INT-4193: Large Group Aggregation Performance

JIRA: https://jira.spring.io/browse/INT-4193

When using the internal `SequenceAwareMessageGroup` within an correlating
message handler, the messages were copied to a new collection before
checking for duplicate sequences in `canAdd()`.

This was unnecessary since we never add anything to this group, if `canAdd()`
returns true, the message is added to the store; this group is discarded.

Instead, use the message collection from the original group; although it is
not modifiable, this is not an issue because we don't need to modify it.

Conflicts:
	spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java
	spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroupFactory.java
	src/reference/asciidoc/aggregator.adoc

* Polishing `aggregator.adoc` to reflect reality

spring-integration-core/src/main/java/org/springframework/integration/aggregator/AbstractCorrelatingMessageHandler.java

@@ -753,13 +753,20 @@ public abstract class AbstractCorrelatingMessageHandler extends AbstractMessageP
 	protected static class SequenceAwareMessageGroup extends SimpleMessageGroup {
 
 		public SequenceAwareMessageGroup(MessageGroup messageGroup) {
-			super(messageGroup);
+			/*
+			 * Since this group is temporary, and never added to, we simply use the
+			 * supplied group's message collection for the lookup rather than creating a
+			 * new group.
+			 */
+			super(messageGroup.getMessages(), null, messageGroup.getGroupId(), messageGroup.getTimestamp(),
+					messageGroup.isComplete(), true);
 		}
 
 		/**
-		 * This method determines whether messages have been added to this group that supersede the given message based on
-		 * its sequence id. This can be helpful to avoid ending up with sequences larger than their required sequence size
-		 * or sequences that are missing certain sequence numbers.
+		 * This method determines whether messages have been added to this group that
+		 * supersede the given message based on its sequence id. This can be helpful to
+		 * avoid ending up with sequences larger than their required sequence size or
+		 * sequences that are missing certain sequence numbers.
 		 */
 		@Override
 		public boolean canAdd(Message<?> message) {

spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroup.java

@@ -67,20 +67,22 @@ public class SimpleMessageGroup implements MessageGroup {
 
 	public SimpleMessageGroup(Collection<? extends Message<?>> messages, Object groupId, long timestamp,
 			boolean complete) {
-		this(new LinkedHashSet<Message<?>>(), messages, groupId, timestamp, complete);
+		this(new LinkedHashSet<Message<?>>(), messages, groupId, timestamp, complete, false);
 	}
 
-	SimpleMessageGroup(Collection<Message<?>> internalStore, Collection<? extends Message<?>> messages, Object groupId,
-			long timestamp, boolean complete) {
+	protected SimpleMessageGroup(Collection<Message<?>> internalStore, Collection<? extends Message<?>> messages,
+			Object groupId, long timestamp, boolean complete, boolean storePreLoaded) {
 		Assert.notNull(internalStore, "'internalStore' must not be null");
-		Assert.notNull(messages, "'messages' must not be null");
 		this.messages = internalStore;
 		this.groupId = groupId;
 		this.timestamp = timestamp;
 		this.complete = complete;
-		for (Message<?> message : messages) {
-			if (message != null) { //see INT-2666
-				addMessage(message);
+		if (!storePreLoaded) {
+			Assert.notNull(messages, "'messages' must not be null");
+			for (Message<?> message : messages) {
+				if (message != null) { //see INT-2666
+					addMessage(message);
+				}
 			}
 		}
 	}

spring-integration-core/src/main/java/org/springframework/integration/store/SimpleMessageGroupFactory.java

@@ -56,7 +56,7 @@ public class SimpleMessageGroupFactory implements MessageGroupFactory {
 	@Override
 	public MessageGroup create(Collection<? extends Message<?>> messages, Object groupId, long timestamp,
 			boolean complete) {
-		return new SimpleMessageGroup(this.type.get(), messages, groupId, timestamp, complete);
+		return new SimpleMessageGroup(this.type.get(), messages, groupId, timestamp, complete, false);
 	}
 
 	@Override

spring-integration-core/src/test/java/org/springframework/integration/store/SimpleMessageGroupTests.java

@@ -26,10 +26,12 @@ import java.lang.reflect.Constructor;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
 
 import org.junit.Test;
 
+import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.support.GenericMessage;
@@ -40,6 +42,7 @@ import org.springframework.util.StopWatch;
  * @author Oleg Zhurakousky
  * @author Dave Syer
  * @author Artem Bilan
+ * @author Gary Russell
  */
 public class SimpleMessageGroupTests {
 
@@ -54,11 +57,12 @@ public class SimpleMessageGroupTests {
 		Constructor<SimpleMessageGroup> ctr = clazz.getDeclaredConstructor(MessageGroup.class);
 		ctr.setAccessible(true);
 		group = ctr.newInstance(group);
+		new DirectFieldAccessor(group).setPropertyValue("messages", new HashSet<Message<?>>());
 	}
 
 	@Test
 	public void shouldFindSupersedingMessagesIfSequenceAware() throws Exception {
-		this.prepareForSequenceAwareMessageGroup();
+		prepareForSequenceAwareMessageGroup();
 		final Message<?> message1 = MessageBuilder.withPayload("test").setSequenceNumber(1).build();
 		final Message<?> message2 = MessageBuilder.fromMessage(message1).setSequenceNumber(1).build();
 		assertThat(group.canAdd(message1), is(true));
@@ -69,7 +73,7 @@ public class SimpleMessageGroupTests {
 
 	@Test
 	public void shouldIgnoreMessagesWithZeroSequenceNumberIfSequenceAware() throws Exception {
-		this.prepareForSequenceAwareMessageGroup();
+		prepareForSequenceAwareMessageGroup();
 		final Message<?> message1 = MessageBuilder.withPayload("test").build();
 		final Message<?> message2 = MessageBuilder.fromMessage(message1).build();
 		assertThat(group.canAdd(message1), is(true));