Spring 源码分析之AbstractApplicationContext源码分析

　　首先我觉得分析ApplicationContext必须从它的实现类开始进行分析，AbstractApplicationContext我觉得是一个不错的选择，那我们就从这里开始逐一分析吧，首先我自己手画了一张图，作为索引吧，其中蓝色的为类，紫色的为接口，箭头 指向的方向是父类或者父接口。

　　因为里面接口和方法过多，所以不做展示，下面具体来进行代码分析。首先我们来看看这句话，MESSAGE_SOURCE_BEAN_NAME。

public static final String MESSAGE_SOURCE_BEAN_NAME = "messageSource";

　　它这句话翻译成中文就是消息资源的bean的一个name,我们暂时把它看成一个普通的beanName,我们来看看有哪些地方引用到了这个属性，首先在initMessageSource方法里面有引用到，我把这些地方标红显示了。

protected void initMessageSource() {
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		if (beanFactory.containsLocalBean(MESSAGE_SOURCE_BEAN_NAME)) {
			this.messageSource = beanFactory.getBean(MESSAGE_SOURCE_BEAN_NAME, MessageSource.class);
			// Make MessageSource aware of parent MessageSource.
			if (this.parent != null && this.messageSource instanceof HierarchicalMessageSource) {
				HierarchicalMessageSource hms = (HierarchicalMessageSource) this.messageSource;
				if (hms.getParentMessageSource() == null) {
					// Only set parent context as parent MessageSource if no parent MessageSource
					// registered already.
					hms.setParentMessageSource(getInternalParentMessageSource());
				}
			}
			if (logger.isTraceEnabled()) {
				logger.trace("Using MessageSource [" + this.messageSource + "]");
			}
		}
		else {
			// Use empty MessageSource to be able to accept getMessage calls.
			DelegatingMessageSource dms = new DelegatingMessageSource();
			dms.setParentMessageSource(getInternalParentMessageSource());
			this.messageSource = dms;
			beanFactory.registerSingleton(MESSAGE_SOURCE_BEAN_NAME, this.messageSource);
			if (logger.isTraceEnabled()) {
				logger.trace("No '" + MESSAGE_SOURCE_BEAN_NAME + "' bean, using [" + this.messageSource + "]");
			}
		}
	}

　　还有一个显示的地方就是在StaticApplicationContext类中的构造器当中有使用到。下面是StaticApplicationContext的类结构图：

	public StaticApplicationContext(@Nullable ApplicationContext parent) throws BeansException {
		super(parent);

		// Initialize and register a StaticMessageSource.
		this.staticMessageSource = new StaticMessageSource();
		getBeanFactory().registerSingleton(MESSAGE_SOURCE_BEAN_NAME, this.staticMessageSource);
	}

　　我们下面再来看一下AbstractApplicationContext这个类的一些Fields,并且来理清一下对象和对象之间的依赖关系，首先是parent -ApplicationContext，我们找到是一个叫做getParent()的方法对这个私有的属性进行了调用，然后又发现了getParentBeanFactory方法也对其进行了间接调用，因为BeanFactory是ApplicationContext的父接口，如下图：

private ApplicationContext parent;

public ApplicationContext getParent() {
   return this.parent;
}

public BeanFactory getParentBeanFactory() {
   return getParent();
}

　在这个类中还有一个属性是environment,这个environment在这里也有getter和setter方法，唯一需要注意的是如果调用getEnvironment()方法在environment为空的情况下会创建一个StandardEnvironment对象。

private ConfigurableEnvironment environment;

public ConfigurableEnvironment getEnvironment() {
   if (this.environment == null) {
      this.environment = createEnvironment();
   }
   return this.environment;
}

　　StandardEnvironment类继承了抽象的AbstractEnvironment，它的类结构图如下所示：

　　还有一个比较重要的属性就是beanFactoryPostProcessors，这事一个ArrayList的数组,Doc当中给出的解释是当onRefresh的时候有用到。我找到了3个方法引用到了这个属性，下面都已标红。

private final List<BeanFactoryPostProcessor> beanFactoryPostProcessors = new ArrayList<>();

public void addBeanFactoryPostProcessor(BeanFactoryPostProcessor postProcessor) {
   Assert.notNull(postProcessor, "BeanFactoryPostProcessor must not be null");
   this.beanFactoryPostProcessors.add(postProcessor);
}

public List<BeanFactoryPostProcessor> getBeanFactoryPostProcessors() {
   return this.beanFactoryPostProcessors;
}

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
   PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

   // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
   // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
   if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
      beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
      beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
   }
}

　　往下面看还有一个属性active,它是线程安全的，用到了CAS技术。它常常和closed这个属性一起使用，我们发现，在如下3个地方有组合引用，我已用相应的颜色标识出来。

• prepareRefresh
• doClose
• assertBeanFactoryActive

private final AtomicBoolean active = new AtomicBoolean();

private final AtomicBoolean closed = new AtomicBoolean();

protected void prepareRefresh() {
   // Switch to active.
   this.startupDate = System.currentTimeMillis();
   this.closed.set(false);
   this.active.set(true);
   ....................
   ....................
}

protected void doClose() {
   // Check whether an actual close attempt is necessary...
   if (this.active.get() && this.closed.compareAndSet(false, true)) {
      if (logger.isDebugEnabled()) {
         logger.debug("Closing " + this);
      }
    ........................
    ........................

   //Switch to inactive.
   this.active.set(false);

}

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");
      }
   }
}

　　我们继续往下看，有一个startupShutdownMonitor的属性，字面意思上面理解就是启动关闭监视器，属性在这个类当中的命名表示了它所发挥的作用，我们来看一下有哪些方法引用到了这个属性。大家不知道发现没有，还有一个“很像”的属性在这里就是shutdownHook,这个和startupShutdownMonitor是配合在一起使用的。shudownHook在这里是一个线程类型的属性。

private final Object startupShutdownMonitor = new Object();

private Thread shutdownHook;

public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {......

}......
}

public void registerShutdownHook() {
   if (this.shutdownHook == null) {
      // No shutdown hook registered yet.
      this.shutdownHook = new Thread() {
         @Override
         public void run() {
            synchronized (startupShutdownMonitor) {
               doClose();
            }
         }
      };
      Runtime.getRuntime().addShutdownHook(this.shutdownHook);
   }
}

public void close() {
   synchronized (this.startupShutdownMonitor) {
      doClose();
      // If we registered a JVM shutdown hook, we don't need it anymore now:
      // We've already explicitly closed the context.
      if (this.shutdownHook != null) {
         try {
            Runtime.getRuntime().removeShutdownHook(this.shutdownHook);
         }
         catch (IllegalStateException ex) {
            // ignore - VM is already shutting down
         }
      }
   }
}

　既然shutdownHook和startupShutdownMonitor一起使用，那么它们之间的关系我们得分析一下,hook顾名思义钩子，说简单点这个就是一个钩子，也算是一个扩展点。我们来仔细分析一下它的几个方法，首先是registerShutdownHook方法：这个方法有一句话特别重要，就是Runtime.getRuntime().addShutdownHook(this.shutdownHook);它实际上在系统层面上把钩子线程添加到了JVM虚拟机。在钩子运行的时候，就会执行doClose方法关闭并销毁applicationContext。需要注意的一点是明白registerShutdownHook方法和close方法的不同点，在close方法中如果发现已经调用registerShutdownHook在JVM层面上注册了钩子，那么就调用Runtime.getRuntime().removeShutdownHook(this.shutdownHook)移除此钩子，另外这个close的实现来自于closable接口的父接口AutoClosable接口方法，而registerShutdownHook则在PropertyResolver当中被定义。

public void registerShutdownHook() {
		if (this.shutdownHook == null) {
			// No shutdown hook registered yet.
			this.shutdownHook = new Thread() {
				@Override
				public void run() {
					synchronized (startupShutdownMonitor) {
						doClose();
					}
				}
			};
			Runtime.getRuntime().addShutdownHook(this.shutdownHook);
		}
	}

public void close() {
   synchronized (this.startupShutdownMonitor) {
      doClose();
      // If we registered a JVM shutdown hook, we don't need it anymore now:
      // We've already explicitly closed the context.
      if (this.shutdownHook != null) {
         try {
            Runtime.getRuntime().removeShutdownHook(this.shutdownHook);
         }
         catch (IllegalStateException ex) {
            // ignore - VM is already shutting down
         }
      }
   }
}

　时间不早了，今天就写到这里吧。