spring源码分析5----bean的实例化

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前面几篇文章介绍了bean被加载到spring中，但没涉及bean的实例化，只是class文件字节流被加载了，没有new成一个对象。bean定义在xml中形式为，bean定义本身有一些属性，常用的有class，id，property，还有name和scope，id和name是有区别的，id取的名字是唯一的，但是name可以给bean赋多个名字。bean定义相关的属性列表：

https://docs.spring.io/spring/docs/4.2.x/spring-framework-reference/html/beans.html#beans-basics

这些bean的属性在org.springframework.beans.facroty.config包下的BeanDefinition类中有对应的方法和属性。另外还有Primary，注入时优先考虑这个bean。要创建一个bean，就离不开这些属性。

BeanDefinition用map存于DefaultListableBeanFactory中，这个map还用是多线程安全的ConcurrentHashMap。

/** Map of bean definition objects, keyed by bean name. */
private final Map beanDefinitionMap = new ConcurrentHashMap<>(256);

BeanDefinition通过registerBeanDefinition方法加到map中。

@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
      throws BeanDefinitionStoreException {

   Assert.hasText(beanName, "Bean name must not be empty");
   Assert.notNull(beanDefinition, "BeanDefinition must not be null");

   if (beanDefinition instanceof AbstractBeanDefinition) {
      try {
         ((AbstractBeanDefinition) beanDefinition).validate();
      }
      catch (BeanDefinitionValidationException ex) {
         throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
               "Validation of bean definition failed", ex);
      }
   }

   BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
   if (existingDefinition != null) { //如果一个bean定义了两次，而且不允许覆盖，抛异常
      if (!isAllowBeanDefinitionOverriding()) {
         throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
      }
      else if (existingDefinition.getRole() < beanDefinition.getRole()) {
         // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
         if (logger.isInfoEnabled()) {
            logger.info("Overriding user-defined bean definition for bean '" + beanName +
                  "' with a framework-generated bean definition: replacing [" +
                  existingDefinition + "] with [" + beanDefinition + "]");
         }
      }
      else if (!beanDefinition.equals(existingDefinition)) {
         if (logger.isDebugEnabled()) {
            logger.debug("Overriding bean definition for bean '" + beanName +
                  "' with a different definition: replacing [" + existingDefinition +
                  "] with [" + beanDefinition + "]");
         }
      }
      else {
         if (logger.isTraceEnabled()) {
            logger.trace("Overriding bean definition for bean '" + beanName +
                  "' with an equivalent definition: replacing [" + existingDefinition +
                  "] with [" + beanDefinition + "]");
         }
      }
      this.beanDefinitionMap.put(beanName, beanDefinition);//没毛病了，就加到map里
   }
   else {
      if (hasBeanCreationStarted()) {
         // Cannot modify startup-time collection elements anymore (for stable iteration)
         synchronized (this.beanDefinitionMap) {
            this.beanDefinitionMap.put(beanName, beanDefinition);
            List updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
            updatedDefinitions.addAll(this.beanDefinitionNames);
            updatedDefinitions.add(beanName);
            this.beanDefinitionNames = updatedDefinitions;
            removeManualSingletonName(beanName);
         }
      }
      else {
         // Still in startup registration phase
         this.beanDefinitionMap.put(beanName, beanDefinition);
         this.beanDefinitionNames.add(beanName);
         removeManualSingletonName(beanName);
      }
      this.frozenBeanDefinitionNames = null;
   }

   if (existingDefinition != null || containsSingleton(beanName)) {
      resetBeanDefinition(beanName);
   }
}

每次贴代码都希望能够加个代码行功能，不过这是不可能的，因为的定位就不太支持技术宅。registerBeanDefinition是外部调用的，在前面的文章里已经谈过了。创建bean的源材料已经有了，下面再看一下，bean的创建。refresh方法中调用了finishBeanFactoryInitialization。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
   // Initialize conversion service for this context.
   if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
         beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
      beanFactory.setConversionService(
            beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
   }

   // Register a default embedded value resolver if no bean post-processor
   // (such as a PropertyPlaceholderConfigurer bean) registered any before:
   // at this point, primarily for resolution in annotation attribute values.
   if (!beanFactory.hasEmbeddedValueResolver()) {
      beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
   }

   // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
   String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
   for (String weaverAwareName : weaverAwareNames) {
      getBean(weaverAwareName);
   }

   // Stop using the temporary ClassLoader for type matching.
   beanFactory.setTempClassLoader(null);

   // Allow for caching all bean definition metadata, not expecting further changes.
   beanFactory.freezeConfiguration();

   // Instantiate all remaining (non-lazy-init) singletons.
   beanFactory.preInstantiateSingletons(); //由factary完成后面的工作
}

这个preInstantiateSingletons方法，是有一个接口里声明的。

/**
 * Ensure that all non-lazy-init singletons are instantiated, also considering
 * {@link org.springframework.beans.factory.FactoryBean FactoryBeans}.
 * Typically invoked at the end of factory setup, if desired.
 * @throws BeansException if one of the singleton beans could not be created.
 * Note: This may have left the factory with some beans already initialized!
 * Call {@link #destroySingletons()} for full cleanup in this case.
 * @see #destroySingletons()
 */
void preInstantiateSingletons() throws BeansException;

看注释，所有非懒初始化的singletons都在这个方法里实例化。

@Override
public void preInstantiateSingletons() throws BeansException {
   if (logger.isTraceEnabled()) {
      logger.trace("Pre-instantiating singletons in " + this);
   }

   // Iterate over a copy to allow for init methods which in turn register new bean definitions.
   // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
   List beanNames = new ArrayList<>(this.beanDefinitionNames); //取出所有的BeanDefinition

   // Trigger initialization of all non-lazy singleton beans...
   for (String beanName : beanNames) {  //遍历BeanDefinition
      RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
      if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {  //注意这里第二第三个条件判断
         if (isFactoryBean(beanName)) { //factory bean处理
            Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
            if (bean instanceof FactoryBean) {
               final FactoryBean factory = (FactoryBean) bean;
               boolean isEagerInit;
               if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                  isEagerInit = AccessController.doPrivileged((PrivilegedAction)
                              ((SmartFactoryBean) factory)::isEagerInit,
                        getAccessControlContext());
               }
               else {
                  isEagerInit = (factory instanceof SmartFactoryBean &&
                        ((SmartFactoryBean) factory).isEagerInit());
               }
               if (isEagerInit) {
                  getBean(beanName);
               }
            }
         }  //下面是普通bean处理
         else {
            getBean(beanName); //这里创建实例
         }
      }
   }

   // Trigger post-initialization callback for all applicable beans...
   for (String beanName : beanNames) {
      Object singletonInstance = getSingleton(beanName);
      if (singletonInstance instanceof SmartInitializingSingleton) { //实现了SmartInitializingSingleton接口的实例
         final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
         if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction