Spring Channel Interceptor - how to return null message without throwing an error

The context: I have a message channel that does a post request to a service A and 2 interceptors - one that does some processing and another one that sends to another service B.

The problem: Where to add a feature toggle such that I can stop sending to A but still send to B? (also 1st interceptor does some processing that is needed before sending to B)

My approach was to add the feature toggle in the 2nd interceptor and return a null message object so that the messageChannel.send won't be executed. This works but also throws an error which is not desirable.

According to spring's message channel documentation:

> The preSend of an interceptor is invoked before the Message is
> actually sent to the channel. This allows for modification of the
> Message if necessary. If this method returns {@code null} then the
> actual send invocation will not occur.

Is there a way to silently kill the sending process or would there be a better approach?

Instead of using a ChannelInterceptor, try a Filter

> Message filters are used to decide whether a Message should be passed along or dropped based on some criteria, such as a message header value or message content itself.

With a Filter you can specify your own implementation of a MessageSelector, which when returning false, will cause the Filter to discard the message.

package exampleCode;

import org.springframework.integration.core.MessageSelector;
import org.springframework.messaging.Message;

public class MySelector implements MessageSelector {

    @Override
    public boolean accept(Message<?> message) {
        // If the message should continue through the filter, return true

        // If the message should be blocked from continuing, return false
    }
}

Then you can configure your selector to be used by a filter

<bean id="MySelector" class="exampleCode.MySelector" />

<int:filter input-channel="someChannel" ref="MySelector"
	output-channel="someOtherChannel" />

Prior to version 3.0, invoking the send method on a PublishSubscribeChannel that had no subscribers returned false. When used in conjunction with a MessagingTemplate, a MessageDeliveryException was thrown. Starting with version 3.0, the behavior has changed such that a send is always considered successful if at least the minimum subscribers are present (and successfully handle the message). This behavior can be modified by setting the minSubscribers property, which defaults to 0.

Reference: https://docs.spring.io/spring-integration/docs/5.0.5.RELEASE/reference/html/messaging-channels-section.html

Also, I think you can implement Conditional Variables with Spring using @Conditional for the interceptors to execute for a given condition.

Reference: https://javapapers.com/spring/spring-conditional-annotation/#:~:text=Spring%20Boot%20module%20makes%20heavy,variables%2C%20irrespective%20of%20its%20value.