Spring IOC源码跟踪记录-基于XML

what is IOC？

IOC（Inversion of Control ）控制反转，是Spring框架最重要的一个特性，提供了一个装载类的容器来为类之间进行解耦，并提供了一系列的扩展接口，使得开发者能够在bean的生命周期里自定义一些行为操做。

在没有IOC以前，类与类之间的耦合关系是这样的。这儿仅仅只有5个类，类之间的耦合关系就如此复杂，不可思议当类的数量随着业务发展而爆炸增多的时候，耦合关系是多么的糟糕。

在有了IOC以后，类与类之间的耦合关系是这样的。全部类都注册在IOC容器上，全部类只和IOC容器耦合，而且IOC容器为全部管理的类提供生命周期的管理。

how IOC do？

能够看出来IOC容器很像咱们生活中的百货商场，咱们须要什么东西均可以去百货商场买到，在没有百货商场之前咱们买菜须要去菜市场，买手机须要去手机店，买电脑须要到电脑城......极大的方便了获取类的方式。接下来我经过Debug源码的方式来追一追整个IOC启动过程的步骤，但愿能揭开这个“百货商场”的神秘面纱。

这是咱们debug的入口，用的是最基础的XML解析的方式。

ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
HelloService service = (HelloService) context.getBean("service");
service.say();

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
      throws BeansException {
   super(parent); //设置parent父容器
   setConfigLocations(configLocations);//让ioc容器感知到xml配置文件
   if (refresh) {
      refresh();
   }
}

作的第一步是super(parent)，能够看出IOC容器是具有父子关系的，顺便提一下，这个特性在SpringMVC中体现出来了，SpringMVC的容器是 Spring容器的子容器，这样的结果就是Controller（SpringMVC容器中的Bean）能够调用Service（Spring容器中的类），而反过来则不行，这样必定程度保证了类之间的单向关系，以及调用方式不可逆，使得容器更加安全。

public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // 为刷新作好准备。
      prepareRefresh();
      // 告诉子类刷新内部bean工厂。
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
      // 准备beanfactory，为使用ioc容器作好准备
      prepareBeanFactory(beanFactory);
      try {
         // 容许beanfactory准备好后作一些事情，扩展点
         postProcessBeanFactory(beanFactory);
         // 将bean注册在ioc容器
         invokeBeanFactoryPostProcessors(beanFactory);
         // 注册拦截bean建立的bean处理器。
         registerBeanPostProcessors(beanFactory);
         // 初始化上下文消息
         initMessageSource();
         // 为这个上下文初始化事件多播器。
         initApplicationEventMulticaster();
         // 初始化特定上下文子类中的其余特殊bean。
         onRefresh();
         // 检查listener类并注册它们
         registerListeners();
         // 实例化全部的lazy-init单例
         finishBeanFactoryInitialization(beanFactory);
         // 最后一步：发布相应的事件
         finishRefresh();
      }
      catch (BeansException ex) {
         if (logger.isWarnEnabled()) {
            logger.warn("Exception encountered during context initialization - " +
                  "cancelling refresh attempt: " + ex);
         }
         // 销毁已经建立的单例
         destroyBeans();
         // 重置active的值
         cancelRefresh(ex);
         // 将异常传播给调用者
         throw ex;
      }
      finally {
         //清空一些元数据的内存，这些元数据生成了单例模式后就再也用不到了
         resetCommonCaches();
      }
   }
}

这里是ioc容器的诞生点 createBeanFactory()

protected final void refreshBeanFactory() throws BeansException {
......

      DefaultListableBeanFactory beanFactory = createBeanFactory();
......
      loadBeanDefinitions(beanFactory);
......
}

再看看loadBeanDefinitions（）方法 ，这里解释下什么是BeanDefinition （全部的Bean在Spring容器中的数据结构都是BeanDefinition，其中包含了跟这个bean相关的全部信息）beanDefinitionReader能够看作是一个IOC容器bean的生产者，能够从外部环境（xml，注解等）获取到bean的信息并装载进去，这个方法对beanDefinitionReader 设置了ResourceLoader，EnitityResolver等等对解析xml文件很重要的类，继续Debug看。

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
   // Create a new XmlBeanDefinitionReader for the given BeanFactory.
   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);
}

ResourceLoader会去解析xml文件，将每一个xml中的每一个元素都解析而后返回一个DOM的文档树便于后续操做。

接着会执行prepareBeanFactory方法，这个方法相似一个制定ioc规则的方法，让哪些接口的bean不注册进去，哪些接口的bean注册进去

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
   // Tell the internal bean factory to use the context's class loader etc.
   beanFactory.setBeanClassLoader(getClassLoader());
   beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
   beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

   // Configure the bean factory with context callbacks.
   beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
   beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
   beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
   beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
   beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
   beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
   beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

   // BeanFactory interface not registered as resolvable type in a plain factory.
   // MessageSource registered (and found for autowiring) as a bean.
   beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
   beanFactory.registerResolvableDependency(ResourceLoader.class, this);
   beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
   beanFactory.registerResolvableDependency(ApplicationContext.class, this);

   // Register early post-processor for detecting inner beans as ApplicationListeners.
   beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

   // Detect a LoadTimeWeaver and prepare for weaving, if found.
   if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
      beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
      // Set a temporary ClassLoader for type matching.
      beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
   }

   // Register default environment beans.
   if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
      beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
   }
   if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
      beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
   }
   if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
      beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
   }
}

以后会对实现了BeanFactoryPostProcessor接口的类处理，该扩展点容许在（容器已经初始完成，可是bean尚未初始化这部分时间进行扩展），体现了Spring的扩展性。

又看到了一个扩展点，BeanPostProcessor，该扩展点容许在bean 初始化以前或者以后的时候进行扩展

String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

接下来回去预实例化没有设置lazy-init的类

beanFactory.preInstantiateSingletons();

进入这个方法体，在debug已经能够看到本身设置的bean的id值了

能够看到除了类以外还有scope，lazyInit，primary等属性

这儿走了一遍getBean的逻辑，不过由于没有初始化，去到的实例都是null，接着会标记即将建立的bean，并将其缓存

/**
 标记已建立的（或即将建立的）指定bean。
这容许bean工厂优化其缓存重复。
建立指定的bean。
 * @param beanName the name of the bean
 */
protected void markBeanAsCreated(String beanName) {
   if (!this.alreadyCreated.contains(beanName)) {
      synchronized (this.mergedBeanDefinitions) {
         if (!this.alreadyCreated.contains(beanName)) {
            // Let the bean definition get re-merged now that we're actually creating
            // the bean... just in case some of its metadata changed in the meantime.
            clearMergedBeanDefinition(beanName);
            this.alreadyCreated.add(beanName);
         }
      }
   }
}

立刻进入实例化bean的代码，其中createBean（）是重点

// Create bean instance.
if (mbd.isSingleton()) {
   sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
      @Override
      public Object getObject() throws BeansException {
         try {
            return createBean(beanName, mbd, args);  
         }
         catch (BeansException ex) {
            // Explicitly remove instance from singleton cache: It might have been put there
            // eagerly by the creation process, to allow for circular reference resolution.
            // Also remove any beans that received a temporary reference to the bean.
            destroySingleton(beanName);
            throw ex;
         }
      }
   });

追了好几层终于看到，bean是经过反射实例化的。

Class<?> resolvedClass = ClassUtils.forName(className, classLoader);

若是是singleton，直接从map里面取，若是没有则经过反射生成，放进map中而后返回，若是是prototype则每次获取都会实例化，返回一个新的。

if (mbd.isSingleton()) {
   sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
      @Override
      public Object getObject() throws BeansException {
         try {
            return createBean(beanName, mbd, args);
         }
         catch (BeansException ex) {
            // Explicitly remove instance from singleton cache: It might have been put there
            // eagerly by the creation process, to allow for circular reference resolution.
            // Also remove any beans that received a temporary reference to the bean.
            destroySingleton(beanName);
            throw ex;
         }
      }
   });
   bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

else if (mbd.isPrototype()) {
   // It's a prototype -> create a new instance.
   Object prototypeInstance = null;
   try {
      beforePrototypeCreation(beanName);
      prototypeInstance = createBean(beanName, mbd, args);
   }
   finally {
      afterPrototypeCreation(beanName);
   }
   bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}

这就是缓存单例bean的map

private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256);

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
   Object singletonObject = this.singletonObjects.get(beanName);
   if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
         singletonObject = this.earlySingletonObjects.get(beanName);
         if (singletonObject == null && allowEarlyReference) {
            ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
            if (singletonFactory != null) {
               singletonObject = singletonFactory.getObject();
               this.earlySingletonObjects.put(beanName, singletonObject);
               this.singletonFactories.remove(beanName);
            }
         }
      }
   }
   return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

总结

ResourceLoader对xml文档树进行读取解析生成Resource文件，Resource文件封装了对xml文件的IO操做，而后BeanDefinitionReader会对Resource文件读取并生成BeanDefinition放在BeanDefinitionRegistry里面，事后beanFactoryPostProcesser会解析占位符，而后SimpleInstantiationStrategy会经过反射实例化，而后BeanWrapper会对非lazy-init的bean进行赋值，在getBean的时候IOC容器经过Map缓存的方式来达到单例的效果。