Direct autowiring of existing collection subtype bean if `ApplicationContext` contains beans of type `T`

[yvasyliev]

The current focus of this issue is described in #30022 (comment).

For background information, the original issue title and description follow.

---

Previous title: "Cannot autowire java.util.Queue<T> if ApplicationContext contains beans of type T"

Affects: spring-context 5.3.25

---

I'm not able to autowire java.util.Queue in console app using spring-context.

There is a quick code example:

public interface Action {
    void doAction();
}

public class Action1 implements Action {
    @Override
    public void doAction() {
        System.out.println(this.getClass().getName());
    }
}

public class Action2 implements Action {
    @Override
    public void doAction() {
        System.out.println(this.getClass().getName());
    }
}

public class ActionManager {
    @Autowired
    private Queue<Action> actionQueue;

    public void doActions() {
        actionQueue.forEach(Action::doAction);
    }
}

@Configuration
public class Config {
    @Bean
    public ActionManager actionManager() {
        return new ActionManager();
    }

    @Bean
    public Queue<Action> actionQueue() {
        Queue<Action> actionQueue = new ArrayDeque<>();
        actionQueue.add(action1());
        actionQueue.add(action2());
        return actionQueue;
    }

    @Bean
    public Action action1() {
        return new Action1();
    }

    @Bean
    public Action action2() {
        return new Action2();
    }
}

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(Config.class);
        ActionManager actionManager = context.getBean(ActionManager.class);
        actionManager.doActions();
    }
}

When I run this code, I receive the next error in console:

Feb 24, 2023 9:16:11 AM org.springframework.context.support.AbstractApplicationContext refresh
WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'actionManager': Unsatisfied dependency expressed through field 'actionQueue'; nested exception is org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue': no matching editors or conversion strategy found
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'actionManager': Unsatisfied dependency expressed through field 'actionQueue'; nested exception is org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue': no matching editors or conversion strategy found
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:660)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1431)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:619)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:955)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:93)
	at app.Main.main(Main.java:8)
Caused by: org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue'; nested exception is java.lang.IllegalStateException: Cannot convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue': no matching editors or conversion strategy found
	at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:76)
	at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:45)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveMultipleBeans(DefaultListableBeanFactory.java:1471)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1349)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1311)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657)
	... 15 more
Caused by: java.lang.IllegalStateException: Cannot convert value of type 'java.util.LinkedHashMap$LinkedValues' to required type 'java.util.Queue': no matching editors or conversion strategy found
	at org.springframework.beans.TypeConverterDelegate.convertIfNecessary(TypeConverterDelegate.java:262)
	at org.springframework.beans.TypeConverterSupport.convertIfNecessary(TypeConverterSupport.java:73)
	... 20 more

Note 1: If I replace @Autowired with @javax.annotation.Resource, the code will work.
Note 2: If I replace java.util.Queue with java.util.Set, the code will work.

Since the issue is reproduced for java.util.Queue only, it seems like a bug.

[sbrannen]

This is a known limitation and effectively a duplicate of #16794. See the discussions in that issue for details.

Known workarounds:

1. As you mentioned, you can use @Resource instead of @Autowired.
2. Alternatively, you can combine @Autowired with @Qualifier("actionQueue") in ActionManager.

---

Aside from those workarounds, there is also the option to remove the Action @Bean methods from your configuration, and your code will then work as expected.

The reason is that Spring will first look for beans of type Action and attempt to create a Queue collection instance from all Action beans; however, as you pointed out that results in an exception since Spring does not know how to convert from java.util.LinkedHashMap$LinkedValues to java.util.Queue.

Thus, when you remove the Action @Bean methods, Spring no longer finds any beans of type Action and instead looks for a bean of type Queue<Action> which it finds and autowires into your ActionManager.

In summary, if you do not need to have the Action instances managed as beans within the ApplicationContext, I would recommend you simply remove them. On the other hand, if you need them to be managed beans, I'd suggest you go with @Resource or the @Autowired/@Qualifier combination.

[sbrannen]

To help you experiment with the options, I've reduced your examples to the following single test class.

The test passes as-is, but if you reinstate the @Bean lines the test will fail as in your original example.

@SpringJUnitConfig
class AutowiredQueueTests {

	@Autowired
	// @Autowired @Qualifier("queue")
	// @Resource
	Queue<String> queue;

	@Test
	void test() {
		assertThat(this.queue).containsExactly("foo", "bar");
	}

	@Configuration
	static class Config {

		// @Bean
		String foo() {
			return "foo";
		}

		// @Bean
		String bar() {
			return "bar";
		}

		@Bean
		Queue<String> queue() {
			Queue<String> queue = new ArrayDeque<>();
			queue.add(foo());
			queue.add(bar());
			return queue;
		}

	}

}

[sbrannen]

As a proof of concept, I inserted the following lines of code...

// If a single bean of the exact collection type exists, do not resolve beans of the elementType into a
// new collection. Rather, fall back to autowiring the single bean of the exact collection type as-is.
if (determineAutowireCandidate(findAutowireCandidates(beanName, type, descriptor), descriptor) != null) {
	return null;
}

... into DefaultListableBeanFactory#resolveMultipleBeans(DependencyDescriptor, String, Set<String>, TypeConverter) here:

spring-framework/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java

Line 1476 in 6825a84

Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType,

That appears to achieve the desired effect and does not cause any other tests in Spring Framework's test suite to fail -- though it is a change in behavior and can cause existing applications to fail or behave differently.

@jhoeller, putting aside the comments you made in #16794 against changing this behavior, what are your thoughts on supporting such use cases in a future version of Spring Framework?

[sbrannen]

As a side note for @yvasyliev and anyone else reading this issue...

If you want Spring to collect beans of a certain type into a collection type that is not supported by Spring out of the box, you can always register your own custom Converter.

For example, if we want Spring to find all beans of type String and collect them into a Queue (with concrete type ArrayDeque), we could modify the previous test case as follows.

@SpringJUnitConfig
class AutowiredQueueTests {

	@Autowired
	Queue<String> queue;

	@Test
	void test() {
		assertThat(this.queue).containsExactly("foo", "bar");
	}

	@Configuration
	static class Config {

		@Bean
		String foo() {
			return "foo";
		}

		@Bean
		String bar() {
			return "bar";
		}

		@Bean
		ConversionService conversionService() {
			ConversionServiceFactoryBean factory = new ConversionServiceFactoryBean();
			factory.setConverters(Set.of(new CollectionToQueueConverter()));
			factory.afterPropertiesSet();
			return factory.getObject();
		}

	}

	@SuppressWarnings("rawtypes")
	static class CollectionToQueueConverter implements Converter<Collection<?>, Queue<?>> {

		@Override
		public Queue<?> convert(Collection<?> source) {
			return new ArrayDeque<>(source);
		}
	}

}

Note, however, that the above CollectionToQueueConverter is very basic (always creates an ArrayDeque) and completely ignores generics. In addition, there is no longer a @Bean Queue<String> queue() method since Spring is now creating the Queue.

[sbrannen]

In light of this issue, #29987, as well as the popularity of issues such as #16794 (and all of the related issues linked from that issue), we have decided to revisit this topic for Spring Framework 6.1.

Specifically, our goal is to support direct injection of a collection bean even if the bean factory contains beans of the type contained in the collection bean.

Things to consider:

• investigate if it makes sense to introduce a simple check as demonstrated in above.
• investigate if it makes sense to support (or require?) @Primary on the collection bean to signal that the existing collection bean should be injected instead of a dynamically created collection consisting of beans of that type in the bean factory.
• investigate if the same kind of support can be provided for map beans.