Spring循环依赖实现过程揭秘

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

private final Map<String, ObjectFactory<?>> singletonFactories = new ConcurrentHashMap<>(16);

@Component
public class A {
    @Autowired
    private B b;
    public B getB() {
        return b;
    }
    public void setB(B b) {
        this.b = b;
    }
}
@Component
public class B {
    @Autowired
    private A a;
    public A getA() {
        return a;
    }
    public void setA(A a) {
        this.a = a;
    }
}
@Configuration
@ComponentScan(basePackages = "com.mashibing.selfcycle")
public class CycleConfig {
}
public class TestCycle {
    public static void main(String[] args) {
        final AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(CycleConfig.class);
        ac.close();
    }
}

// org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
// 为避免后期循环依赖，可以在bean初始化完成前将创建实例的ObjectFactory加入工厂
// bean是实例化后的bean对象
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
	// 默认最终公开的对象是bean,通过createBeanInstance创建出来的普通对象
	Object exposedObject = bean;
	// mbd的systhetic属性：设置此bean定义是否是"synthetic"，一般是指只有aop相关的pointCut配置或者Advice配置才会将 synthetic设置为true
	// 如果mdb不是synthetic且此工厂拥有InstantiationAwareBeanPostProcessor
	if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
		// 遍历工厂内的所有后处理器
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
			// 如果bp是SmartInstantiationAwareBeanPostProcessor实例
			if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
				SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
				// 让exposedObject经过每个SmartInstantiationAwareBeanPostProcessor的包装
				exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
			}
		}
	}
}

	public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
			throws BeansException {
		return beanFactory.getBean(beanName);
	}

DefaultSingleBeanRegistry.java
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	// Quick check for existing instance without full singleton lock
	// 从单例对象缓存中获取beanName对应的单例对象
	Object singletonObject = this.singletonObjects.get(beanName);
	// 如果单例对象缓存中没有，并且该beanName对应的单例bean正在创建中
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		//从早期单例对象缓存中获取单例对象（之所称成为早期单例对象，是因为earlySingletonObjects里
		// 的对象的都是通过提前曝光的ObjectFactory创建出来的，还未进行属性填充等操作）
		singletonObject = this.earlySingletonObjects.get(beanName);
		// 如果在早期单例对象缓存中也没有，并且允许创建早期单例对象引用
		if (singletonObject == null && allowEarlyReference) {
			// 如果为空，则锁定全局变量并进行处理
			synchronized (this.singletonObjects) {
				// Consistent creation of early reference within full singleton lock
				singletonObject = this.singletonObjects.get(beanName);
				if (singletonObject == null) {
					singletonObject = this.earlySingletonObjects.get(beanName);
					if (singletonObject == null) {
						// 当某些方法需要提前初始化的时候则会调用addSingletonFactory方法将对应的ObjectFactory初始化策略存储在singletonFactories
						ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
						if (singletonFactory != null) {
							// 如果存在单例对象工厂，则通过工厂创建一个单例对象
							singletonObject = singletonFactory.getObject();
							// 记录在缓存中，二级缓存和三级缓存的对象不能同时存在
							this.earlySingletonObjects.put(beanName, singletonObject);
							// 从三级缓存中移除
							this.singletonFactories.remove(beanName);
						}
					}
				}
			}
		}
	}
	return singletonObject;
}

	protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			// 将映射关系添加到单例对象的高速缓存中
			this.singletonObjects.put(beanName, singletonObject);
			// 移除beanName在单例工厂缓存中的数据
			this.singletonFactories.remove(beanName);
			// 移除beanName在早期单例对象的高速缓存的数据
			this.earlySingletonObjects.remove(beanName);
			// 将beanName添加到已注册的单例集中
			this.registeredSingletons.add(beanName);
		}
	}