Spring循环依赖使用setter

Question

我已经读过，为了避免循环依赖，我可以在 setter 上使用 @Autowired 而不是构造器。

如果是这样，为什么会失败？

@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;
    }
}

如果我将 A 类定义为类似于 B 的类，一切正常，但上面的应该已经可以工作了，那为什么不行呢？我处于无法摆脱 A 类的构造函数中的依赖关系的情况。在这种情况下我该怎么办？

编辑:我正在使用 Spring 3.2.1.RELEASE

编辑2:我的异常(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

我认为 Spring 还不够聪明，无法意识到必须以特定顺序加载这种关系以避免看起来像无法解析的循环引用。它通常通过在构造单例实例之后但在完全填充它们之前缓存它们来处理循环引用，以便它可以在需要的地方注入(inject)实例，即使它尚未填充，因为它知道稍后会完成设置。

发生的情况是它首先尝试创建 A，但它甚至无法在没有 B 的情况下构造一个，因此它无法缓存 A 实例。然后它尝试创建一个 B 并看到它需要一个 A，但它知道它正在创建 A 的过程中但没有用于依赖的缓存实例，所以它失败了。您需要强制它首先使用 @DependsOn("dependencyTest.B") 之类的东西创建 B 实例。然后可以进行单例缓存，因为可以在不需要 A 的情况下构造 B，因此在创建 A 时可以使用未填充的 B 实例。