Spring循环依赖产生与解决

static class BeanA {
    // 1. 属性循环依赖
    BeanB beanB = new BeanB();
    // 2. 构造器循环依赖
    public BeanA(BeanB beanB) {
        this.beanB = beanB;
    }
}
static class BeanB {
    // 1. 属性循环依赖
    BeanA beanA = new BeanA();
    // 2. 构造器循环依赖
    public BeanB(BeanA beanA) {
        this.beanA = beanA;
    }
}
public static void main(String[] args) {
    // 1. 属性循环依赖
    BeanA beanA = new BeanA(); // StackOverflowError
    // 2. 构造器循环依赖
    new BeanA(new BeanB(new BeanA(new BeanB(  ))));
}

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    // ...
    // Fail if we're already creating this bean instance:
    // We're assumably within a circular reference.
    if (isPrototypeCurrentlyInCreation(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
    }
    // ...
    else if (mbd.isPrototype()) {
        // It's a prototype -> create a new instance.
        Object prototypeInstance = null;
        try {
            beforePrototypeCreation(beanName);
            prototypeInstance = createBean(beanName, mbd, args);
        }
        finally {
            afterPrototypeCreation(beanName);
        }
        bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
    }
    // ...
}
protected boolean isPrototypeCurrentlyInCreation(String beanName) {
    Object curVal = this.prototypesCurrentlyInCreation.get();
    return (curVal != null &&
            (curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName))));
}
protected void beforePrototypeCreation(String beanName) {
    Object curVal = this.prototypesCurrentlyInCreation.get();
    if (curVal == null) {
        this.prototypesCurrentlyInCreation.set(beanName);
    }
    else if (curVal instanceof String) {
        Set<String> beanNameSet = new HashSet<>(2);
        beanNameSet.add((String) curVal);
        beanNameSet.add(beanName);
        this.prototypesCurrentlyInCreation.set(beanNameSet);
    }
    else {
        Set<String> beanNameSet = (Set<String>) curVal;
        beanNameSet.add(beanName);
    }
}

static class BeanA {
    BeanB beanB;
    public BeanA(BeanB beanB) {
        this.beanB = beanB;
    }
}
static class BeanB {
    BeanA beanA;
    public BeanB(BeanA beanA) {
        this.beanA = beanA;
    }
}
public static void main(String[] args) {
    // 1. 属性循环依赖
    BeanA beanA = new BeanA();
    BeanB beanB = new BeanB();
    beanA.beanB = beanB;
    beanB.beanA = beanA;
    // 2. 构造器循环依赖
    new BeanA(new BeanB(new BeanA(new BeanB(  ))));
}