GH-8981: Add UnicastingDispatcher.failoverStrategy option

Fixes: #8981

Sometime the simple `boolean failover` on the `MessageChannel` (default `true`)
is not enough to be sure that we can dispatch to the next handler or not.
Such a decision can be made using `ErrorMessageExceptionTypeRouter`, but that would
require an overhaul for the whole integration flow

* Introduce a simple `Predicate<Exception> failoverStrategy` into `UnicastingDispatcher`
and all its `MessageChannel` implementation consumers to allow to make a decision about next failover
according to a thrown exception from the current `MessageHandler`
* Expose `failoverStrategy` on the `DirectChannel`, `ExecutorChannel` & `PartitionedChannel`,
and add it into respective Java DSL specs
* Fix involved tests to rely on the `failoverStrategy` property from now on
* Document the new feature

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -16,6 +16,8 @@
 
 package org.springframework.integration.channel;
 
+import java.util.function.Predicate;
+
 import org.springframework.integration.dispatcher.LoadBalancingStrategy;
 import org.springframework.integration.dispatcher.RoundRobinLoadBalancingStrategy;
 import org.springframework.integration.dispatcher.UnicastingDispatcher;
@@ -60,12 +62,26 @@ public class DirectChannel extends AbstractSubscribableChannel {
 	/**
 	 * Specify whether the channel's dispatcher should have failover enabled.
 	 * By default, it will. Set this value to 'false' to disable it.
+	 * Overrides {@link #setFailoverStrategy(Predicate)} option.
+	 * In other words: or this, or that option has to be set.
 	 * @param failover The failover boolean.
 	 */
 	public void setFailover(boolean failover) {
 		this.dispatcher.setFailover(failover);
 	}
 
+	/**
+	 * Configure a strategy whether the channel's dispatcher should have failover enabled
+	 * for the exception thrown.
+	 * Overrides {@link #setFailover(boolean)} option.
+	 * In other words: or this, or that option has to be set.
+	 * @param failoverStrategy The failover boolean.
+	 * @since 6.3
+	 */
+	public void setFailoverStrategy(Predicate<Exception> failoverStrategy) {
+		this.dispatcher.setFailoverStrategy(failoverStrategy);
+	}
+
 	/**
 	 * Specify the maximum number of subscribers supported by the
 	 * channel's dispatcher.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -17,6 +17,7 @@
 package org.springframework.integration.channel;
 
 import java.util.concurrent.Executor;
+import java.util.function.Predicate;
 
 import org.springframework.integration.dispatcher.LoadBalancingStrategy;
 import org.springframework.integration.dispatcher.RoundRobinLoadBalancingStrategy;
@@ -49,7 +50,7 @@ public class ExecutorChannel extends AbstractExecutorChannel {
 
 	private final LoadBalancingStrategy loadBalancingStrategy;
 
-	private boolean failover = true;
+	private Predicate<Exception> failoverStrategy = (exception) -> true;
 
 	/**
 	 * Create an ExecutorChannel that delegates to the provided
@@ -88,8 +89,20 @@ public class ExecutorChannel extends AbstractExecutorChannel {
 	 * @param failover The failover boolean.
 	 */
 	public void setFailover(boolean failover) {
-		this.failover = failover;
-		getDispatcher().setFailover(failover);
+		setFailoverStrategy((exception) -> failover);
+	}
+
+	/**
+	 * Configure a strategy whether the channel's dispatcher should have failover enabled
+	 * for the exception thrown.
+	 * Overrides {@link #setFailover(boolean)} option.
+	 * In other words: or this, or that option has to be set.
+	 * @param failoverStrategy The failover boolean.
+	 * @since 6.3
+	 */
+	public void setFailoverStrategy(Predicate<Exception> failoverStrategy) {
+		this.failoverStrategy = failoverStrategy;
+		getDispatcher().setFailoverStrategy(failoverStrategy);
 	}
 
 	@Override
@@ -107,7 +120,7 @@ public class ExecutorChannel extends AbstractExecutorChannel {
 			this.executor = new ErrorHandlingTaskExecutor(this.executor, errorHandler);
 		}
 		UnicastingDispatcher unicastingDispatcher = new UnicastingDispatcher(this.executor);
-		unicastingDispatcher.setFailover(this.failover);
+		unicastingDispatcher.setFailoverStrategy(this.failoverStrategy);
 		if (this.maxSubscribers == null) {
 			this.maxSubscribers = getIntegrationProperties().getChannelsMaxUnicastSubscribers();
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 the original author or authors.
+ * Copyright 2023-2024 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.
@@ -18,6 +18,7 @@ package org.springframework.integration.channel;
 
 import java.util.concurrent.ThreadFactory;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
 import org.springframework.integration.dispatcher.LoadBalancingStrategy;
@@ -99,6 +100,18 @@ public class PartitionedChannel extends AbstractExecutorChannel {
 		getDispatcher().setFailover(failover);
 	}
 
+	/**
+	 * Configure a strategy whether the channel's dispatcher should have failover enabled
+	 * for the exception thrown.
+	 * Overrides {@link #setFailover(boolean)} option.
+	 * In other words: or this, or that option has to be set.
+	 * @param failoverStrategy The failover boolean.
+	 * @since 6.3
+	 */
+	public void setFailoverStrategy(Predicate<Exception> failoverStrategy) {
+		getDispatcher().setFailoverStrategy(failoverStrategy);
+	}
+
 	/**
 	 * Provide a {@link LoadBalancingStrategy} for the {@link PartitionedDispatcher}.
 	 * @param loadBalancingStrategy The load balancing strategy implementation.

spring-integration-core/src/main/java/org/springframework/integration/dispatcher/PartitionedDispatcher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 the original author or authors.
+ * Copyright 2023-2024 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.
@@ -28,6 +28,7 @@ import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 import org.springframework.integration.util.ErrorHandlingTaskExecutor;
 import org.springframework.lang.Nullable;
@@ -67,7 +68,7 @@ public class PartitionedDispatcher extends AbstractDispatcher {
 
 	private ThreadFactory threadFactory = new CustomizableThreadFactory("partition-thread-");
 
-	private boolean failover = true;
+	private Predicate<Exception> failoverStrategy = (exception) -> true;
 
 	@Nullable
 	private LoadBalancingStrategy loadBalancingStrategy;
@@ -108,7 +109,20 @@ public class PartitionedDispatcher extends AbstractDispatcher {
 	 * @param failover The failover boolean.
 	 */
 	public void setFailover(boolean failover) {
-		this.failover = failover;
+		setFailoverStrategy((exception) -> failover);
+	}
+
+	/**
+	 * Configure a strategy whether the channel's dispatcher should have failover enabled
+	 * for the exception thrown.
+	 * Overrides {@link #setFailover(boolean)} option.
+	 * In other words: or this, or that option has to be set.
+	 * @param failoverStrategy The failover boolean.
+	 * @since 6.3
+	 */
+	public void setFailoverStrategy(Predicate<Exception> failoverStrategy) {
+		Assert.notNull(failoverStrategy, "'failoverStrategy' must not be null");
+		this.failoverStrategy = failoverStrategy;
 	}
 
 	/**
@@ -179,7 +193,7 @@ public class PartitionedDispatcher extends AbstractDispatcher {
 		this.executors.add(executor);
 		DelegateDispatcher delegateDispatcher =
 				new DelegateDispatcher(new ErrorHandlingTaskExecutor(executor, this.errorHandler));
-		delegateDispatcher.setFailover(this.failover);
+		delegateDispatcher.setFailoverStrategy(this.failoverStrategy);
 		delegateDispatcher.setLoadBalancingStrategy(this.loadBalancingStrategy);
 		delegateDispatcher.setMessageHandlingTaskDecorator(this.messageHandlingTaskDecorator);
 		return delegateDispatcher;

spring-integration-core/src/main/java/org/springframework/integration/dispatcher/UnicastingDispatcher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -21,6 +21,7 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.Set;
 import java.util.concurrent.Executor;
+import java.util.function.Predicate;
 
 import org.springframework.integration.MessageDispatchingException;
 import org.springframework.integration.support.utils.IntegrationUtils;
@@ -58,7 +59,7 @@ public class UnicastingDispatcher extends AbstractDispatcher {
 
 	private final Executor executor;
 
-	private boolean failover = true;
+	private Predicate<Exception> failoverStrategy = (exception) -> true;
 
 	private LoadBalancingStrategy loadBalancingStrategy;
 
@@ -77,10 +78,25 @@ public class UnicastingDispatcher extends AbstractDispatcher {
 	 * Specify whether this dispatcher should failover when a single
 	 * {@link MessageHandler} throws an Exception. The default value is
 	 * <code>true</code>.
+	 * Overrides {@link #setFailoverStrategy(Predicate)} option.
+	 * In other words: or this, or that option has to be set.
 	 * @param failover The failover boolean.
 	 */
 	public void setFailover(boolean failover) {
-		this.failover = failover;
+		setFailoverStrategy((exception) -> failover);
+	}
+
+	/**
+	 * Configure a strategy whether the channel's dispatcher should have failover enabled
+	 * for the exception thrown.
+	 * Overrides {@link #setFailover(boolean)} option.
+	 * In other words: or this, or that option has to be set.
+	 * @param failoverStrategy The failover boolean.
+	 * @since 6.3
+	 */
+	public void setFailoverStrategy(Predicate<Exception> failoverStrategy) {
+		Assert.notNull(failoverStrategy, "'failoverStrategy' must not be null");
+		this.failoverStrategy = failoverStrategy;
 	}
 
 	/**
@@ -154,10 +170,15 @@ public class UnicastingDispatcher extends AbstractDispatcher {
 				}
 				exceptions.add(runtimeException);
 				boolean isLast = !handlerIterator.hasNext();
-				if (!isLast && this.failover) {
+				boolean failover = this.failoverStrategy.test(ex);
+
+				if (!isLast && failover) {
 					logExceptionBeforeFailOver(ex, handler, message);
 				}
-				handleExceptions(exceptions, message, isLast);
+
+				if (isLast || !failover) {
+					handleExceptions(exceptions, message);
+				}
 			}
 		}
 		return success;
@@ -187,22 +208,12 @@ public class UnicastingDispatcher extends AbstractDispatcher {
 		}
 	}
 
-	/**
-	 * Handles Exceptions that occur while dispatching. If this dispatcher has
-	 * failover enabled, it will only throw an Exception when the handler list
-	 * is exhausted. The 'isLast' flag will be <em>true</em> if the
-	 * Exception occurred during the final iteration of the MessageHandlers.
-	 * If failover is disabled for this dispatcher, it will re-throw any
-	 * Exception immediately.
-	 */
-	private void handleExceptions(List<RuntimeException> allExceptions, Message<?> message, boolean isLast) {
-		if (isLast || !this.failover) {
-			if (allExceptions.size() == 1) {
-				throw allExceptions.get(0);
-			}
-			throw new AggregateMessageDeliveryException(message,
-					"All attempts to deliver Message to MessageHandlers failed.", allExceptions);
+	private void handleExceptions(List<RuntimeException> allExceptions, Message<?> message) {
+		if (allExceptions.size() == 1) {
+			throw allExceptions.get(0);
 		}
+		throw new AggregateMessageDeliveryException(message,
+				"All attempts to deliver Message to MessageHandlers failed.", allExceptions);
 	}
 
 }

spring-integration-core/src/main/java/org/springframework/integration/dsl/DirectChannelSpec.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2020 the original author or authors.
+ * Copyright 2016-2024 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.
@@ -28,8 +28,8 @@ public class DirectChannelSpec extends LoadBalancingChannelSpec<DirectChannelSpe
 	@Override
 	protected DirectChannel doGet() {
 		this.channel = new DirectChannel(this.loadBalancingStrategy);
-		if (this.failover != null) {
-			this.channel.setFailover(this.failover);
+		if (this.failoverStrategy != null) {
+			this.channel.setFailoverStrategy(this.failoverStrategy);
 		}
 		if (this.maxSubscribers != null) {
 			this.channel.setMaxSubscribers(this.maxSubscribers);

spring-integration-core/src/main/java/org/springframework/integration/dsl/ExecutorChannelSpec.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2020 the original author or authors.
+ * Copyright 2016-2024 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.
@@ -36,8 +36,8 @@ public class ExecutorChannelSpec extends LoadBalancingChannelSpec<ExecutorChanne
 	@Override
 	protected ExecutorChannel doGet() {
 		this.channel = new ExecutorChannel(this.executor, this.loadBalancingStrategy);
-		if (this.failover != null) {
-			this.channel.setFailover(this.failover);
+		if (this.failoverStrategy != null) {
+			this.channel.setFailoverStrategy(this.failoverStrategy);
 		}
 		if (this.maxSubscribers != null) {
 			this.channel.setMaxSubscribers(this.maxSubscribers);

spring-integration-core/src/main/java/org/springframework/integration/dsl/LoadBalancingChannelSpec.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2019 the original author or authors.
+ * Copyright 2016-2024 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.
@@ -16,6 +16,8 @@
 
 package org.springframework.integration.dsl;
 
+import java.util.function.Predicate;
+
 import org.springframework.integration.channel.AbstractMessageChannel;
 import org.springframework.integration.dispatcher.LoadBalancingStrategy;
 import org.springframework.integration.dispatcher.RoundRobinLoadBalancingStrategy;
@@ -34,7 +36,7 @@ public abstract class LoadBalancingChannelSpec<S extends MessageChannelSpec<S, C
 
 	protected LoadBalancingStrategy loadBalancingStrategy = new RoundRobinLoadBalancingStrategy(); // NOSONAR
 
-	protected Boolean failover; // NOSONAR
+	protected Predicate<Exception> failoverStrategy; // NOSONAR
 
 	protected Integer maxSubscribers; // NOSONAR
 
@@ -46,8 +48,20 @@ public abstract class LoadBalancingChannelSpec<S extends MessageChannelSpec<S, C
 		return _this();
 	}
 
-	public S failover(Boolean failoverToSet) {
-		this.failover = failoverToSet;
+	public S failover(boolean failoverToSet) {
+		return failoverStrategy((exception) -> failoverToSet);
+	}
+
+	/**
+	 * Configure a strategy whether the channel's dispatcher should have failover enabled
+	 * for the exception thrown.
+	 * Overrides {@link #failover(boolean)} option.
+	 * In other words: or this, or that option has to be set.
+	 * @param failoverStrategy The failover boolean.
+	 * @since 6.3
+	 */
+	public S failoverStrategy(Predicate<Exception> failoverStrategy) {
+		this.failoverStrategy = failoverStrategy;
 		return _this();
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/dsl/PartitionedChannelSpec.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 the original author or authors.
+ * Copyright 2023-2024 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.
@@ -63,8 +63,8 @@ public class PartitionedChannelSpec extends LoadBalancingChannelSpec<Partitioned
 			this.channel = new PartitionedChannel(this.partitionCount);
 		}
 		this.channel.setLoadBalancingStrategy(this.loadBalancingStrategy);
-		if (this.failover != null) {
-			this.channel.setFailover(this.failover);
+		if (this.failoverStrategy != null) {
+			this.channel.setFailoverStrategy(this.failoverStrategy);
 		}
 		if (this.maxSubscribers != null) {
 			this.channel.setMaxSubscribers(this.maxSubscribers);

spring-integration-core/src/test/java/org/springframework/integration/channel/PartitionedChannelTests.java

@@ -21,6 +21,8 @@ import java.util.HashSet;
 import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.IntStream;
 
 import org.assertj.core.api.InstanceOfAssertFactories;
@@ -76,7 +78,16 @@ public class PartitionedChannelTests {
 
 		MultiValueMap<String, Message<?>> partitionedMessages = new LinkedMultiValueMap<>();
 
-		partitionedChannel.subscribe((message) -> partitionedMessages.add(Thread.currentThread().getName(), message));
+		Lock partitionsLock = new ReentrantLock();
+		partitionedChannel.subscribe((message) -> {
+			partitionsLock.lock();
+			try {
+				partitionedMessages.add(Thread.currentThread().getName(), message);
+			}
+			finally {
+				partitionsLock.unlock();
+			}
+		});
 
 		partitionedChannel.send(MessageBuilder.withPayload("test1").setHeader("partitionKey", "1").build());
 		partitionedChannel.send(MessageBuilder.withPayload("test2").setHeader("partitionKey", "2").build());

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -19,9 +19,9 @@ package org.springframework.integration.channel.config;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.function.Predicate;
 
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.beans.factory.annotation.Autowired;
@@ -37,18 +37,20 @@ import org.springframework.integration.util.ErrorHandlingTaskExecutor;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHandler;
-import org.springframework.test.context.ContextConfiguration;
-import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.mockito.Mockito.mock;
 
 /**
  * @author Mark Fisher
  * @author Oleg Zhurakousky
+ * @author Artem Bilan
+ *
  * @since 1.0.3
  */
-@RunWith(SpringJUnit4ClassRunner.class)
-@ContextConfiguration
+@SpringJUnitConfig
 public class DispatchingChannelParserTests {
 
 	@Autowired
@@ -60,50 +62,60 @@ public class DispatchingChannelParserTests {
 	@Test
 	public void taskExecutorOnly() {
 		MessageChannel channel = channels.get("taskExecutorOnly");
-		assertThat(channel.getClass()).isEqualTo(ExecutorChannel.class);
+		assertThat(channel).isInstanceOf(ExecutorChannel.class);
 		Object executor = getDispatcherProperty("executor", channel);
-		assertThat(executor.getClass()).isEqualTo(ErrorHandlingTaskExecutor.class);
+		assertThat(executor).isInstanceOf(ErrorHandlingTaskExecutor.class);
 		assertThat(new DirectFieldAccessor(executor).getPropertyValue("executor"))
 				.isSameAs(context.getBean("taskExecutor"));
-		assertThat((Boolean) getDispatcherProperty("failover", channel)).isTrue();
-		assertThat(getDispatcherProperty("loadBalancingStrategy", channel).getClass())
-				.isEqualTo(RoundRobinLoadBalancingStrategy.class);
+		@SuppressWarnings("unchecked")
+		Predicate<Exception> failoverStrategy = (Predicate<Exception>) getDispatcherProperty("failoverStrategy", channel);
+		assertThat(failoverStrategy.test(mock())).isTrue();
+		assertThat(getDispatcherProperty("loadBalancingStrategy", channel))
+				.isInstanceOf(RoundRobinLoadBalancingStrategy.class);
 	}
 
 	@Test
 	public void failoverFalse() {
 		MessageChannel channel = channels.get("failoverFalse");
-		assertThat(channel.getClass()).isEqualTo(DirectChannel.class);
-		assertThat((Boolean) getDispatcherProperty("failover", channel)).isFalse();
-		assertThat(getDispatcherProperty("loadBalancingStrategy", channel).getClass())
-				.isEqualTo(RoundRobinLoadBalancingStrategy.class);
+		assertThat(channel).isInstanceOf(DirectChannel.class);
+		@SuppressWarnings("unchecked")
+		Predicate<Exception> failoverStrategy = (Predicate<Exception>) getDispatcherProperty("failoverStrategy", channel);
+		assertThat(failoverStrategy.test(mock())).isFalse();
+		assertThat(getDispatcherProperty("loadBalancingStrategy", channel))
+				.isInstanceOf(RoundRobinLoadBalancingStrategy.class);
 	}
 
 	@Test
 	public void failoverTrue() {
 		MessageChannel channel = channels.get("failoverTrue");
-		assertThat(channel.getClass()).isEqualTo(DirectChannel.class);
-		assertThat((Boolean) getDispatcherProperty("failover", channel)).isTrue();
-		assertThat(getDispatcherProperty("loadBalancingStrategy", channel).getClass())
-				.isEqualTo(RoundRobinLoadBalancingStrategy.class);
+		assertThat(channel).isInstanceOf(DirectChannel.class);
+		@SuppressWarnings("unchecked")
+		Predicate<Exception> failoverStrategy = (Predicate<Exception>) getDispatcherProperty("failoverStrategy", channel);
+		assertThat(failoverStrategy.test(mock())).isTrue();
+		assertThat(getDispatcherProperty("loadBalancingStrategy", channel))
+				.isInstanceOf(RoundRobinLoadBalancingStrategy.class);
 	}
 
 	@Test
 	public void loadBalancerDisabled() {
 		MessageChannel channel = channels.get("loadBalancerDisabled");
-		assertThat(channel.getClass()).isEqualTo(DirectChannel.class);
-		assertThat((Boolean) getDispatcherProperty("failover", channel)).isTrue();
+		assertThat(channel).isInstanceOf(DirectChannel.class);
+		@SuppressWarnings("unchecked")
+		Predicate<Exception> failoverStrategy = (Predicate<Exception>) getDispatcherProperty("failoverStrategy", channel);
+		assertThat(failoverStrategy.test(mock())).isTrue();
 		assertThat(getDispatcherProperty("loadBalancingStrategy", channel)).isNull();
 	}
 
 	@Test
 	public void loadBalancerDisabledAndTaskExecutor() {
 		MessageChannel channel = channels.get("loadBalancerDisabledAndTaskExecutor");
-		assertThat(channel.getClass()).isEqualTo(ExecutorChannel.class);
-		assertThat((Boolean) getDispatcherProperty("failover", channel)).isTrue();
+		assertThat(channel).isInstanceOf(ExecutorChannel.class);
+		@SuppressWarnings("unchecked")
+		Predicate<Exception> failoverStrategy = (Predicate<Exception>) getDispatcherProperty("failoverStrategy", channel);
+		assertThat(failoverStrategy.test(mock())).isTrue();
 		assertThat(getDispatcherProperty("loadBalancingStrategy", channel)).isNull();
 		Object executor = getDispatcherProperty("executor", channel);
-		assertThat(executor.getClass()).isEqualTo(ErrorHandlingTaskExecutor.class);
+		assertThat(executor).isInstanceOf(ErrorHandlingTaskExecutor.class);
 		assertThat(new DirectFieldAccessor(executor).getPropertyValue("executor"))
 				.isSameAs(context.getBean("taskExecutor"));
 	}
@@ -111,12 +123,14 @@ public class DispatchingChannelParserTests {
 	@Test
 	public void roundRobinLoadBalancerAndTaskExecutor() {
 		MessageChannel channel = channels.get("roundRobinLoadBalancerAndTaskExecutor");
-		assertThat(channel.getClass()).isEqualTo(ExecutorChannel.class);
-		assertThat((Boolean) getDispatcherProperty("failover", channel)).isTrue();
-		assertThat(getDispatcherProperty("loadBalancingStrategy", channel).getClass())
-				.isEqualTo(RoundRobinLoadBalancingStrategy.class);
+		assertThat(channel).isInstanceOf(ExecutorChannel.class);
+		@SuppressWarnings("unchecked")
+		Predicate<Exception> failoverStrategy = (Predicate<Exception>) getDispatcherProperty("failoverStrategy", channel);
+		assertThat(failoverStrategy.test(mock())).isTrue();
+		assertThat(getDispatcherProperty("loadBalancingStrategy", channel))
+				.isInstanceOf(RoundRobinLoadBalancingStrategy.class);
 		Object executor = getDispatcherProperty("executor", channel);
-		assertThat(executor.getClass()).isEqualTo(ErrorHandlingTaskExecutor.class);
+		assertThat(executor).isInstanceOf(ErrorHandlingTaskExecutor.class);
 		assertThat(new DirectFieldAccessor(executor).getPropertyValue("executor"))
 				.isSameAs(context.getBean("taskExecutor"));
 	}
@@ -126,19 +140,15 @@ public class DispatchingChannelParserTests {
 		MessageChannel channel = channels.get("lbRefChannel");
 		LoadBalancingStrategy lbStrategy = TestUtils.getPropertyValue(channel, "dispatcher.loadBalancingStrategy",
 				LoadBalancingStrategy.class);
-		assertThat(lbStrategy instanceof SampleLoadBalancingStrategy).isTrue();
+		assertThat(lbStrategy).isInstanceOf(SampleLoadBalancingStrategy.class);
 	}
 
 	@Test
 	public void loadBalancerRefFailWithLoadBalancer() {
-
-		try {
-			new ClassPathXmlApplicationContext("ChannelWithLoadBalancerRef-fail-config.xml", this.getClass()).close();
-		}
-		catch (BeanDefinitionParsingException e) {
-			assertThat(e.getMessage()).contains("'load-balancer' and 'load-balancer-ref' are mutually exclusive");
-		}
-
+		assertThatExceptionOfType(BeanDefinitionParsingException.class)
+				.isThrownBy(() -> new ClassPathXmlApplicationContext("ChannelWithLoadBalancerRef-fail-config.xml",
+						getClass()))
+				.withMessageContaining("'load-balancer' and 'load-balancer-ref' are mutually exclusive");
 	}
 
 	private static Object getDispatcherProperty(String propertyName, MessageChannel channel) {
@@ -153,5 +163,7 @@ public class DispatchingChannelParserTests {
 		public Iterator<MessageHandler> getHandlerIterator(Message<?> message, Collection<MessageHandler> handlers) {
 			return handlers.iterator();
 		}
+
 	}
+
 }

spring-integration-core/src/test/java/org/springframework/integration/dispatcher/FailOverDispatcherTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 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.
@@ -20,7 +20,7 @@ import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 
-import org.junit.Test;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.integration.MessageRejectedException;
@@ -32,6 +32,7 @@ import org.springframework.messaging.MessageHandler;
 import org.springframework.messaging.support.GenericMessage;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -69,12 +70,8 @@ public class FailOverDispatcherTests {
 		MessageHandler target = new CountingTestEndpoint(counter, false);
 		dispatcher.addHandler(target);
 		dispatcher.addHandler(target);
-		try {
-			dispatcher.dispatch(new GenericMessage<>("test"));
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(MessageRejectedException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test")));
 		assertThat(counter.get()).as("target should not have duplicate subscriptions").isEqualTo(1);
 	}
 
@@ -89,12 +86,8 @@ public class FailOverDispatcherTests {
 		dispatcher.addHandler(target2);
 		dispatcher.addHandler(target3);
 		dispatcher.removeHandler(target2);
-		try {
-			dispatcher.dispatch(new GenericMessage<>("test"));
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(AggregateMessageDeliveryException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test")));
 		assertThat(counter.get()).isEqualTo(2);
 	}
 
@@ -108,46 +101,31 @@ public class FailOverDispatcherTests {
 		dispatcher.addHandler(target1);
 		dispatcher.addHandler(target2);
 		dispatcher.addHandler(target3);
-		try {
-			dispatcher.dispatch(new GenericMessage<>("test1"));
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(AggregateMessageDeliveryException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test1")));
 		assertThat(counter.get()).isEqualTo(3);
 		dispatcher.removeHandler(target2);
-		try {
-			dispatcher.dispatch(new GenericMessage<>("test2"));
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(AggregateMessageDeliveryException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test2")));
 		assertThat(counter.get()).isEqualTo(5);
 		dispatcher.removeHandler(target1);
-		try {
-			dispatcher.dispatch(new GenericMessage<>("test3"));
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(MessageRejectedException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test3")));
 		assertThat(counter.get()).isEqualTo(6);
 	}
 
-	@Test(expected = MessageDeliveryException.class)
+	@Test
 	public void removeConsumerLastTargetCausesDeliveryException() {
 		UnicastingDispatcher dispatcher = new UnicastingDispatcher();
 		final AtomicInteger counter = new AtomicInteger();
 		MessageHandler target = new CountingTestEndpoint(counter, false);
 		dispatcher.addHandler(target);
-		try {
-			dispatcher.dispatch(new GenericMessage<>("test1"));
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(MessageRejectedException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test1")));
 		assertThat(counter.get()).isEqualTo(1);
 		dispatcher.removeHandler(target);
-		dispatcher.dispatch(new GenericMessage<>("test2"));
+		assertThatExceptionOfType(MessageDeliveryException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test2")));
 	}
 
 	@Test
@@ -188,15 +166,26 @@ public class FailOverDispatcherTests {
 		dispatcher.addHandler(target1);
 		dispatcher.addHandler(target2);
 		dispatcher.addHandler(target3);
-		try {
-			assertThat(dispatcher.dispatch(new GenericMessage<>("test"))).isFalse();
-		}
-		catch (Exception e) {
-			// ignore
-		}
+		assertThatExceptionOfType(AggregateMessageDeliveryException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test")));
 		assertThat(counter.get()).as("each target should have been invoked").isEqualTo(3);
 	}
 
+	@Test
+	public void failoverStrategyRejects() {
+		UnicastingDispatcher dispatcher = new UnicastingDispatcher();
+		dispatcher.setFailoverStrategy((exception) -> !(exception instanceof MessageRejectedException));
+		AtomicInteger counter = new AtomicInteger();
+		MessageHandler target1 = new CountingTestEndpoint(counter, false);
+		MessageHandler target2 = new CountingTestEndpoint(counter, true);
+		MessageHandler target3 = new CountingTestEndpoint(counter, true);
+		dispatcher.addHandler(target1);
+		dispatcher.addHandler(target2);
+		dispatcher.addHandler(target3);
+		assertThatExceptionOfType(MessageRejectedException.class)
+				.isThrownBy(() -> dispatcher.dispatch(new GenericMessage<>("test")));
+		assertThat(counter.get()).as("only first should have been invoked").isEqualTo(1);
+	}
 
 	private static ServiceActivatingHandler createConsumer(Object object) {
 		ServiceActivatingHandler handler = new ServiceActivatingHandler(object);

src/reference/antora/modules/ROOT/pages/channel/configuration.adoc

@@ -112,6 +112,10 @@ XML::
 ----
 ======
 
+Starting with version 6.3, all the `MessageChannel` implementations based on the `UnicastingDispatcher` can be configured with a `Predicate<Exception> failoverStrategy` instead of plain `failover` option.
+This predicate makes a decision to failover or not to the next `MessageHandler` based on an exception thrown from the current one.
+The more complex error analysis should be done using xref:router/implementations.adoc#router-implementations-exception-router[`ErrorMessageExceptionTypeRouter`].
+
 [[channel-datatype-channel]]
 == Datatype Channel Configuration
 

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -23,6 +23,9 @@ The `MessageHistory` header is now mutable, append-only container.
 And all the subsequent tracks don't create new message - only their entry is added to existing message history header.
 See xref:message-history.adoc[Message History Chapter] for more information.
 
+All the `MessageChannel` implementations based on the `UnicastingDispatcher` now can be configured with a `Predicate<Exception> failoverStrategy` for dynamic decision for the failover on the exception thrown from the current `MessageHandler`.
+See xref:channel/configuration.adoc[Message Channel Configuration] for more information.
+
 [[x6.3-security-changes]]
 === Security Support Changes