GH-3067: Use default NullChannel instance if poss.

Resolves #3067

Also add missing receive counter.

* Late binding of null discard channel; checkstyle

* Fix test; fall back to new NullChannel(); always evaluate tx expressions

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

@@ -39,6 +39,7 @@
 import org.springframework.expression.Expression;
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
 import org.springframework.integration.channel.NullChannel;
+import org.springframework.integration.context.IntegrationContextUtils;
 import org.springframework.integration.expression.ExpressionUtils;
 import org.springframework.integration.handler.AbstractMessageProducingHandler;
 import org.springframework.integration.handler.DiscardingMessageHandler;
@@ -55,6 +56,7 @@
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageDeliveryException;
 import org.springframework.messaging.MessageHandlingException;
+import org.springframework.messaging.core.DestinationResolutionException;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 
@@ -315,7 +317,7 @@ protected void onInit() {
 		super.onInit();
 		Assert.state(!(this.discardChannelName != null && this.discardChannel != null),
 				"'discardChannelName' and 'discardChannel' are mutually exclusive.");
-		BeanFactory beanFactory = this.getBeanFactory();
+		BeanFactory beanFactory = getBeanFactory();
 		if (beanFactory != null) {
 			if (this.outputProcessor instanceof BeanFactoryAware) {
 				((BeanFactoryAware) this.outputProcessor).setBeanFactory(beanFactory);
@@ -328,10 +330,6 @@ protected void onInit() {
 			}
 		}
 
-		if (this.discardChannel == null) {
-			this.discardChannel = new NullChannel();
-		}
-
 		if (this.releasePartialSequences) {
 			Assert.isInstanceOf(SequenceSizeReleaseStrategy.class, this.releaseStrategy, () ->
 					"Release strategy of type [" + this.releaseStrategy.getClass().getSimpleName() +
@@ -392,8 +390,21 @@ protected ReleaseStrategy getReleaseStrategy() {
 	@Override
 	public MessageChannel getDiscardChannel() {
 		String channelName = this.discardChannelName;
+		if (channelName == null && this.discardChannel == null) {
+			channelName = IntegrationContextUtils.NULL_CHANNEL_BEAN_NAME;
+		}
 		if (channelName != null) {
-			this.discardChannel = getChannelResolver().resolveDestination(channelName);
+			try {
+				this.discardChannel = getChannelResolver().resolveDestination(channelName);
+			}
+			catch (DestinationResolutionException ex) {
+				if (channelName.equals(IntegrationContextUtils.NULL_CHANNEL_BEAN_NAME)) {
+					this.discardChannel = new NullChannel();
+				}
+				else {
+					throw ex;
+				}
+			}
 			this.discardChannelName = null;
 		}
 		return this.discardChannel;

spring-integration-core/src/main/java/org/springframework/integration/channel/NullChannel.java

@@ -27,6 +27,7 @@
 import org.springframework.integration.support.management.ConfigurableMetricsAware;
 import org.springframework.integration.support.management.DefaultMessageChannelMetrics;
 import org.springframework.integration.support.management.IntegrationManagedResource;
+import org.springframework.integration.support.management.metrics.CounterFacade;
 import org.springframework.integration.support.management.metrics.MetricsCaptor;
 import org.springframework.integration.support.management.metrics.TimerFacade;
 import org.springframework.lang.Nullable;
@@ -68,6 +69,8 @@ public class NullChannel implements PollableChannel,
 
 	private TimerFacade successTimer;
 
+	private CounterFacade receiveCounter;
+
 	@Override
 	public void setBeanName(String beanName) {
 		this.beanName = beanName;
@@ -268,6 +271,7 @@ public Message<?> receive() {
 		if (this.loggingEnabled) {
 			this.logger.debug("receive called on null channel");
 		}
+		incrementReceiveCounter();
 		return null;
 	}
 
@@ -276,6 +280,27 @@ public Message<?> receive(long timeout) {
 		return receive();
 	}
 
+	private void incrementReceiveCounter() {
+		if (this.metricsCaptor != null) {
+			if (this.receiveCounter == null) {
+				this.receiveCounter = buildReceiveCounter();
+			}
+			this.receiveCounter.increment();
+		}
+	}
+
+	private CounterFacade buildReceiveCounter() {
+		CounterFacade counterFacade = this.metricsCaptor
+				.counterBuilder(RECEIVE_COUNTER_NAME)
+				.tag("name", getComponentName() == null ? "unknown" : getComponentName())
+				.tag("type", "channel")
+				.tag("result", "success")
+				.tag("exception", "none")
+				.description("Messages received")
+				.build();
+		return counterFacade;
+	}
+
 	@Override
 	public String toString() {
 		return (this.beanName != null) ? this.beanName : super.toString();
@@ -286,6 +311,9 @@ public void destroy() {
 		if (this.successTimer != null) {
 			this.successTimer.remove();
 		}
+		if (this.receiveCounter != null) {
+			this.receiveCounter.remove();
+		}
 	}
 
 }

spring-integration-core/src/main/java/org/springframework/integration/transaction/ExpressionEvaluatingTransactionSynchronizationProcessor.java

@@ -21,9 +21,9 @@
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.Expression;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
-import org.springframework.integration.channel.NullChannel;
 import org.springframework.integration.context.IntegrationObjectSupport;
 import org.springframework.integration.expression.ExpressionUtils;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.util.Assert;
@@ -66,11 +66,11 @@ public class ExpressionEvaluatingTransactionSynchronizationProcessor extends Int
 
 	private volatile Expression afterRollbackExpression;
 
-	private volatile MessageChannel beforeCommitChannel = new NullChannel();
+	private volatile MessageChannel beforeCommitChannel;
 
-	private volatile MessageChannel afterCommitChannel = new NullChannel();
+	private volatile MessageChannel afterCommitChannel;
 
-	private volatile MessageChannel afterRollbackChannel = new NullChannel();
+	private volatile MessageChannel afterRollbackChannel;
 
 	public void setIntegrationEvaluationContext(EvaluationContext evaluationContext) {
 		this.evaluationContext = evaluationContext;
@@ -129,8 +129,8 @@ public void processAfterRollback(IntegrationResourceHolder holder) {
 		doProcess(holder, this.afterRollbackExpression, this.afterRollbackChannel, "afterRollback");
 	}
 
-	private void doProcess(IntegrationResourceHolder holder, Expression expression, MessageChannel messageChannel,
-			String expressionType) {
+	private void doProcess(IntegrationResourceHolder holder, Expression expression,
+			@Nullable MessageChannel messageChannel, String expressionType) {
 
 		Message<?> message = holder.getMessage();
 		if (message != null) {
@@ -141,7 +141,7 @@ private void doProcess(IntegrationResourceHolder holder, Expression expression,
 				}
 				EvaluationContext evaluationContextToUse = prepareEvaluationContextToUse(holder);
 				Object value = expression.getValue(evaluationContextToUse, message);
-				if (value != null) {
+				if (value != null && messageChannel != null) {
 					if (logger.isDebugEnabled()) {
 						logger.debug("Sending expression result message to " + messageChannel + " " +
 								"as part of '" + expressionType + "' transaction synchronization");
@@ -171,7 +171,7 @@ private void doProcess(IntegrationResourceHolder holder, Expression expression,
 					}
 				}
 			}
-			else {
+			else if (messageChannel != null) {
 				if (logger.isDebugEnabled()) {
 					logger.debug("Sending received message to " + messageChannel + " as part of '" +
 							expressionType + "' transaction synchronization");

spring-integration-core/src/test/java/org/springframework/integration/config/ResequencerParserTests.java

@@ -30,7 +30,6 @@
 import org.springframework.integration.aggregator.MethodInvokingReleaseStrategy;
 import org.springframework.integration.aggregator.ReleaseStrategy;
 import org.springframework.integration.aggregator.ResequencingMessageHandler;
-import org.springframework.integration.channel.NullChannel;
 import org.springframework.integration.endpoint.EventDrivenConsumer;
 import org.springframework.integration.store.MessageGroup;
 import org.springframework.integration.store.SimpleMessageGroup;
@@ -60,7 +59,6 @@ void testDefaultResequencerProperties() {
 		ResequencingMessageHandler resequencer = TestUtils.getPropertyValue(endpoint, "handler",
 				ResequencingMessageHandler.class);
 		assertThat(getPropertyValue(resequencer, "outputChannel")).isNull();
-		assertThat(getPropertyValue(resequencer, "discardChannel") instanceof NullChannel).isTrue();
 		assertThat(getPropertyValue(
 				resequencer, "messagingTemplate.sendTimeout"))
 				.as("The ResequencerEndpoint is not set with the appropriate timeout value").isEqualTo(-1L);

spring-integration-core/src/test/java/org/springframework/integration/config/annotation/AggregatorAnnotationTests.java

@@ -30,7 +30,6 @@
 import org.springframework.integration.aggregator.MethodInvokingCorrelationStrategy;
 import org.springframework.integration.aggregator.MethodInvokingReleaseStrategy;
 import org.springframework.integration.aggregator.SimpleSequenceSizeReleaseStrategy;
-import org.springframework.integration.channel.NullChannel;
 import org.springframework.integration.endpoint.EventDrivenConsumer;
 import org.springframework.integration.test.util.TestUtils;
 import org.springframework.messaging.MessageHandler;
@@ -39,6 +38,7 @@
  * @author Marius Bogoevici
  * @author Mark Fisher
  * @author Artem Bilan
+ * @author Gary Russell
  */
 public class AggregatorAnnotationTests {
 
@@ -51,7 +51,6 @@ public void testAnnotationWithDefaultSettings() {
 		assertThat(getPropertyValue(aggregator, "releaseStrategy") instanceof SimpleSequenceSizeReleaseStrategy)
 				.isTrue();
 		assertThat(getPropertyValue(aggregator, "outputChannel")).isNull();
-		assertThat(getPropertyValue(aggregator, "discardChannel") instanceof NullChannel).isTrue();
 		assertThat(getPropertyValue(aggregator, "messagingTemplate.sendTimeout")).isEqualTo(-1L);
 		assertThat(getPropertyValue(aggregator, "sendPartialResultOnExpiry")).isEqualTo(false);
 		context.close();

spring-integration-core/src/test/java/org/springframework/integration/endpoint/PseudoTransactionalMessageSourceTests.java

@@ -96,10 +96,8 @@ public Message<String> receive() {
 			}
 		});
 
-		MessageChannel afterCommitChannel = TestUtils.getPropertyValue(syncProcessor, "afterCommitChannel",
-				MessageChannel.class);
-		assertThat(afterCommitChannel).isInstanceOf(NullChannel.class);
-
+		MessageChannel afterCommitChannel = new NullChannel();
+		syncProcessor.setAfterCommitChannel(afterCommitChannel);
 		Log logger = TestUtils.getPropertyValue(afterCommitChannel, "logger", Log.class);
 
 		logger = Mockito.spy(logger);

spring-integration-file/src/main/java/org/springframework/integration/file/config/FileTailInboundChannelAdapterFactoryBean.java

@@ -45,39 +45,43 @@
 public class FileTailInboundChannelAdapterFactoryBean extends AbstractFactoryBean<FileTailingMessageProducerSupport>
 		implements BeanNameAware, SmartLifecycle, ApplicationEventPublisherAware {
 
-	private volatile String nativeOptions;
+	private String nativeOptions;
 
-	private volatile boolean enableStatusReader = true;
+	private boolean enableStatusReader = true;
 
-	private volatile Long idleEventInterval;
+	private Long idleEventInterval;
 
-	private volatile File file;
+	private File file;
 
-	private volatile TaskExecutor taskExecutor;
+	private TaskExecutor taskExecutor;
 
-	private volatile TaskScheduler taskScheduler;
+	private TaskScheduler taskScheduler;
 
-	private volatile Long delay;
+	private Long delay;
 
-	private volatile Long fileDelay;
+	private Long fileDelay;
 
-	private volatile Boolean end;
+	private Boolean end;
 
-	private volatile Boolean reopen;
+	private Boolean reopen;
 
-	private volatile FileTailingMessageProducerSupport tailAdapter;
+	private FileTailingMessageProducerSupport tailAdapter;
 
-	private volatile String beanName;
+	private String beanName;
 
-	private volatile MessageChannel outputChannel;
+	private MessageChannel outputChannel;
 
-	private volatile MessageChannel errorChannel;
+	private MessageChannel errorChannel;
 
-	private volatile Boolean autoStartup;
+	private String outputChannelName;
 
-	private volatile Integer phase;
+	private String errorChannelName;
 
-	private volatile ApplicationEventPublisher applicationEventPublisher;
+	private Boolean autoStartup;
+
+	private Integer phase;
+
+	private ApplicationEventPublisher applicationEventPublisher;
 
 	public void setNativeOptions(String nativeOptions) {
 		if (StringUtils.hasText(nativeOptions)) {
@@ -141,10 +145,18 @@ public void setOutputChannel(MessageChannel outputChannel) {
 		this.outputChannel = outputChannel;
 	}
 
+	public void setOutputChannelName(String outputChannelName) {
+		this.outputChannelName = outputChannelName;
+	}
+
 	public void setErrorChannel(MessageChannel errorChannel) {
 		this.errorChannel = errorChannel;
 	}
 
+	public void setErrorChannelName(String errorChannelName) {
+		this.errorChannelName = errorChannelName;
+	}
+
 	public void setAutoStartup(boolean autoStartup) {
 		this.autoStartup = autoStartup;
 	}
@@ -238,6 +250,8 @@ protected FileTailingMessageProducerSupport createInstance() {
 			.acceptIfNotNull(this.autoStartup, adapter::setAutoStartup)
 			.acceptIfNotNull(this.phase, adapter::setPhase)
 			.acceptIfNotNull(this.applicationEventPublisher, adapter::setApplicationEventPublisher)
+			.acceptIfNotNull(this.outputChannelName, adapter::setOutputChannelName)
+			.acceptIfNotNull(this.errorChannelName, adapter::setErrorChannelName)
 			.acceptIfNotNull(beanFactory, adapter::setBeanFactory);
 		adapter.afterPropertiesSet();
 		this.tailAdapter = adapter;

spring-integration-file/src/main/java/org/springframework/integration/file/dsl/TailAdapterSpec.java

@@ -20,7 +20,6 @@
 
 import org.springframework.beans.factory.support.DefaultListableBeanFactory;
 import org.springframework.core.task.TaskExecutor;
-import org.springframework.integration.channel.NullChannel;
 import org.springframework.integration.dsl.MessageProducerSpec;
 import org.springframework.integration.file.config.FileTailInboundChannelAdapterFactoryBean;
 import org.springframework.integration.file.tail.FileTailingMessageProducerSupport;
@@ -32,6 +31,7 @@
  * A {@link MessageProducerSpec} for file tailing adapters.
  *
  * @author Artem Bilan
+ * @author Gary Russell
  *
  * @since 5.0
  */
@@ -186,9 +186,6 @@ public TailAdapterSpec errorChannel(MessageChannel errorChannel) {
 
 	@Override
 	protected FileTailingMessageProducerSupport doGet() {
-		if (this.outputChannel == null) {
-			this.factoryBean.setOutputChannel(new NullChannel());
-		}
 		FileTailingMessageProducerSupport tailingMessageProducerSupport = null;
 		try {
 			this.factoryBean.afterPropertiesSet();