Proper way to define java.util.Queue bean in Spring?

I faced with a strange error when I tried to inject a java.util.Queue bean using Spring Core.

I made a quick code example:

@Configuration public class Main {
    @Autowired private Queue<Service> serviceQueue;

    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(Main.class);
        Main bean = context.getBean(Main.class);
        System.out.println(bean.serviceQueue.size());
    }

    @Bean public Queue<Service> serviceQueue() {
        Queue<Service> serviceQueue = new ArrayDeque<>();
        serviceQueue.add(serviceImpl1());
        serviceQueue.add(serviceImpl2());
        return serviceQueue;
    }

    @Bean public Service serviceImpl1() { return new ServiceImpl1(); }
    @Bean public Service serviceImpl2() { return new ServiceImpl2(); }

    public interface Service {}
    public static class ServiceImpl1 implements Service {}
    public static class ServiceImpl2 implements Service {}
}

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

Feb 23, 2023 8:59:48 PM 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 'main': Unsatisfied dependency expressed through field 'serviceQueue'; 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 'main': Unsatisfied dependency expressed through field 'serviceQueue'; 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$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:955)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:93)
	at app.Main.main(Main.java:16)
Caused by: org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.util.LinkedHashMap$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

Could you please explain why I can't create a Queue like this? If I replace Queue with List, everything will work.

Thank you.

Let me summarize the insights from chat in this answer and add a few things.

Why does it work with List?

When autowiring a List<X>, Spring automatically gives you a List containing all Spring-managed beans that can be assigned to X as described in this article.

So, Spring doesn't give you the List you created but it creates its own List containing all registered beans of type Service and doesn't give you the one created by you.

The error with Queue

Knowing that, we can also explain the error you are receiving:

> 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

It tried to autowire the List<Service> containing all registered Service beans but that wasn't a Queue.

@Resource to the rescue

You can explicitely specify the bean to be autowired using javax.annotation.Resource (Spring 5/Spring Boot 2/Java EE) or jakarta.annotation.Resource (Spring 6/Spring Boot 3/Jakarta EE):

@Resource(name = "serviceQueue")
private Queue<Service> serviceQueue;

@Bean("serviceQueue")
public Queue<Service> serviceQueue() {
    Queue<Service> serviceQueue = new ArrayDeque<>();
    serviceQueue.add(serviceImpl1());
    serviceQueue.add(serviceImpl2());
    return serviceQueue;
}

This tells Spring explicitly to use the Queue defined by you.

Alternatives

Constructor injection and @Order

If you don't need the same Queue in multiple places, you could use constructor autowiring and create the Queue in the constructor:

@Configuration
public class Main {
    private Queue<Service> serviceQueue;
    @Autowired
    public Main(List<Service> services){
        serviceQueue = new ArrayDequeue<>(services);
    }
}

@Bean
@Order(1)
public Service serviceImpl1() {
    return new ServiceImpl1();
}
@Bean
@Order(2)
public Service serviceImpl2() {
    return new ServiceImpl2();
}

Don't make the implementations beans

You could also try to remove @Bean from serviceImpl1() and serviceImpl2() so that Spring doesn't attempt to create a List of it by itself.

Note that I haven't tested this approach.