Improve BeanFactory/ObjectProvider to select the only one default candidate among non-default candidates

[quaff]

We can inject the default candidate when other candidates are non-default, but there is no way to get the default candidate from BeanFactory unless we know the bean name, sometimes the bean name is volatile or not visible to application, for example registered by Spring Boot's AutoConfiguration.

import static org.assertj.core.api.Assertions.assertThat;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;

@SpringJUnitConfig
class SimpleTests {

	@Autowired
	private BeanFactory beanFactory;

	@Autowired
	private Service defaultService;

	@Autowired
	@Qualifier("additionalService")
	private Service additionalService;

	@Test
	void testGetBean() {
		assertThat(beanFactory.getBean(Service.class)).isSameAs(defaultService);
		// failed due to NoUniqueBeanDefinitionException
	}

	@Test
	void testGetBeanProvider() {
		assertThat(beanFactory.getBeanProvider(Service.class).getObject()).isSameAs(defaultService);
		// failed due to NoUniqueBeanDefinitionException
	}

	@Test
	void testGetBeanWithName() {
		assertThat(beanFactory.getBean("additionalService", Service.class)).isSameAs(additionalService);
	}

	@Configuration
	static class Config {

		@Bean(name = "auto-generated-name-unknown-to-application")
		Service defaultService() {
			return new Service();
		}

		@Bean(defaultCandidate = false)
		Service additionalService() {
			return new Service();
		}

	}

	static class Service {

	}

}

[quaff]

My proposal: https://github.com/spring-projects/spring-framework/compare/main...quaff:patch-99?expand=1

[jhoeller]

@quaff good point, this is a general concern that can be taken into account in getBean(Class) just like the primary flag, even if no injection point with qualifier annotations is involved. However, this is semantically not quite "effectively primary", it is rather a last step of resolving a unique default among non-default candidates. I'll roll this into 6.2.4 (scheduled for release this Thursday).