kind of user mostly.
  *
- * @param <T> the type of the auditing instance
+ * @param <T> the type of the auditing instance.
  * @author Oliver Gierke
  */
 public interface AuditorAware<T> {
@@ -28,7 +28,7 @@ public interface AuditorAware<T> {
 	/**
 	 * Returns the current auditor of the application.
 	 *
-	 * @return the current auditor
+	 * @return the current auditor.
 	 */
 	Optional<T> getCurrentAuditor();
 }

src/main/java/org/springframework/data/domain/ReactiveAuditorAware.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2008-2020 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.domain;
+
+import reactor.core.publisher.Mono;
+
+/**
+ * Interface for components that are aware of the application's current auditor. This will be some kind of user mostly.
+ *
+ * @param <T> the type of the auditing instance.
+ * @author Mark Paluch
+ * @since 2.4
+ */
+public interface ReactiveAuditorAware<T> {
+
+	/**
+	 * Returns the current auditor of the application.
+	 *
+	 * @return the current auditor. If the mono does not emit a value, the auditor is considered to be unknown.
+	 */
+	Mono<T> getCurrentAuditor();
+}