Spring circular dependency using setters

Question

I have read that to avoid circular dependencies I can use @Autowired on setters instead of constructors.

If so, why does this fail?

@Component
private static class A {
    @Autowired
    public A(B b) {
    }
}

@Component
private static class B {
    private A a;

    public B() {
    }

    @Autowired
    public void setA(A a) {
        this.a = a;
    }
}

If I define the A class similarly to B, everything is ok, but the above one should already work, so why it doesn't? I am in situation where I cannot get rid of dependency in constructor of A class. What can I do in this case?

EDIT: I am using Spring 3.2.1.RELEASE

EDIT2: My exception:

Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'dependencyTest.A' defined in file [/home/adam/workspaces/testproject/target/classes/mypackage/test/DependencyTest$A.class]: Unsatisfied dependency expressed through constructor argument with index 0 of type [mypackage.test.DependencyTest$B]: : Error creating bean with name 'dependencyTest.B': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire method: public void mypackage.test.DependencyTest$B.setA(mypackage.test.DependencyTest$A); nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'dependencyTest.A': Requested bean is currently in creation: Is there an unresolvable circular reference?; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dependencyTest.B': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire method: public void mypackage.test.DependencyTest$B.setA(mypackage.test.DependencyTest$A); nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'dependencyTest.A': Requested bean is currently in creation: Is there an unresolvable circular reference?
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:730)
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:196)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1049)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:953)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:490)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:461)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:626)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:932)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:479)
    at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)
    at mypackage.test.DependencyTest.main(DependencyTest.java:10)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dependencyTest.B': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire method: public void mypackage.test.DependencyTest$B.setA(mypackage.test.DependencyTest$A); nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'dependencyTest.A': Requested bean is currently in creation: Is there an unresolvable circular reference?
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:288)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1120)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:522)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:461)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:891)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:834)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:749)
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:795)
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:723)
    ... 14 more
Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire method: public void mypackage.test.DependencyTest$B.setA(mypackage.test.DependencyTest$A); nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'dependencyTest.A': Requested bean is currently in creation: Is there an unresolvable circular reference?
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredMethodElement.inject(AutowiredAnnotationBeanPostProcessor.java:601)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:285)
    ... 26 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'dependencyTest.A': Requested bean is currently in creation: Is there an unresolvable circular reference?
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:327)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:217)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:891)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:834)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:749)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredMethodElement.inject(AutowiredAnnotationBeanPostProcessor.java:558)
    ... 28 more

Answer 1

I think Spring just isn't quite clever enough to realize that this kind of relationship has to be loaded in a specific order to avoid looking like an unresolvable circular reference. It typically handles circular references by caching singleton instances after constructing them but before fully populating them so that it can inject the instance where needed, even if it's not populated yet, since it knows it will finish setting it up later.

What's happening is that it's trying to create the A first, but it can't even construct one without a B, so it can't cache an A instance. Then it tries to create a B and sees that it needs an A, but it knows it's in the process of creating the A but doesn't have a cached instance to use for the dependency, so it fails. You need to force it to create the B instance first with something like a @DependsOn("dependencyTest.B"). Then the singleton caching can take place because a B can be constructed without needing an A, so the unpopulated B instance will be available when creating the A.

Answer 2

The beans are instantiated first, then injected into each other. That's why you are facing this situation. When A is being instantiated, it needs B.

Using setters, instead of constructors, would resolve this issue, as the instantiation phase is done without adding dependencies.