spring源码解析之IOC容器（四）——属性注入

　　上一篇跟踪了bean的创建过程，接下来，我们继续跟踪bean的属性填充的过程。先回到doCreateBean方法，代码如下：

  1 protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
  2             throws BeanCreationException {
  3 
  4         // Instantiate the bean.
  5         BeanWrapper instanceWrapper = null;
  6         if (mbd.isSingleton()) {
  7             //如果是单例，先把缓存中的同名Bean清除
  8             instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
  9         }
 10         //如果缓存中没有
 11         if (instanceWrapper == null) {
 12             //则创建一个实例
 13             instanceWrapper = createBeanInstance(beanName, mbd, args);
 14         }
 15         final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
 16         Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
 17         mbd.resolvedTargetType = beanType;
 18 
 19         // Allow post-processors to modify the merged bean definition.
 20         synchronized (mbd.postProcessingLock) {
 21             if (!mbd.postProcessed) {
 22                 try {
 23                     //使用后置处理器进行处理
 24                     applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
 25                 }
 26                 catch (Throwable ex) {
 27                     throw new BeanCreationException(mbd.getResourceDescription(), beanName,
 28                             "Post-processing of merged bean definition failed", ex);
 29                 }
 30                 mbd.postProcessed = true;
 31             }
 32         }
 33 
 34         // Eagerly cache singletons to be able to resolve circular references
 35         // even when triggered by lifecycle interfaces like BeanFactoryAware.
 36         //这里是对单例的循环引用的处理，单例&&允许循环依赖&&正在被创建同时满足，才为true
 37         boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
 38                 isSingletonCurrentlyInCreation(beanName));
 39         if (earlySingletonExposure) {
 40             if (logger.isDebugEnabled()) {
 41                 logger.debug("Eagerly caching bean '" + beanName +
 42                         "' to allow for resolving potential circular references");
 43             }
 44             addSingletonFactory(beanName, new ObjectFactory