单例在Spring的同一个容器中只会被创建一次，后续再获取bean，就直接从单例缓存中获取了。
先尝试从SingletonObjects缓存中加载实例；如果不成功则尝试从earlySingletonObjects中加载已经在创建完成之前提前暴露的单例Bean；如果失败了，则从singletonFactories中获取beanName对应的ObjectFactory，然后在调用ObjectFactory#getObject()方法创建对应的Bean，并放到earlySingletonObjects中，并从SingletonFactories中remove掉这个ObjectFactory。而对于后续的所有操作都只是为了循环依赖检测时候使用，也就是allowEarlyReference为true的情况下才使用。
为了解决循环依赖的问题，Spring中创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提前曝光加入到缓存中， 一旦下一个Bean创建时需要依赖上一个bean则直接使用ObjectFactory。
1. 从缓存中获取单例bean
beans.factory.support.DefaultSingletonBeanRegistry
@Override
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
/**
* Return the (raw) singleton object registered under the given name.
* <p>Checks already instantiated singletons and also allows for an early
* reference to a currently created singleton (resolving a circular reference).
* @param beanName the name of the bean to look for
* @param allowEarlyReference whether early references should be created or not
* @return the registered singleton object, or {@code null} if none found
*/
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 != NULL_OBJECT ? singletonObject : null);
}
简单解释一下用于存储bean的不同的map
- singletonObjects
用于保存BeanName和创建bean实例之间的关系，bean name --> bean instance
- singletonFactories
用于保存BeanName和创建bean的工厂之间的关系，bean name --> ObjectFactory
- earlySingletonObjects
保运BeanName和创建bean实例之间的关系，与singletonObjects的不同在于，当一个单例bean被放到这里面后，name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引用。
- registeredSingletons
用来保存当前所有已注册的单例bean
