基于 Kotlin 的 Spring RabbitListener 产生无限循环，试图发回 `kotlin.Unit`

Question

我们在 Kotlin 中实现了一个 RabbitListener。该方法返回 Unit，所以我不希望 Spring 发送结果，但我们收到以下错误消息。看起来 Spring 尝试将 Unit 解释为结果并返回一个兔子消息：

2021-04-16 16:39:21 
msg="Execution of Rabbit message listener failed." thread="org.springframework.amqp.rabbit.RabbitListenerEndpointContainer#0-2" level=WARN logger="org.springframework.amqp.rabbit.listener.ConditionalRejectingErrorHandler" exception="org.springframework.amqp.rabbit.support.ListenerExecutionFailedException: Listener threw exception
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.wrapToListenerExecutionFailedExceptionIfNeeded(AbstractMessageListenerContainer.java:1746)
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.doInvokeListener(AbstractMessageListenerContainer.java:1636)
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.actualInvokeListener(AbstractMessageListenerContainer.java:1551)
    at jdk.internal.reflect.GeneratedMethodAccessor64.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.base/java.lang.reflect.Method.invoke(Unknown Source)
    at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:198)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163)
    at brave.spring.rabbit.TracingRabbitListenerAdvice.invoke(TracingRabbitListenerAdvice.java:108)
    at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
    at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:215)
    at org.springframework.amqp.rabbit.listener.$Proxy146.invokeListener(Unknown Source)
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.invokeListener(AbstractMessageListenerContainer.java:1539)
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.doExecuteListener(AbstractMessageListenerContainer.java:1530)
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.executeListener(AbstractMessageListenerContainer.java:1474)
    at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.doReceiveAndExecute(SimpleMessageListenerContainer.java:967)
    at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.receiveAndExecute(SimpleMessageListenerContainer.java:913)
    at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer.access$1600(SimpleMessageListenerContainer.java:83)
    at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer$AsyncMessageProcessingConsumer.mainLoop(SimpleMessageListenerContainer.java:1288)
    at org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer$AsyncMessageProcessingConsumer.run(SimpleMessageListenerContainer.java:1194)
    at java.base/java.lang.Thread.run(Unknown Source)
Caused by: org.springframework.amqp.rabbit.listener.adapter.ReplyFailureException: Failed to send reply with payload 'InvocationResult [returnValue=kotlin.Unit, returnType=class java.lang.Object, bean=com.xxx.EventHandler@4aaf6902, method=public java.lang.Object com.xxx.EventHandler.receiveMessage(org.springframework.amqp.core.Message)]'
    at org.springframework.amqp.rabbit.listener.adapter.AbstractAdaptableMessageListener.doHandleResult(AbstractAdaptableMessageListener.java:476)
    at org.springframework.amqp.rabbit.listener.adapter.AbstractAdaptableMessageListener.handleResult(AbstractAdaptableMessageListener.java:400)
    at org.springframework.amqp.rabbit.listener.adapter.MessagingMessageListenerAdapter.invokeHandlerAndProcessResult(MessagingMessageListenerAdapter.java:152)
    at org.springframework.amqp.rabbit.listener.adapter.MessagingMessageListenerAdapter.onMessage(MessagingMessageListenerAdapter.java:135)
    at org.springframework.amqp.rabbit.listener.AbstractMessageListenerContainer.doInvokeListener(AbstractMessageListenerContainer.java:1632)
    ... 20 common frames omitted
Caused by: java.lang.IllegalArgumentException: SimpleMessageConverter only supports String, byte[] and Serializable payloads, received: kotlin.Unit
    at org.springframework.amqp.support.converter.SimpleMessageConverter.createMessage(SimpleMessageConverter.java:164)
    at org.springframework.amqp.support.converter.AbstractMessageConverter.createMessage(AbstractMessageConverter.java:88)
    at org.springframework.amqp.support.converter.AbstractMessageConverter.toMessage(AbstractMessageConverter.java:70)
    at org.springframework.amqp.rabbit.listener.adapter.AbstractAdaptableMessageListener.convert(AbstractAdaptableMessageListener.java:519)
    at org.springframework.amqp.rabbit.listener.adapter.MessagingMessageListenerAdapter.buildMessage(MessagingMessageListenerAdapter.java:257)
    at org.springframework.amqp.rabbit.listener.adapter.AbstractAdaptableMessageListener.doHandleResult(AbstractAdaptableMessageListener.java:464)
    ... 24 common frames omitted
" time="2021-04-16 14:39:21,726"

这里是 RabbitLister 的简化版本：

@Component
class EventHandler {

    @RabbitListener(queues = ["someQueue"])
    fun receiveMessage(message: Message): Unit {
        log.debug("Received message")
    }
}

我们使用 Spring Boot 2.4.4、Spring Rabbit 2.3.6 和 Spring Framework 5.3.6

调试问题，得到如下结果：

Answer 1

你用的是什么版本？

这对我来说很好用（有和没有Unit 返回类型）...

@RabbitListener(queues = ["foo"])
open fun listen(data: String?): Unit {
    println(data)
}

即使使用: Unit，我也看到void 作为返回类型，null：