DATACMNS-1162 - Improved support for immutable aggregates.

AbstractAggregateRoot is now generified to be able to use its concrete type on methods returning an instance of itself. Added new methods andEvent(…) and andEventsFrom(…) to easily transfer events from another aggregate instance. Added unit tests for fundamental functionality of the base class.

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

@@ -33,7 +33,7 @@ import org.springframework.util.Assert;
  * @author Christoph Strobl
  * @since 1.13
  */
-public class AbstractAggregateRoot {
+public class AbstractAggregateRoot<A extends AbstractAggregateRoot<A>> {
 
 	private transient final @Transient List<Object> domainEvents = new ArrayList<>();
 
@@ -41,7 +41,8 @@ public class AbstractAggregateRoot {
 	 * Registers the given event object for publication on a call to a Spring Data repository's save methods.
 	 * 
 	 * @param event must not be {@literal null}.
-	 * @return
+	 * @return the event that has been added.
+	 * @see #andEvent(Object)
 	 */
 	protected <T> T registerEvent(T event) {
 
@@ -67,4 +68,34 @@ public class AbstractAggregateRoot {
 	protected Collection<Object> domainEvents() {
 		return Collections.unmodifiableList(domainEvents);
 	}
+
+	/**
+	 * Adds all events contained in the given aggregate to the current one.
+	 * 
+	 * @param aggregate must not be {@literal null}.
+	 * @return the aggregate
+	 */
+	protected final A andEventsFrom(A aggregate) {
+
+		Assert.notNull(aggregate, "Aggregate must not be null!");
+
+		this.domainEvents.addAll(aggregate.domainEvents());
+
+		return (A) this;
+	}
+
+	/**
+	 * Adds the given event to the aggregate for later publication when calling a Spring Data repository's save-method.
+	 * Does the same as {@link #registerEvent(Object)} but returns the aggregate instead of the event.
+	 * 
+	 * @param event must not be {@literal null}.
+	 * @return the aggregate
+	 * @see #registerEvent(Object)
+	 */
+	protected final A andEvent(Object event) {
+
+		registerEvent(event);
+
+		return (A) this;
+	}
 }

src/test/java/org/springframework/data/domain/AbstractAggregateRootUnitTests.java

@@ -0,0 +1,106 @@
+/*
+ * 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.domain;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.junit.Test;
+
+/**
+ * Unit tests for {@link AbstractAggregateRoot}.
+ * 
+ * @author Oliver Gierke
+ */
+public class AbstractAggregateRootUnitTests {
+
+	@Test // DATACMNS-928
+	public void registersEvent() {
+
+		Object event = new Object();
+
+		SampleAggregate aggregate = new SampleAggregate();
+		aggregate.registerEvent(event);
+
+		assertThat(aggregate.domainEvents()).containsExactly(event);
+	}
+
+	@Test // DATACMNS-928
+	public void clearsEvents() {
+
+		Object event = new Object();
+
+		SampleAggregate aggregate = new SampleAggregate();
+		aggregate.registerEvent(event);
+
+		assertThat(aggregate.domainEvents()).isNotEmpty();
+
+		aggregate.clearDomainEvents();
+
+		assertThat(aggregate.domainEvents()).isEmpty();
+	}
+
+	@Test // DATACMNS-928, DATACMNS-1162
+	public void copiesEventsFromExistingAggregate() {
+
+		SampleAggregate aggregate = new SampleAggregate();
+		aggregate.registerEvent(new Object());
+
+		SampleAggregate result = new SampleAggregate().andEventsFrom(aggregate);
+
+		assertThat(result.domainEvents()).isEqualTo(aggregate.domainEvents());
+	}
+
+	@Test // DATACMNS-928, DATACMNS-1162
+	public void addsEventAndReturnsAggregate() {
+
+		Object first = new Object();
+		Object second = new Object();
+
+		SampleAggregate aggregate = new SampleAggregate();
+		aggregate.registerEvent(first);
+
+		SampleAggregate result = aggregate.andEvent(second);
+
+		assertThat(result).isSameAs(aggregate);
+		assertThat(result.domainEvents()).containsExactly(first, second);
+	}
+
+	@Test // DATACMNS-928, DATACMNS-1162
+	@SuppressWarnings("null")
+	public void rejectsNullEvent() {
+
+		assertThatExceptionOfType(IllegalArgumentException.class) //
+				.isThrownBy(() -> new SampleAggregate().andEvent(null));
+	}
+
+	@Test // DATACMNS-928
+	@SuppressWarnings("null")
+	public void rejectsNullEventForRegistration() {
+
+		assertThatExceptionOfType(IllegalArgumentException.class) //
+				.isThrownBy(() -> new SampleAggregate().registerEvent(null));
+	}
+
+	@Test // DATACMNS-928, DATACMNS-1162
+	@SuppressWarnings("null")
+	public void rejectsNullAggregate() {
+
+		assertThatExceptionOfType(IllegalArgumentException.class) //
+				.isThrownBy(() -> new SampleAggregate().andEventsFrom(null));
+	}
+
+	static class SampleAggregate extends AbstractAggregateRoot<SampleAggregate> {}
+}