spring源码分析3----refresh方法

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前两篇讲了xml的读取，这一篇继续讲xml的读取是被谁调用的。涉及的类主要是AbstractApplicationContext，以及类中的refresh方法。AbstractApplicationContext这个类代码比较多，挑重点。下面来看一下refresh方法。

@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();
      }
   }
}

refresh方法是在ClassPathXmlApplicationContext中被调用的，它做的工作很多，这一篇中先聚焦到xml文件的读取解析和bean注册。后续还会讲到这个方法的。在ClassPathXmlApplicationContext的构造方法中，调用了refresh，代码如下：

public ClassPathXmlApplicationContext(String[] paths, Class clazz, @Nullable ApplicationContext parent)
      throws BeansException {

   super(parent);
   Assert.notNull(paths, "Path array must not be null");
   Assert.notNull(clazz, "Class argument must not be null");
   this.configResources = new Resource[paths.length];
   for (int i = 0; i < paths.length; i++) {
      this.configResources[i] = new ClassPathResource(paths[i], clazz);
   }
   refresh(); //就是这里调的
}

refresh方法里调了那么多方法，具体是哪一个去做了加载读取xml的工作呢，是obtainFreshBeanFactory这个方法。

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
   refreshBeanFactory();
   return getBeanFactory();
}

obtainFreshBeanFactory这个方法又调了refreshBeanFactory方法，后者是在AbstractRefreshableApplicationContext中实现的。

@Override
protected final void refreshBeanFactory() throws BeansException {
   if (hasBeanFactory()) {
      destroyBeans();
      closeBeanFactory();
   }
   try {
      DefaultListableBeanFactory beanFactory = createBeanFactory(); //创建了类厂
      beanFactory.setSerializationId(getId());
      customizeBeanFactory(beanFactory);
      loadBeanDefinitions(beanFactory); //注意这一行
      synchronized (this.beanFactoryMonitor) {
         this.beanFactory = beanFactory; //保存了类厂
      }
   }
   catch (IOException ex) {
      throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
   }
}

在这个方法中，调用了loadBeanDefinitions，也就是前两篇一直在分析的东西。另外这个方法还创建了类厂，并把它保存起来了。

在AbstractApplicationContext类中，还有一大块代码是对beanFactory方法加了一层包装，举个例子，从里面随便找一个方法出来。

@Override
public boolean isSingleton(String name) throws NoSuchBeanDefinitionException {
   assertBeanFactoryActive();
   return getBeanFactory().isSingleton(name);
}

加这一层实际上是对一些条件进行判定，避免使用时每次都要用户自己去写一遍。

protected void assertBeanFactoryActive() {
   if (!this.active.get()) {
      if (this.closed.get()) {
         throw new IllegalStateException(getDisplayName() + " has been closed already");
      }
      else {
         throw new IllegalStateException(getDisplayName() + " has not been refreshed yet");
      }
   }
}

assertBeanFacrotyActive实在是判定beanFacroty是否处于可用状态。this.active和this.closed是两个原子操作类型。它们对应的是这两行代码。

/** Flag that indicates whether this context is currently active. */
private final AtomicBoolean active = new AtomicBoolean();

/** Flag that indicates whether this context has been closed already. */
private final AtomicBoolean closed = new AtomicBoolean();

/*

类型都是AtomicBoolean，原子布尔类型，免加锁。在prepareRefresh方法中，两个变量被设置。

this.closed.set(false);
this.active.set(true);

这代表beanFactory已经可用了。

最后提一下AbstractApplicationContext中出现的ApplicationEvent，spring利用观察者（或者说回调）来实现事件通信，而Event就是用来保存通信内容的。事件通信留待下一篇讲。

写在最后：接单，有后台活java/cpp/lua/go联系[email protected]。不上班了也要有点收入才行。