@SendTo 报 Multiple destinations cannot be specified

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废话不多说，今天我们来说一个关于 Spring Cloud Stream 的问题。目前 Spring Cloud Stream 使用的用户还不是很多，所以抛出一些异常后，很多人素手无策。

详细异常如下：

java.lang.IllegalArgumentException: Multiple destinations cannot be specified at org.springframework.util.Assert.isTrue(Assert.java:118) at org.springframework.cloud.stream.binding.StreamListenerMethodUtils.getOutboundBindingTargetName(StreamListenerMethodUtils.java:146) at org.springframework.cloud.stream.binding.StreamListenerAnnotationBeanPostProcessor$DefaultStreamListenerSetupMethodOrchestrator.orchestrateStreamListenerSetupMethod(StreamListenerAnnotationBeanPostProcessor.java:349) at org.springframework.cloud.stream.binding.StreamListenerAnnotationBeanPostProcessor.doPostProcess(StreamListenerAnnotationBeanPostProcessor.java:195) at org.springframework.cloud.stream.binding.StreamListenerAnnotationBeanPostProcessor.lambda$postProcessAfterInitialization$0(StreamListenerAnnotationBeanPostProcessor.java:167) at java.lang.Iterable.forEach(Iterable.java:75) at org.springframework.cloud.stream.binding.StreamListenerAnnotationBeanPostProcessor.injectAndPostProcessDependencies(StreamListenerAnnotationBeanPostProcessor.java:285) at org.springframework.cloud.stream.binding.StreamListenerAnnotationBeanPostProcessor.afterSingletonsInstantiated(StreamListenerAnnotationBeanPostProcessor.java:105) at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:863) at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:863) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:546) at org.springframework.boot.web.servle

这是错误使用 @SendTo 导致的。比如下面的用法：

@StreamListener(SampleBinding.INPUT1)
@SendTo({SampleBinding.XTTBLOG_OUTPUT, SampleBinding.XTTBLOG_OUTPUT})
public String handleM(String sampleMessage){
    log.info("xttblog Received message=" + sampleMessage.toString());
    sampleMessage = sampleMessage.toUpperCase();
    return sampleMessage;
}

@SendTo 注解虽然支持配置一个数组，但是对某些消息组件支持不是很友好，会抛出这个异常。可以采用硬编码或是其他动态转发方案。

举例一个正确用法：

@StreamListener(SampleBinding.INPUT1)
@SendTo(SampleBinding.XTTBLOG_OUTPUT)
public String handleM(String sampleMessage){
    log.info("xttblog Received message=" + sampleMessage.toString());
    sampleMessage = sampleMessage.toUpperCase();
    return sampleMessage;
}

另外需要注意的是，@StreamListener 和 @SendTo 组合使用时，不能配置 condition。

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