protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// 方法1）从三个map中获取单例类
Object sharedInstance = getSingleton(beanName);
// 省略无关代码
}
else {
// 如果是多例的循环引用，则直接报错
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 省略若干无关代码
try {
// Create bean instance.
if (mbd.isSingleton()) {
// 方法2) 获取单例对象
sharedInstance = getSingleton(beanName, () -> {
try { //方法3) 创建ObjectFactory中getObject方法的返回值
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
}
// 省略若干无关代码
return (T) bean;
}
该方法比较长，对于解决循环引用来说，上面标出来的3个方法起到了至关重要的作用，下面我们挨个攻克。
3.1） getSingleton(beanName)方法： 注意该方法跟方法2）是重载方法，名字一样内部逻辑却大相径庭。
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);// 步骤A
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);// 步骤B
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);// 步骤C
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}