Failed to start bean 'stompBrokerRelayMessageHandler'; nested exception is java.lang.NoClassDefFoundError: reactor/io/codec/Codec

最新版本的Spring需要reactor 2.0，看看你的POM有一个明确的1.1.6依赖。

解决：

<dependency>
    <groupId>org.projectreactor</groupId>
    <artifactId>reactor-core</artifactId>
    <version>1.1.6.RELEASE</version>
</dependency>

<dependency>
    <groupId>org.projectreactor</groupId>
    <artifactId>reactor-net</artifactId>
    <version>1.1.6.RELEASE</version>
</dependency>

<dependency>
    <groupId>org.projectreactor</groupId>
    <artifactId>reactor-tcp</artifactId>
    <version>1.0.1.RELEASE</version>
</dependency>

<dependency>
    <groupId>org.projectreactor</groupId>
    <artifactId>reactor-spring</artifactId>
    <version>1.0.1.RELEASE</version>
</dependency>