Two special channels are defined within the application context by default: errorChannel and nullChannel. The 'nullChannel' (an instance of NullChannel) acts like /dev/null, logging any message sent to it at the DEBUG level and returning immediately. The special treatment is applied for an org.reactivestreams.Publisher payload of a transmitted message: it is subscribed to in this channel immediately, to initiate reactive stream processing, although the data is discarded. An error thrown from a reactive stream processing (see Subscriber.onError(Throwable)) is logged under the warn level for possible investigation. If there is need to do anything with such an error, the ReactiveRequestHandlerAdvice with a Mono.doOnError() customization can be applied to the message handler producing Mono reply into this nullChannel. Any time you face channel resolution errors for a reply that you do not care about, you can set the affected component’s output-channel attribute to 'nullChannel' (the name, 'nullChannel', is reserved within the application context).spring-doc.cn

The 'errorChannel' is used internally for sending error messages and may be overridden with a custom configuration. This is discussed in greater detail in Error Handling.spring-doc.cn

See also Message Channels in the Java DSL chapter for more information about message channel and interceptors.spring-doc.cn