The error message is unhelpful when spring.rabbitmq.host is configured with a comma-separated value

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/amqp/RabbitProperties.java

@@ -27,6 +27,7 @@
 import org.springframework.amqp.rabbit.connection.CachingConnectionFactory.CacheMode;
 import org.springframework.amqp.rabbit.connection.CachingConnectionFactory.ConfirmType;
 import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.boot.context.properties.source.InvalidConfigurationPropertyValueException;
 import org.springframework.boot.convert.DurationUnit;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.StringUtils;
@@ -43,6 +44,7 @@
  * @author Artsiom Yudovin
  * @author Franjo Zilic
  * @author Eddú Meléndez
+ * @author Rafael Carvalho
  * @since 1.0.0
  */
 @ConfigurationProperties(prefix = "spring.rabbitmq")
@@ -203,6 +205,10 @@ public String getAddresses() {
 	 */
 	public String determineAddresses() {
 		if (CollectionUtils.isEmpty(this.parsedAddresses)) {
+			if (this.host.contains(",")) {
+				throw new InvalidConfigurationPropertyValueException("spring.rabbitmq.host", this.host,
+						"Invalid character ','. Value must be a single host. For multiple hosts, use property 'spring.rabbitmq.addresses' instead.");
+			}
 			return this.host + ":" + determinePort();
 		}
 		List<String> addressStrings = new ArrayList<>();

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/amqp/RabbitPropertiesTests.java

@@ -22,15 +22,18 @@
 import org.springframework.amqp.rabbit.config.SimpleRabbitListenerContainerFactory;
 import org.springframework.amqp.rabbit.listener.DirectMessageListenerContainer;
 import org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer;
+import org.springframework.boot.context.properties.source.InvalidConfigurationPropertyValueException;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 /**
  * Tests for {@link RabbitProperties}.
  *
  * @author Dave Syer
  * @author Andy Wilkinson
  * @author Stephane Nicoll
+ * @author Rafael Carvalho
  */
 class RabbitPropertiesTests {
 
@@ -338,4 +341,13 @@ void determineUsernameWithoutPassword() {
 		assertThat(this.properties.determinePassword()).isEqualTo("guest");
 	}
 
+	@Test
+	void hostPropertyMustBeSingleHost() {
+		this.properties.setHost("my-rmq-host.net,my-rmq-host-2.net");
+		assertThat(this.properties.getHost()).isEqualTo("my-rmq-host.net,my-rmq-host-2.net");
+		assertThatThrownBy(this.properties::determineAddresses)
+			.isInstanceOf(InvalidConfigurationPropertyValueException.class)
+			.hasMessageContaining("spring.rabbitmq.host");
+	}
+
 }