5.2缓存中获取单例bean

DefaultSingletonBeanRegistry
public Object getSingleton(String beanName) {
    //参数true设置标识允许早期依赖(提前引用)
    return getSingleton(beanName, true);
}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // Quick check for existing instance without full singleton lock
    //检查缓存中是否存在实例
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
    	//缓存中没有实例但是又在创建中，那么可能是循环依赖，解决循环依赖的方式是
		//Spring创建bean的原则是不等bean创建完成就会创建bean的ObjectFactory提早曝光
        singletonObject = this.earlySingletonObjects.get(beanName);
        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) {
                            //调用预先设定的getObject方法
                            singletonObject = singletonFactory.getObject();
                            //记录在缓存中，earlySingletonObjects和singletonFactories互斥
                            this.earlySingletonObjects.put(beanName, singletonObject);
                            this.singletonFactories.remove(beanName);
                        }
                    }
                }
            }
        }
    }
    return singletonObject;
}

• singletonObjects:用于保存BeanName和创建bean实例直接的关系，bean name--> bean instance.
• singletonFactories:用于保存BeanName和创建bean的工厂之间的关系，beanname-->ObjectFactory.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
• earlySingletonObjects:也是保存BeanName和创建bean实例之间的关系，与singletonObjects的不同之处在于，当一个单例bean被放到这里面后，那么当bean还在创建过程中，就可以通过getBean方法获取到了，其目的是用来检测循环应用的。
• registeredSingletons：用来保存当前所有已注册的bean。