【Spring成神之路】从源码角度深度刨析Spring循环依赖

在Spring框架中，循环依赖是指两个或多个Bean相互依赖对方，形成闭环，导致无法正常创建Bean的情况。Spring解决循环依赖的关键在于提前暴露一个Bean的引用，使得后续Bean可以引用该Bean，从而打破循环依赖。

以下是Spring解决循环依赖的核心步骤：

1. 创建Bean实例，但不初始化属性。
2. 暴露一个代理对象（Bean的原型），使得其他Bean能引用该Bean。
3. 初始化Bean，设置属性。

以下是Spring处理循环依赖的核心源码：

public class DefaultSingletonBeanRegistry {
    // 存储Bean名称和Object的关系
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>();
 
    // 存储Bean名称和ObjectFactory的关系，用于创建Bean
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>();
 
    // 对应的Bean正在创建中
    private final Set<String> singletonsCurrentlyInCreation = Collections.synchronizedSet(new HashSet<>());
 
    // 获取Bean，如果Bean正在创建中，则从singletonFactories获取早期的引用
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && allowEarlyReference) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && this.singletonsCurrentlyInCreation.contains(beanName)) {
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }
 
    // 注册单例Bean，将Bean添加到singletonFactories
    protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
        synchronized (this.singletonObjects) {
            if (!this.singletonObjects.containsKey(beanName)) {
                this.singletonFactories.put(beanName, singletonFactory);
                this.singletonsCurrentlyInCreation.add(beanName);
            }
        }
    }
 
    // 注册单例Bean，将Bean添加到singletonObjects
    protected void addSingleton(String beanName, Object singletonObject) {
        synchronized (this.singletonObjects) {
            this.singletonObjects.put(beanName, singletonObject);
            this.singletonsCurrentlyInCreation.remove(beanName);
            this.singletonFactories.remov