Bean创建三级缓存

通过三级缓存解决循环依赖问题. 使用到的地方主要是 DefaultSingletonBeanRegistry

关键源码部分

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {

// 1级缓存, 用于存放已属性赋值,初始化后的 单例bean
private final Map<String, Object> singletonObjects = new ConcurrentHashMap(256);

// 3级缓存 存储创建BEAN的工厂
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap(16);

// 2级缓存 用于存放已经实例化,还未做代理属性赋值的操作 单例
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap(16);

// 已经注册到单例池的bean name
private final Set<String> registeredSingletons = new LinkedHashSet(256);

   // 通过 beanName 寻找单例
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        // 从一级缓存中拿 
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && this.isSingletonCurrentlyInCreation(beanName)) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
              // 锁住一级缓存
                synchronized(this.singletonObjects) {
                   // 1级
                    singletonObject = this.singletonObjects.get(beanName);
                    if (singletonObject == null) {
                        // 2级
                        singletonObject = this.earlySingletonObjects.get(beanName);
                        if (singletonObject == null) {
                            // 3级
                            ObjectFactory<?> singletonFactory = (ObjectFactory)this.singletonFactories.get(beanName);
                            if (singletonFactory != null) {
                               // 创建一个bean的实例 
                                singletonObject = singletonFactory.getObject();
                                // 将其放入二级缓存
                                this.earlySingletonObjects.put(beanName, singletonObject);
                                // 从三级缓存中移除
                                this.singletonFactories.remove(beanName);
                            }
                        }
                    }
                }
            }
        }

        return singletonObject;
    }
}