add support for multiple ContainerCustomizer (spring-amqp)

[rfelgent]

Hello community,

I have a feature request.

Background: I have splitted up my ContainerCustomizer logic into multiple parts in order to increase focus (one takes care of timeouts, another one takes care of consumer concurency and so on).

Currently, SpringBoots auto-configuration for ContainerCustomizer allows only one instance at a time.

@Bean(name = "rabbitListenerContainerFactory")
	@ConditionalOnMissingBean(name = "rabbitListenerContainerFactory")
	@ConditionalOnProperty(prefix = "spring.rabbitmq.listener", name = "type", havingValue = "simple",
			matchIfMissing = true)
	SimpleRabbitListenerContainerFactory simpleRabbitListenerContainerFactory(
			SimpleRabbitListenerContainerFactoryConfigurer configurer, ConnectionFactory connectionFactory,
			ObjectProvider<ContainerCustomizer<SimpleMessageListenerContainer>> simpleContainerCustomizer) {
		SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();
		configurer.configure(factory, connectionFactory);
		simpleContainerCustomizer.ifUnique(factory::setContainerCustomizer);   // here is the "problem"
		return factory;
	}

My currenct work around is to provide one ContainerCustomizier as an adapter which delegates the work to each ContainerCustomizier, like this:

 ...
     // SpringBoot defaults
     // simpleContainerCustomizer.ifUnique(factory::setContainerCustomizer);
      List<ContainerCustomizer<SimpleMessageListenerContainer>> containerCustomizers = simpleContainerCustomizer.orderedStream().collect(Collectors.toList());
      if (!containerCustomizers.isEmpty()) {
        factory.setContainerCustomizer(container -> containerCustomizers.forEach(customizer -> customizer.configure(container))); // here comes the wrapper/delegate logic
      }

I know that I could encapsulate all me customization into one ContainerCustomizer but,

• I would like to keep things simple/small and consistent by splitting along its purpose/usage (catchword: seperation of concerns)
• there are other customizers in SpringBoot auto-configure which do allow multiple implementations at the same time, e.g. WebClientCustomizer

Please let me know if my feature request to add logic that can handle multiple ContainerCustomizer` is feasable/useful.

If so, I could provide a pull request.

[wilkinsona]

We've taken our lead from Spring AMQP which supports a single customizer. We could make your workaround part of Boot but that feels like we'd be going in a direction that Spring AMQP has chosen not to support. In short, I think you should raise this with with Spring AMQP. If the team decides that AbstractRabbitListenerContainerFactory should support more than one ContainerCustomizer we can then adapt to that in Boot.

I'm going to close this issue for now as there's nothing that we can do or would want to do in Boot at this time. If a change is made in Spring AMQP we can re-open and update Boot accordingly.

/cc @garyrussell

[garyrussell]

A simple solution, which retains the separation of concerns would be to implement a CompositeContainerCustmizer which takes a collection of delegate customizers; that pattern is in use elsewhere in the portfolio.

If you would like to implement that, and contribute it to spring-amqp, no Boot changes would be required (as long as you don't declare the delegates as beans).

[rfelgent]

@garyrussell this kind of delegate/adapter code is my current solution and I was thinking of moving such logic too spring-boot auto-config.

It is not clear too me why the composite/adapter code must reside inside spring-amqp?! Why an Adapter class in spring-amqp and not just a list?
And yes, my delegated classes are spring managed beans - that is my current requirement.

@garyrussell and @wilkinsona please make a final decision where to put the new code: either spring-boot or spring-amqp (or do both projects require changes?!)

I can then create a pull request.

Thank you for your time.

[garyrussell]

If the composite customizer resides in spring-amqp, it would be available to users that don't use Spring Boot (as well as Boot users). It makes more sense too, because the interface resides there.

It would be a simple addition there (plus some small doc updates) and would not require any boot changes - the Boot team are extremely busy right now.

[rfelgent]

fyi: the PR for introducing CompositeContainerCustomizer was accepted in spring-amqp and is available since v2.4.8

As soon as spring-boot (2.7.x or 3.0.x) references this new release, I will make a PR for spring-boot.

[garyrussell]

Why do you need changes to Boot? Just add the composite customizer as a single bean, don't declare the delegates as beans, or mark the composite as @Primary, and Boot will auto configure it with existing code.

[rfelgent]

ah... very nice idea @garyrussell.

the thing is, that your solution is a little bit different compared to the configuration of other customizers in spring-boot, e.g. the WebClientCustomizer. The auto-configuration of WebClientCustomizer supports multiple WebClientCustomizer beans at a time. I do not know how "other" customizers behave or should behave - I came accross the ContainerCustomizer which behaves differently. It took me some time to figure out why none of my n (n > 1) ContainerCustomizer beans was found until I checked the auto-config

simpleContainerCustomizer.ifUnique(factory::setContainerCustomizer);   // here is the "problem"

I would love to have "one" way to handle all kinds of customizers and I thought, that spring boot autoconfig is the abstraction to facilate the customizer handling (see my 1st comment of this issue).

[garyrussell]

I understand it is a different technique.

My personal preference is the composite approach; it is simpler and you are in total control of the sequence in which customizers are called. With the Boot approach you have to implement Ordered or add an @Order annotation, to ensure they are invoked in the right sequence.