Spring源码剖析（二）-Bean对象的三级缓存.pdfVIP

Spring源码剖析（⼆）-Bean对象的三级缓存 前⾯讲到Spring在创建Bean的过程中使⽤了三级缓存来对Bean对象进⾏缓存。这个三级缓存是⽤来解决对象的循环依赖问题。 什么是对象的循环依赖？ 举个例⼦： class A { private B b; private C c; } class B { private A a; } class C { private A a; } class A中引⽤了B和C，⽽B和C中⼜引⽤了A。 那么在spring中创建Bean对象时，需要先创建对象，然后对其属性进⾏填充。当对A进⾏属性填充时，发现需要创建B和C，⽽在创建B和C时， ⼜发现需要创建A。此时就出现了循环依赖问题了。 怎么解决循环依赖？ public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry { /** Cache of singleton objects: bean name to bean instance. */ private final MapString, Object singletonObjects = new ConcurrentHashMap(256); /** Cache of singleton factories: bean name to ObjectFactory. */ private final MapString, ObjectFactory? singletonFactories = new HashMap(16); /** Cache of early singleton objects: bean name to bean instance. */ private final MapString, Object earlySingletonObjects = new HashMap(16); spring利⽤singletonObjects, earlySingletonObjects, singletonFactories三级缓存去解决的。 spring2.png 我们假设现在有这样的场景AService依赖BService，BService依赖AService: 1. AService⾸先实例化，实例化通过ObjectFactory半成品暴露在三级缓存中 2. 填充属性BService，发现BService还未进⾏过加载，就会先去加载BService 3. 再加载BService的过程中，实例化，也通过ObjectFactory半成品暴露在三级缓存 4. 填充属性AService的时候，这时候能够从三级缓存中拿到半成品的ObjectFactory。 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();