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

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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<String, Object> singletonObjects = new ConcurrentHashMap<>(256); /** Cache of singleton factories: bean name --> ObjectFactory */ private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); /** Cache of early singleton objects: bean name --> bean instance */ private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); /** Set of registered singletons, containing the bean names in registration order */ private final Set<String> 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); } } }