五、spring源码阅读之ClassPathXmlApplicationContext加载beanFactory

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring-config.xml");

Spring加载xml数据的切入点是通过ClassPathXmlApplicationContext类进行切入的。该类是面向xml文件。类似的spring还提供了面向注解的解析类AnnotationConfigApplicationContext等。

进入

new ClassPathXmlApplicationContext("spring-config.xml");

之后代码如下：

public ClassPathXmlApplicationContext(
      String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
      throws BeansException {

super(parent);
   setConfigLocations(configLocations);
   if (refresh) {
      refresh();
   }
}

Spring容器的加载、xml文件的解析器以及单例、非懒加载的类等都是由refresh（）方法完成。Refresh代码如下所示：

@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      //准备刷新环境列入对系统属性或者环境变量进行准备及验证
      prepareRefresh();

// Tell the subclass to refresh the internal bean factory.
      /* 调用子类AbstractRefreshableApplicationContext 刷新DefaultListableBeanFactory工厂
       * 功能描述  1、创建核心组件ConfigurableListableBeanFactory
       * 2、获取xml、注解等方式的配置，将其转换为原数据放入ConfigurableListableBeanFactory中
       * 3、xml读取
       */
      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.
         /* 创建单例或者非懒加载实例*/
         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();
      }
   }
}

如代码所示xml数据的读取主要是在obtainFreshBeanFactory()方法中进行处理的。而AbstractApplicationContext类中并没有对obtainFreshBeanFactory()方法做具体的实现，而是将其交由子类AbstractRefreshableApplicationContext去实现

Xml的读取主要处理2个问题

1、准备容器，用于存储解析后的数据

2、准备documentReader，用于解析spring配置文件

代码如下：

@Override
protected final void refreshBeanFactory() throws BeansException {
   /**
    * DefaultListableBeanFactory被实例化至少一次时将销毁spring创建的对象，并且关闭DefaultListableBeanFactory实例
    */
   if (hasBeanFactory()) {
      destroyBeans();
      closeBeanFactory();
   }
   try {
      DefaultListableBeanFactory beanFactory = createBeanFactory();//创建spring的核心组件
      //为了序列化指定id
      beanFactory.setSerializationId(getId());
      //定制beanFactory，设置相关属性。
      customizeBeanFactory(beanFactory);
      //初始化xml读取器、读取原数据切入点  交由子类处理
      loadBeanDefinitions(beanFactory);
      synchronized (this.beanFactoryMonitor) {
         this.beanFactory = beanFactory;
      }
   }
   catch (IOException ex) {
      throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
   }
}

该方法主要完成以下几个工作：

1、校验DefaultListableBeanFactory被实例化至少一次时将销毁spring创建的对象，并且关闭DefaultListableBeanFactory实例

2、创建spring的核心组件DefaultListableBeanFactory的具体实例并设置相应参数，也就是beanFactory。

3、初始化xml读取器、读取xml数据切入点  交由子类处理。

4、将beanFactory设定为全局变量。

AbstractRefreshableApplicationContext并没有对loadBeanDefinitions()方法进行具体的实现，而是将其交由子类去做处理，这样做的好处是针对不同的spring配置方式采用不同的解析方式进行解析（策略模式）。

Ps:此处只针对xml文件的配置（AbstractXmlApplicationContext）解析进行说明

上面部分代码完成了对beanFactory的定制，并且对xml文件的解析做了切入loadBeanDefinitions();

@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
   // Create a new XmlBeanDefinitionReader for the given BeanFactory.
   /*依据beanFactory创建新的XmlBeanDefinitionReader */
   XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

// Configure the bean definition reader with this context's
   // resource loading environment.
   beanDefinitionReader.setEnvironment(this.getEnvironment());
   beanDefinitionReader.setResourceLoader(this);
   beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

// Allow a subclass to provide custom initialization of the reader,
   // then proceed with actually loading the bean definitions.
   initBeanDefinitionReader(beanDefinitionReader);
   loadBeanDefinitions(beanDefinitionReader);
}

以上代码完成以下功能

1、为特定的beanFactory准备相应的beanDefinitionReader 并完成相应设置

2、切入loadBeanDefinitions（）；

Xml文件的解析是交由专门的解析类AbstractBeanDefinitionReader进行处理。

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
   Resource[] configResources = getConfigResources();
   if (configResources != null) {
      reader.loadBeanDefinitions(configResources);
   }
   String[] configLocations = getConfigLocations();
   if (configLocations != null) {
      reader.loadBeanDefinitions(configLocations);
   }
}

Ps：读者可参考此图