Q:

Failed to start bean 'stompBrokerRelayMessageHandler'; nested exception is java.lang.NoClassDefFoundError: io/netty/util/concurrent/EventExecutor

I'm setting up Springboot 2.1.4 with RabbitMQ. The project works fine with a simple broker, but on switching to a Stomp broker relay with RabbitMQ, application startup fails with the following error.
2019-04-19 18:32:36.841  INFO 80706 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Shutting down ExecutorService 'clientOutboundChannelExecutor'
2019-04-19 18:32:36.841  INFO 80706 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Shutting down ExecutorService 'clientInboundChannelExecutor'
2019-04-19 18:32:36.845  INFO 80706 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]
2019-04-19 18:32:36.860  INFO 80706 --- [           main] ConditionEvaluationReportLoggingListener : 
Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2019-04-19 18:32:36.871 ERROR 80706 --- [           main] o.s.boot.SpringApplication               : Application run failed
org.springframework.context.ApplicationContextException: Failed to start bean 'stompBrokerRelayMessageHandler'; nested exception is java.lang.NoClassDefFoundError: io/netty/util/concurrent/EventExecutor
    at org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:185) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.DefaultLifecycleProcessor.access$200(DefaultLifecycleProcessor.java:53) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.DefaultLifecycleProcessor$LifecycleGroup.start(DefaultLifecycleProcessor.java:360) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.DefaultLifecycleProcessor.startBeans(DefaultLifecycleProcessor.java:158) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.DefaultLifecycleProcessor.onRefresh(DefaultLifecycleProcessor.java:122) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:893) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.finishRefresh(ServletWebServerApplicationContext.java:163) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:552) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:142) ~[spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775) [spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:316) [spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) [spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) [spring-boot-2.1.4.RELEASE.jar:2.1.4.RELEASE]
    at com.sand.SpringWebSockets.SpringWebSocketsApplication.main(SpringWebSocketsApplication.java:10) [classes/:na]
Caused by: java.lang.NoClassDefFoundError: io/netty/util/concurrent/EventExecutor
    at org.springframework.messaging.simp.stomp.StompBrokerRelayMessageHandler.initTcpClient(StompBrokerRelayMessageHandler.java:435) ~[spring-messaging-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.messaging.simp.stomp.StompBrokerRelayMessageHandler.startInternal(StompBrokerRelayMessageHandler.java:401) ~[spring-messaging-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.messaging.simp.broker.AbstractBrokerMessageHandler.start(AbstractBrokerMessageHandler.java:186) ~[spring-messaging-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    at org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:182) ~[spring-context-5.1.6.RELEASE.jar:5.1.6.RELEASE]
    ... 14 common frames omitted
Caused by: java.lang.ClassNotFoundException: io.netty.util.concurrent.EventExecutor
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_161]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_161]
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338) ~[na:1.8.0_161]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_161]
    ... 18 common frames omitted

A:

There is one more dependency, you need to add.
May be you are missing this.
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-reactor-netty</artifactId>
</dependency>

