Spring之IOC

详细介绍Spring的IOC

IOC

beans和context两个包是IOC的基础，BeanFactory接口提供了管理bean的机制，而 ApplicationContext是BeanFactory的一个子接口，它增加了AOP的整合，资源国际化，事件发布，以及应用层的context，比如WebApplicationContext。

简单点的说，BeanFactory提供了配置框架和基本功能，ApplicationContext增加了企业开发需要的特性，Spring的IOC容器一般也就是指ApplicationContext。

Bean

bean的标识符必须唯一，一般情况下只有一个标识符，但可以有多个名称

在xml配置中，id, name 都是指的标识符，bean可以定义多个名称，在name属性中指定(逗号，分号或者空格分隔多个别名)

如果一个bean没有定义ID，则将会以它的simple name作为名字(首字母小写，如果多个大写字母开头，则保持原样)

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<alias name="fromName" alias="toName"/>

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容器的启动流程

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@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      prepareRefresh();

      // Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      // Prepare the bean factory for use in this context.
      prepareBeanFactory(beanFactory);

      try {
         // Allows post-processing of the bean factory in context subclasses.
         postProcessBeanFactory(beanFactory);

         // Invoke factory processors registered as beans in the context.
         invokeBeanFactoryPostProcessors(beanFactory);

         // Register bean processors that intercept bean creation.
         registerBeanPostProcessors(beanFactory);

         // Initialize message source for this context.
         initMessageSource();

         // Initialize event multicaster for this context.
         initApplicationEventMulticaster();

         // Initialize other special beans in specific context subclasses.
         onRefresh();

         // Check for listener beans and register them.
         registerListeners();

         // Instantiate all remaining (non-lazy-init) singletons.
         finishBeanFactoryInitialization(beanFactory);

         // Last step: publish corresponding event.
         finishRefresh();
      }

      catch (BeansException ex) {
         if (logger.isWarnEnabled()) {
            logger.warn("Exception encountered during context initialization - " +
                  "cancelling refresh attempt: " + ex);
         }

         // Destroy already created singletons to avoid dangling resources.
         destroyBeans();

         // Reset 'active' flag.
         cancelRefresh(ex);

         // Propagate exception to caller.
         throw ex;
      }

      finally {
         // Reset common introspection caches in Spring's core, since we
         // might not ever need metadata for singleton beans anymore...
         resetCommonCaches();
      }
   }
}

加载过程

加载过程分为三个步骤

  1. 资源定位
  2. 解析DefaultBeanDefinitionDocumentReader
  3. 注册
资源定位

image-20210503072417867

就如上图所示，一直是在调用父类的方法，直到ResourceLoader，这个类中有个getResource方法，可以将外部的资源，读取为Resource类。

解析
注册