Update the 3.5 release notes with more information about generics and bean-based conditions

[vmeunier]

Hello,
I saw the different issues #45938 and #45709

The issue here is different from those. I set up a really stupid example that is not our use-case, but it illustrates the goal.

Consider a class GenericClass<T>.

Working in 3.4.6 :

    @Bean
    public GenericClass<String> genericBean1() {
        return new GenericClass<>("Hello, Generics!");
    }

    @Bean
    @ConditionalOnMissingBean
    public GenericClass<Integer> genericBean2() {
        return new GenericClass<>(7);
    }

	@Bean
	public CommandLineRunner init(GenericClass<?> genericBean) {
		return args -> System.out.println("GenericClass with value: " + genericBean.getValue());
	}

Starting from 3.5.0 :

Parameter 0 of method init in com.example.generics.demo.DemoApplication required a single bean, but 2 were found:
	- genericBean1: defined by method 'genericBean1' in class path resource [com/example/generics/demo/DemoConfiguration.class]
	- genericBean2: defined by method 'genericBean2' in class path resource [com/example/generics/demo/DemoConfiguration2.class]

The only "workaround" in 3.5+ is to change the bean definitions with the wildcard ? instead of the concrete type.

For example :

    @Bean
    @ConditionalOnMissingBean
    public GenericClass<?> genericBean2() {
        return new GenericClass<>(7);
    }

I have not seen anything regarding this in the changelog.

Is it still possible to define @ConditionalOnMissingBean with a type that uses Generics where the concrete type used in the generic might defer ?

[wilkinsona]

Have you tried @ConditionalOnMissingBean(GenericClass.class)?

[vmeunier]

Nope, and it works if I add it

It gives a way easier "workaround" if I simply add that instead of using wildcards.

[wilkinsona]

Thanks for trying it. This is then working as designed. There's a very brief mention of this change in the miscellaneous section of the release notes for 3.5, but I think they'd benefit from a dedicated section with some examples. We can use this issue to add such a section.

[vmeunier]

Thanks for the information !

I agree it would be worth mentionning that @ConditionalOnMissingBean on a generic type will by default check the generics, and if you don't care about the actual type you can specify the GenericClass directly in the condition. (which was the default behaviour in SpringBoot 3.4-)