Question

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

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，那么一切都还可以，但上面的那个应该已经有效，那为什么它不会呢？我处于A类的构造函数中无法摆脱依赖的情况。在这种情况下我该怎么办？

编辑：我使用的是Spring 3.2.1.RELEASE

EDIT2：我的例外：

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并不是很聪明地意识到这种关系必须以特定的顺序加载，以避免看起来像一个无法解决的循环引用。它通常通过在构造它们之后缓存单个实例来处理循环引用，但是在完全填充它们之前它可以在需要的地方注入实例，即使它还没有填充，因为它知道它将在以后完成它的设置。 / p>

正在发生的事情是它首先尝试创建A，但它甚至不能构建一个没有B的A，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，但它知道它在创建A的过程中但没有一个缓存实例用于依赖，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。

Answer 2

首先实例化bean，然后相互注入。这就是你面对这种情况的原因。在实例化A时，它需要B。

使用setter而不是构造函数可以解决此问题，因为实例化阶段在不添加依赖项的情况下完成。

使用setter的Spring循环依赖

2 个答案: