(一)Spring IoC源码-2.bean的加载-02从缓存中获取单例bean

本文已收录于【Spring源码札记专栏】。

关联文章：
(一)Spring IoC源码-2.bean的加载-01整体概览
(一)Spring IoC源码-2.bean的加载-02从缓存中获取单例bean
(一)Spring IoC源码-2.bean的加载-03从bean实例中获取对象
(一)Spring IoC源码-2.bean的加载-04创建bean

从缓存中获取单例bean是通过DefaultSingletonBeanRegistry.getSingleton(String)实现的。

@Override
public Object getSingleton(String beanName) {
    return getSingleton(beanName, true);
}

@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    //首先从singletonObjects中获取
    Object singletonObject = this.singletonObjects.get(beanName);
    //如果没有获取到，且bean正在创建过程中（isSingletonCurrentlyInCreation()为true），则尝试从singletonFactories中获取
    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);
}

单例bean在同一个Spring容器中只创建一次，获取bean的时候，尝试从缓存加载bean。首先从singletonObjects中获取，如果没有获取到，且bean正在创建过程中（isSingletonCurrentlyInCreation()为true），则尝试从earlySingletonObjects中获取，如果还获取不到，就求助于singletonFactories。因为spring创建单例bean的时候，存在循环依赖的问题。比如创建bean a的时候发现bean a引用了bean b，此时会去创建bean b，但又发现bean b引用了bean c，所以此时会去创建bean c，在创建bean c的过程中发现bean c引用bean a。为了避免循环依赖，Spring采取了一种将正在创建的bean实例提早暴露加入到singletonFactories缓存中，一旦下一个bean创建的时候需要依赖上个bean，则直接使用singletonFactories来获取bean。

/** Cache of singleton objects: bean name --> bean instance */
private final Map singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map> singletonFactories = new HashMap<>(16);

/** Cache of early singleton objects: bean name --> bean instance */
private final Map earlySingletonObjects = new HashMap<>(16);

/** Set of registered singletons, containing the bean names in registration order */
private final Set registeredSingletons = new LinkedHashSet<>(256);

提前暴露bean实例到缓存的时机是在bean实例创建（调用构造方法）之后，初始化bean实例（属性注入）之前。具体在AbstractAutowireCapableBeanFactory类的protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {...}方法中。在该方法中调用了DefaultSingletonBeanRegistry类的addSingletonFactory方法提前暴露bean实例到缓存。

protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {
    Assert.notNull(singletonFactory, "Singleton factory must not be null");
    synchronized (this.singletonObjects) {
        if (!this.singletonObjects.containsKey(beanName)) {
            this.singletonFactories.put(beanName, singletonFactory);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
    }
}