Spring 源碼分析之AbstractApplicationContext源碼分析

首先我覺得分析ApplicationContext必須從它的實現類開始進行分析，AbstractApplicationContext我覺得是一個不錯的選擇，那我們就從這里開始逐一分析吧，首先我自己手畫了一張圖，作為索引吧，其中藍色的為類，紫色的為接口，箭頭指向的方向是父類或者父接口。
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因為里面接口和方法過多，所以不做展示，下面具體來進行代碼分析。首先我們來看看這句話，MESSAGE_SOURCE_BEAN_NAME。

public static final String MESSAGE_SOURCE_BEAN_NAME = "messageSource";

它這句話翻譯成中文就是消息資源的bean的一個name,我們暫時把它看成一個普通的beanName,我們來看看有哪些地方引用到了這個屬性，首先在initMessageSource方法里面有引用到，我把這些地方標紅顯示了。

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 + "]");}}}

還有一個顯示的地方就是在StaticApplicationContext類中的構造器當中有使用到。下面是StaticApplicationContext的類結構圖：
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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);
}

我們下面再來看一下AbstractApplicationContext這個類的一些Fields,并且來理清一下對象和對象之間的依賴關系，首先是parent -ApplicationContext，我們找到是一個叫做getParent()的方法對這個私有的屬性進行了調用，然后又發現了getParentBeanFactory方法也對其進行了間接調用，因為BeanFactory是ApplicationContext的父接口，如下圖：
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private ApplicationContext parent;
public ApplicationContext getParent() {return this.parent;
}
public BeanFactory getParentBeanFactory() {return getParent();
}

在這個類中還有一個屬性是environment,這個environment在這里也有getter和setter方法，唯一需要注意的是如果調用getEnvironment()方法在environment為空的情況下會創建一個StandardEnvironment對象。

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

StandardEnvironment類繼承了抽象的AbstractEnvironment，它的類結構圖如下所示：
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還有一個比較重要的屬性就是beanFactoryPostProcessors，這事一個ArrayList的數組,Doc當中給出的解釋是當onRefresh的時候有用到。我找到了3個方法引用到了這個屬性，下面都已標紅。

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()));   
}
}

往下面看還有一個屬性active,它是線程安全的，用到了CAS技術。它常常和closed這個屬性一起使用，我們發現，在如下3個地方有組合引用，我已用相應的顏色標識出來。

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");   } }}

我們繼續往下看，有一個startupShutdownMonitor的屬性，字面意思上面理解就是啟動關閉監視器，屬性在這個類當中的命名表示了它所發揮的作用，我們來看一下有哪些方法引用到了這個屬性。大家不知道發現沒有，還有一個“很像”的屬性在這里就是shutdownHook,這個和startupShutdownMonitor是配合在一起使用的。shudownHook在這里是一個線程類型的屬性。

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，那么它們之間的關系我們得分析一下,hook顧名思義鉤子，說簡單點這個就是一個鉤子，也算是一個擴展點。我們來仔細分析一下它的幾個方法，首先是registerShutdownHook方法：這個方法有一句話特別重要，就是Runtime.getRuntime().addShutdownHook(this.shutdownHook);它實際上在系統層面上把鉤子線程添加到了JVM虛擬機。在鉤子運行的時候，就會執行doClose方法關閉并銷毀applicationContext。需要注意的一點是明白registerShutdownHook方法和close方法的不同點，在close方法中如果發現已經調用registerShutdownHook在JVM層面上注冊了鉤子，那么就調用Runtime.getRuntime().removeShutdownHook(this.shutdownHook)移除此鉤子，另外這個close的實現來自于closable接口的父接口AutoClosable接口方法，而registerShutdownHook則在PropertyResolver當中被定義。

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

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}}}
}