簡介

package org.springframework.beans.factory.support;

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
    /** Cache of singleton objects: bean name to bean instance. */
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

    /** Cache of singleton factories: bean name to ObjectFactory. */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

    /** Cache of early singleton objects: bean name to bean instance. */
    private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
}

• singletonObjects：一級緩存，調(diào)用 getBean() 可以直接獲取初始化完成的 單例對象。
• earlySingletonObjects：二級緩存，已經(jīng)被 AOP 增強，但是還有屬性沒有注入完。
• singletonFactories：遍歷所有的 BeanPostProcessor 后置處理器，通過 SmartInstantiationAwareBeanPostProcessor 類型的后置處理器進行 AOP 增強。

緩存初始化時機

• singletonFactories：實例化對象（createBeanInstance 方法）之后，填充屬性（populateBean 方法）之前。

    protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {
        // 1. 實例化 Bean 對象
        BeanWrapper instanceWrapper = createBeanInstance(beanName, mbd, args);
        
        // 2. 把用 lambda 表達式 表示的 ObjectFactory，添加到 singletonFactories 緩存
        addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        
        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            // 3. 填充 Bean 對象的屬性  
            populateBean(beanName, mbd, instanceWrapper);
            exposedObject = initializeBean(beanName, exposedObject, mbd);
        }
        return exposedObject;
    }

• earlySingletonObjects：存在循環(huán)引用時才會初始化。在 getSingleton() 方法中，通過 singletonFactories 獲取到 AOP 對象后填入。

    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) {
                    // 1. 獲取到 ObjectFactory 緩存
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        // 2. 通過 `getEarlyBeanReference()` 方法得到 AOP 增強的代理對象
                        singletonObject = singletonFactory.getObject();

                        // 3. 將 AOP 增強的代碼對象存放到 earlySingletonObjects 緩存中
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }

• singletonObjects：單例對象 初始化完成（屬性填充完畢，以及執(zhí)行完所有的 Spring 初始化生命周期方法后）填入。

參考

• 跳出源碼地獄，Spring 巧用三級緩存解決循環(huán)依賴-原理篇