Spring解决循环依赖的原理源码解析

@Component
public class A {
    @Autowired
    private B b;
}
@Component
public class B {
    @Autowired
    private A a;
}

public Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 1. 先从一级缓存中取
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        // 2. 一级没有，且 Bean 正在创建中，则尝试二级缓存
        singletonObject = this.earlySingletonObjects.get(beanName);
        if (singletonObject == null && allowEarlyReference) {
            // 3. 二级没有，就从三级缓存拿 ObjectFactory 生成早期引用
            ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
            if (singletonFactory != null) {
                singletonObject = singletonFactory.getObject();
                // 提前曝光的对象放到二级缓存
                this.earlySingletonObjects.put(beanName, singletonObject);
                this.singletonFactories.remove(beanName);
            }
        }
    }
    return singletonObject;
}

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, Object[] args) {
    // 1. 实例化 Bean（构造方法）
    BeanWrapper instanceWrapper = createBeanInstance(beanName, mbd, args);
    Object bean = instanceWrapper.getWrappedInstance();
    // 2. 是否需要提前暴露？
    boolean earlySingletonExposure = (mbd.isSingleton() && allowCircularReferences
        && isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
        // 加入三级缓存
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
    // 3. 填充属性（setter/field 注入）
    populateBean(beanName, mbd, instanceWrapper);
    // 4. 初始化（各种回调、AOP 代理）
    bean = initializeBean(beanName, bean, mbd);
    // 5. 放入一级缓存，清理二、三级缓存
    registerSingleton(beanName, bean);
    return bean;
}

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
    if (!this.singletonObjects.containsKey(beanName)) {
        this.singletonFactories.put(beanName, singletonFactory);
        this.earlySingletonObjects.remove(beanName);
    }
}

spring.main.allow-circular-references=true

public A(@Lazy B b) { this.b = b; }

@Autowired
private ObjectProvider<B> bProvider;