Spring容器-ApplicationContext的启动过程

Posted 2020-09-23 andypeker

 

转载自： http://blog.163.com/[email protected]/blog/static/118777042009410248557/

这片博客信息量很大，言简意赅、简明扼要地说清楚了Spring容器的启动过程，前面红色的“打比方”可以忽略。。。

 

 

Spring容器像一台构造精妙的机器，我们通过配置文件向机器传达控制信息，机器就能够按照设定的模式进行工作。如果我们将Spring容器比喻为一辆汽车，可以将BeanFactory看成汽车的发动机，而ApplicationContext则是 整辆汽车，它不但包括发动机，还包括离合器、变速器以及底盘、车身、电气设备等其他组件。在ApplicationContext内，各个组件按部就班、有条不紊地完成汽车的各项功能。

ApplicationContext内部封装了一个BeanFactory对象，来实现对容器的操作，初始化完成之后，BeanFactory封装了bean的信息，而ApplicationContext通过访问这个对象获取bean的对象信息（BeanDefinition/Bean对象，都是由BeanFactory实际创建并管理的），为了实现接口的统一，ApplicationContext也实现了一系列的BeanFactory接口(可以说ApplicationContext对BeanFactory对象实现一种代理)。ApplicationContext建立在BeanFactory的基础之上，对配置对象的管理最终还是交于一个DefaultListableBeanFactory来完成（装配地址/访问等），而ApplicationContext在应用这个DefaultListableBeanFactory对象的基础上，不仅实现了BeanFactory接口提供的功能方法，并且黏合了一些面向应用的功能，如资源/国际化支持/框架事件支持等，并且将一些原先需要手动设置到BeanFactory的属性通过配置文件中配置的形式代替（如工厂后处理器BeanPostProcessor/InstantiationAwareBeanPostProcessor）

同样，因为对于BeanDefinition和bean对象的管理是由上下文持有的beanfactory对象完成的，用户不需要拥有这样的接口，因此，ApplicationContext的接口体系中并没有BeanDefinitionRegistry，SingletonBeanRegistry以及AutowireCapableBeanFactory接口(ApplicationContext可以访问一些接口方法在上述接口中也定义，但这些方法提供者为BeanFactory体系中的其他接口，BeanFactory接口体系中的接口之间有重复定义方法的)。

 

内部工作机制(Spring容器ApplicationContext的初始化                                                                                    

（一） 首先来看创建ApplicationContext ，以ClassPathXmlApplicationContext为例：

ApplicationContext = new ClassPathXmlApplicationContext(xmlPath);  

源码如下：

public class ClassPathXmlApplicationContext extends AbstractXmlApplicationContext {

public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
                  this(new String[] {configLocation});
       }

public ClassPathXmlApplicationContext(String[] configLocations) throws BeansException {
                  this(configLocations, (ApplicationContext) null);
       }

//。。。。。。省略几个重载的构造函数

public ClassPathXmlApplicationContext(String[] configLocations, ApplicationContext parent)
                                                                                                                         throws BeansException {

          super(parent);
                 this.configLocations = configLocations;

          //IoC容器的初始化过程，其初始化过程的大致步骤由AbstractApplicationContext来定义   
                 refresh();
       }

关键之处在于refresh方法，此方法继承于ClassPathXmlApplicationContext的间接父类：

public abstract class AbstractApplicationContext extends DefaultResourceLoader
                                                        implements ConfigurableApplicationContext, DisposableBean {

Spring的AbstractApplicationContext是ApplicationContext抽象实现类，该抽象类的refresh()方法定义了Spring容器在加载配置文件后的各项处理过程，这些处理过程清晰刻画了Spring容器启动时所执行的各项操作（创建Spring容器如ClassPathXmlApplicationContext）。下面，我们来看一下refresh()内部定义了哪些执行逻辑：

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();--------（1）

             // 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);-------------------------------------（2）

                   // Register bean processors that intercept bean creation
                          registerBeanPostProcessors(beanFactory);---------------------------------------------（3）

                   // Initialize message source for this context.
                          initMessageSource();-------------------------------------------------------------------------（4）

                   // Initialize event multicaster for this context.
                          initApplicationEventMulticaster();-----------------------------------------------------------（5）

                   // Initialize other special beans in specific context subclasses.
                          onRefresh();------------------------------------------------------------------------------------（6）

                   // Check for listener beans and register them.
                          registerListeners();----------------------------------------------------------------------------（7）

                   // Instantiate singletons this late to allow them to access the message source.
                          beanFactory.preInstantiateSingletons();--------------------------------------------------（8）

                   // Last step: publish corresponding event.
                          publishEvent(new ContextRefreshedEvent(this));---------------------------------------（9）
                    } catch (BeansException ex) {
                          // Destroy already created singletons to avoid dangling resources.
                          beanFactory.destroySingletons();
                          throw ex;
                   }
             }
       }

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
             refreshBeanFactory();
             ConfigurableListableBeanFactory beanFactory = getBeanFactory();

      return beanFactory;
       } 

protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;

public abstract ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;

 

1．初始化BeanFactory：根据配置文件实例化BeanFactory，getBeanFactory()方法由具体子类实现。在这一步里，Spring将配置文件的信息解析成为一个个的BeanDefinition对象并装入到容器的Bean定义注册表（BeanDefinitionRegistry）中，但此时Bean还未初始化；obtainFreshBeanFactory()会调用自身的refreshBeanFactory(),而refreshBeanFactory()方法由子类AbstractRefreshableApplicationContext实现，该方法返回了一个创建的DefaultListableBeanFactory对象，这个对象就是由ApplicationContext管理的BeanFactory容器对象。

这一步的操作相当于，如果我们在自己的应用代码中不用ApplicationContext而直接用BeanFactory时创建BeanFactory对象的操作

核心代码如下：

public abstract class AbstractRefreshableApplicationContext extends AbstractApplicationContext { }

 /** 该ApplicationContext管理的BeanFactory容器对象*/
       private DefaultListableBeanFactory beanFactory;

protected final void refreshBeanFactory() throws BeansException {
              // Shut down previous bean factory, if any.
              ConfigurableListableBeanFactory oldBeanFactory = null;
              synchronized (this.beanFactoryMonitor) {
                      oldBeanFactory = this.beanFactory;
              }
              if (oldBeanFactory != null) {
                      oldBeanFactory.destroySingletons();
                      synchronized (this.beanFactoryMonitor) {
                             this.beanFactory = null;
                      }
              }

       // Initialize fresh bean factory.
              try {

               // 创建容器对象
                      DefaultListableBeanFactory beanFactory = createBeanFactory();

               // Customize the internal bean factory used by this context
                      customizeBeanFactory(beanFactory);

               // 装载配置文件，并传入相关联的BeanFactory对象，作为BeanDefinition的容器
                      loadBeanDefinitions(beanFactory);
                      synchronized (this.beanFactoryMonitor) {
                            this.beanFactory = beanFactory;
                      }
              }  catch (IOException ex) {
                      throw new ApplicationContextException(
                                  "I/O error parsing XML document for application context [" + getDisplayName() + "]", ex);
              }
       }

// 创建Spring默认的容器对象

protected DefaultListableBeanFactory createBeanFactory() {
              return new DefaultListableBeanFactory(getInternalParentBeanFactory());
       }

// 该方法为一个钩子方法，子类可以覆盖它对当前上下文管理的BeanFactory提供客户化操作，也可以忽略

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
       }

// 装载配置文件的方法，需要子类实现

protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
                                                                                                     throws IOException, BeansException;

 

对于上面装载配置文件的方法，由其子类扩展实现：

public abstract class AbstractXmlApplicationContext extends AbstractRefreshableApplicationContext  {}

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws IOException {
             // 使用XMLBeanDefinitionReader来载入bean定义信息的XML文件，传入关联的BeanFactory

      XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

      // 这里配置reader的环境，其中ResourceLoader是我们用来定位bean定义信息资源位置的
             // 因为上下文本身实现了ResourceLoader接口，所以可以直接把上下文作为ResourceLoader传递入

      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);

      // 这里转到定义好的XmlBeanDefinitionReader中对载入bean信息进行处理 
             loadBeanDefinitions(beanDefinitionReader);
       } 

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);
             }
       }

reader.loadBeanDefinitions(configLocations);涉及到XmlBeanDefinitionReader 工具类的使用（以后整理）

2．调用工厂后处理器：根据反射机制从BeanDefinitionRegistry中找出所有BeanFactoryPostProcessor类型的Bean，并调用其postProcessBeanFactory()接口方法；

经过第一步加载配置文件，已经把配置文件中定义的所有bean装载到BeanDefinitionRegistry这个Beanfactory中，对于ApplicationContext应用来说这个BeanDefinitionRegistry类型的BeanFactory就是Spring默认的DefaultListableBeanFactory

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
                                                           implements ConfigurableListableBeanFactory, BeanDefinitionRegistry

在这些被装载的bean中，若有类型为BeanFactoryPostProcessor的bean（配置文件中配置的），则将对应的BeanDefinition生成BeanFactoryPostProcessor对象

容器扫描BeanDefinitionRegistry中的BeanDefinition，使用java反射自动识别出Bean工厂后处理器（实现BeanFactoryPostProcessor接口）的bean，然后调用这些bean工厂后处理器对BeanDefinitionRegistry中的BeanDefinition进行加工处理，可以完成以下两项工作(当然也可以有其他的操作，用户自己定义)：

1 对使用到占位符的<bean>元素标签进行解析，得到最终的配置值，这意味着对一些半成品式的BeanDefinition对象进行加工处理并取得成品的BeanDefinition对象。2 对BeanDefinitionRegistry中的BeanDefinition进行扫描，通过Java反射机制找出所有属性编辑器的Bean（实现java.beans.PropertyEditor接口的Bean），并自动将它们注册到Spring容器的属性编辑器注册表中（PropertyEditorRegistry），这个Spring提供了实现：CustomEditorConfigurer，它实现了BeanFactoryPostProcessor，用它来在此注册自定义属性编辑器；

AbstractApplicationContext中的代码如下：

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
             // Invoke factory processors registered with the context instance.
             for (Iterator it = getBeanFactoryPostProcessors().iterator(); it.hasNext();) {
                   BeanFactoryPostProcessor factoryProcessor = (BeanFactoryPostProcessor) it.next();
                   factoryProcessor.postProcessBeanFactory(beanFactory);
             }

      // Do not initialize FactoryBeans here: We need to leave all regular beans
             // 通过ApplicatinContext管理的beanfactory获取已经注册的BeanFactoryPostProcessor类型的bean的名字

             String[] factoryProcessorNames =
                              beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

      // Separate between BeanFactoryPostProcessors that implement the Ordered
             // interface and those that do not.
             List orderedFactoryProcessors = new ArrayList();
             List nonOrderedFactoryProcessorNames = new ArrayList();
             for (int i = 0; i < factoryProcessorNames.length; i++) {
                   if (isTypeMatch(factoryProcessorNames[i], Ordered.class)) {

                   // 调用beanfactory的getBean取得所有的BeanFactoryPostProcessor对象
                         orderedFactoryProcessors.add(beanFactory.getBean(factoryProcessorNames[i]));
                   }
                   else {
                         nonOrderedFactoryProcessorNames.add(factoryProcessorNames[i]);
                   }
             }

      // First, invoke the BeanFactoryPostProcessors that implement Ordered.
             Collections.sort(orderedFactoryProcessors, new OrderComparator());
             for (Iterator it = orderedFactoryProcessors.iterator(); it.hasNext();) {
                  BeanFactoryPostProcessor factoryProcessor = (BeanFactoryPostProcessor) it.next();

           // 执行BeanFactoryPostProcessor的方法，传入当前持有的beanfactory对象，以获取要操作的 

           // BeanDefinition
                  factoryProcessor.postProcessBeanFactory(beanFactory);
             }
             // Second, invoke all other BeanFactoryPostProcessors, one by one.
             for (Iterator it = nonOrderedFactoryProcessorNames.iterator(); it.hasNext();) {
                  String factoryProcessorName = (String) it.next();
                  ((BeanFactoryPostProcessor) getBean(factoryProcessorName)).

                                                                      postProcessBeanFactory(beanFactory);
             }
       }

 

BeanFactoryPostProcessor接口代码如下，实际的操作由用户扩展并配置（扩展点，如何扩展？）

package org.springframework.beans.factory.config;

public interface BeanFactoryPostProcessor {

     /**
             * Modify the application context‘s internal bean factory after its standard
            */
             void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

 

3．注册Bean后处理器：根据反射机制从BeanDefinitionRegistry中找出所有BeanPostProcessor类型的Bean，并将它们注册到容器Bean后处理器的注册表中；

AbstractApplicatinContext中对应代码如下：

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
              String[] processorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

         // Register BeanPostProcessorChecker that logs an info message when
                int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 +  

                                                                                                                        processorNames.length;
                beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory,  

                                                                                                                beanProcessorTargetCount));
                List orderedProcessors = new ArrayList();
                List nonOrderedProcessorNames = new ArrayList();  

         for (int i = 0; i < processorNames.length; i++) {
                      if (isTypeMatch(processorNames[i], Ordered.class)) {
                           orderedProcessors.add(getBean(processorNames[i]));
                      }
                      else {
                           nonOrderedProcessorNames.add(processorNames[i]);
                      }  
                }

         // First, register the BeanPostProcessors that implement Ordered.
                Collections.sort(orderedProcessors, new OrderComparator());
                for (Iterator it = orderedProcessors.iterator(); it.hasNext();) {

               // 注册bean后处理器，该方法定义于ConfigurableBeanFactory接口
                      beanFactory.addBeanPostProcessor((BeanPostProcessor) it.next());
                }
                // Second, register all other BeanPostProcessors, one by one.
                for (Iterator it = nonOrderedProcessorNames.iterator(); it.hasNext();) {
                      String processorName = (String) it.next();
                      beanFactory.addBeanPostProcessor((BeanPostProcessor) getBean(processorName));
                }
       }

整段代码类似于第三步的调用工厂后处理器，区别之处在于，工厂后处理器在获取后立即调用，而Bean后处理器在获取后注册到上下文持有的beanfactory中，供以后操作调用（在用户获取bean的过程中，对已经完成属性设置工作的Bean进行后续加工，他加工的是bean，而工厂后处理器加工的是BeanDefinition）

BeanPostProcessor 接口代码如下，实际的操作由用户扩展并配置（扩展点，如何扩展？）

package org.springframework.beans.factory.config;

public interface BeanPostProcessor {
             Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
             Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;

}

 

4．初始化消息源：初始化容器的国际化信息资源；

源代码如下：

protected void initMessageSource() {
                // 补充
       }

 

5．初始化应用上下文事件广播器；

（观察者模式中的具体主题角色，持有观察者角色的集合，称为注册表）

AbstractApplciationContext拥有一个applicationEventMulticaster 成员变量，applicationEventMulticaster 提供了容器监听器的注册表，成其为事件广播器。在第七步中将会将事件监听器装入其中

AbstractApplicationContext中的代码如下：

private ApplicationEventMulticaster applicationEventMulticaster;

protected void initApplicationEventMulticaster() {

        // "applicationEventMulticaster"，先看配置文件中有无配置该类型类（用户扩展 扩展点，如何扩展）
               if (containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
                    this.applicationEventMulticaster = (ApplicationEventMulticaster)
                    getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
               }
               else {

              // 若没有，则应用Spring框架提供的事件广播器实例
                     this.applicationEventMulticaster = new SimpleApplicationEventMulticaster();
               }
       }
       public boolean containsLocalBean(String name) {
               return getBeanFactory().containsLocalBean(name);
       }

public Object getBean(String name, Class requiredType) throws BeansException {
              return getBeanFactory().getBean(name, requiredType);
       }

Spring初始化事件广播器，用户可以在配置文件中为容器定义一个自定义的事件广播器（bean的名称要为"applicationEventMulticaster"），只要实现ApplicationEventMulticaster就可以了，Spring在此会根据beanfactory自动获取。如果没有找到外部配置的事件广播器，Spring使用SimpleApplicationEventMulticaster作为事件广播器。

 

6．初始化其他特殊的Bean：这是一个钩子方法，子类可以借助这个钩子方法执行一些特殊的操作：

如AbstractRefreshableWebApplicationContext就使用该钩子方法执行初始化ThemeSource的操作；

protected void onRefresh() throws BeansException {（扩展点，如何扩展）

package org.springframework.context;
            // For subclasses: do nothing by default.
       }

 

7．注册事件监听器；（观察者模式中的观察者角色）

Spring根据上下文持有的beanfactory对象，从它的BeanDefinitionRegistry中找出所有实现org.springfamework.context.ApplicationListener的bean，将BeanDefinition对象生成bean，注册为容器的事件监听器，实际的操作就是将其添加到事件广播器所提供的监听器注册表中

AbstractApplicationContext中的代码如下：

/** Statically specified listeners */

private List applicationListeners = new ArrayList();

public List getApplicationListeners() {
              return this.applicationListeners;
       }

protected void registerListeners() {
              // Register statically specified listeners first.
              for (Iterator it = getApplicationListeners().iterator(); it.hasNext();) {
                     addListener((ApplicationListener) it.next());
              }
              // 获取ApplicationListener类型的所有bean，即事件监听器
              // uninitialized to let post-processors apply to them!
              Collection listenerBeans = getBeansOfType(ApplicationListener.class, true, false).values();
              for (Iterator it = listenerBeans.iterator(); it.hasNext();) {

              // 将事件监听器装入第五步初始化的事件广播器
                     addListener((ApplicationListener) it.next());
              }
       }

public Map getBeansOfType(Class type, boolean includePrototypes, boolean allowEagerInit)
                                                                                                                          throws BeansException {

         return getBeanFactory().getBeansOfType(type, includePrototypes, allowEagerInit);
       }

protected void addListener(ApplicationListener listener) {
                getApplicationEventMulticaster().addApplicationListener(listener);
       }

ApplicationListener 的源代码如下：（扩展点，如何扩展）

package org.springframework.context;

import java.util.EventListener;

/**
        * Interface to be implemented by application event listeners.
        * @see org.springframework.context.event.ApplicationEventMulticaster
       */
       public interface ApplicationListener extends EventListener {
                void onApplicationEvent(ApplicationEvent event);

}

 

8．初始化singleton的Bean：实例化所有singleton的Bean，并将它们放入Spring容器的缓存中；这就是和直接在应用中使用BeanFactory的区别之处，在创建ApplicationContext对象时，不仅创建了一个BeanFactory对象，并且还应用它实例化所有单实例的bean。（在spring的配置文件中，bean默认为单例，除非在bean的配置中显式指定scope="prototype"）

AbstractApplicationContext中的代码如下：

beanFactory.preInstantiateSingletons();

关于BeanFactory体系的代码参照。。。。。。

 

9．发布上下文刷新事件：在此处时容器已经启动完成，发布容器refresh事件（ContextRefreshedEvent）

创建上下文刷新事件，事件广播器负责将些事件广播到每个注册的事件监听器中。

publishEvent(new ContextRefreshedEvent(this));

public void publishEvent(ApplicationEvent event) {
              Assert.notNull(event, "Event must not be null");

       // 在此获取事件广播器，并调用其方法发布事件：调用所有注册的监听器的方法
              getApplicationEventMulticaster().multicastEvent(event);
              if (this.parent != null) {
                    this.parent.publishEvent(event);
              }
       }

 

至此，ApplicationContext对象就完成了初始化工作：创建BeanFactory来装配BeanDefiniton，加工处理BeanDefiniton，注册了bean后处理器，初始化了消息资源，初始化了应用上下文事件广播器，注册了事件监听器，初始化了所有singleton的bean，最后发布上下文刷新事件