spring三級緩存： * singletonObjects 一級緩存，用于保存實例化、注入、初始化完成的bean實例 * earlySingletonObjects 二級緩存，用于保存**實例化(但未初始化)**完成的bean實例 * singletonFactories 三級緩存，用于保存bean創建工廠(ObjectFactory)，以便于后面擴展有機會創建代理對象 ``` /** Cache of singleton objects: bean name --> bean instance */ private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256); /** Cache of singleton factories: bean name --> ObjectFactory */ private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16); /** Cache of early singleton objects: bean name --> bean instance */ private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16); ``` 獲取單例Bean的源碼如下： 1. 先從一級緩存singletonObjects中去獲取。（如果獲取到就直接return） 2. 如果獲取不到或者對象正在創建中（isSingletonCurrentlyInCreation()），那就再從二級緩存earlySingletonObjects中獲取。（如果獲取到就直接return） 3. 如果還是獲取不到，且允許singletonFactories（allowEarlyReference=true）通過getObject()獲取。就從三級緩存singletonFactory.getObject()獲取。（如果獲取到了就從singletonFactories中移除，并且放進earlySingletonObjects） > 加入`singletonFactories`三級緩存的前提是執行了構造器，所以構造器的循環依賴沒法解決 ``` public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry { ... @Override @Nullable public Object getSingleton(String beanName) { return getSingleton(beanName, true); } @Nullable protected Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { singletonObject = singletonFactory.getObject(); this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return singletonObject; } ... public boolean isSingletonCurrentlyInCreation(String beanName) { return this.singletonsCurrentlyInCreation.contains(beanName); } protected boolean isActuallyInCreation(String beanName) { return isSingletonCurrentlyInCreation(beanName); } ... } ``` ``` // 它可以將創建對象的步驟封裝到ObjectFactory中 交給自定義的Scope來選擇是否需要創建對象來靈活的實現scope。 具體參見Scope接口 @FunctionalInterface public interface ObjectFactory<T> { T getObject() throws BeansException; } ``` 經過ObjectFactory.getObject()后，此時放進了二級緩存**`earlySingletonObjects`**內。這個時候對象已經實例化了，**`雖然還不完美`**，但是對象的引用已經可以被其它引用了