Spring源码阅读（三）

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上一讲我们谈到单例生产关键方法getSingleton。getSingleton方法由DefaultSingletonBeanRegistry类实现。我们的抽象工厂AbstractBeanFactory继承了FactoryBeanRegistrySupport，而FactoryBeanRegistrySupport则继承了DefaultSingletonBeanRegistry，AbstractBeanFactory注册实例的工作实际上由DefaultSingletonBeanRegistry实施，不过其中生产单例的操作还是回调了AbstractBeanFactory中的createBean方法。

DefaultSingletonBeanRegistry类结构

可以看到，DefaultSingletonBeanRegistry实现了

（1）单例注册SingletonBeanRegistry

（2）别名注册SimpleAliasRegistry

同时，DefaultSingletonBeanRegistry需要传入ObjectFactory实现实例的创建，传入DisposableBean实现实例的销毁。

/**
* Return the (raw) singleton object registered under the given name,
* creating and registering a new one if none registered yet.
* 返回一个单例，不存在则创建并注册
* @param beanName the name of the bean
* @param singletonFactory the ObjectFactory to lazily create the singleton
* 单例创建工厂
* with, if necessary
* @return the registered singleton object
*/
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
// 单例已经存在则直接返回
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
// 单例创建前刷新状态
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
// 调用单例创建工厂创建单例
try {
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 单例创建后刷新状态
afterSingletonCreation(beanName);
}
// 单例注册
if (newSingleton) {
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}