BeanDefinition 的合并

• 1.BeanDefinition 有父子关系，有个属性叫parentName,通过parentName可以设置BeanDefinition的父子关系
• 2.为什么要设置BeanDefinition的父子关系，因为子类可以继承父类的属性，抽出共性，跟java的继承有点像
• 3.BeanDefinition 的合并过程就是将父子关系的 BeanDefinition 属性合并起来，如果有相同的就使用子类的属性值 例子：

  
            AnnotationConfigApplicationContext applicationContext
           = new AnnotationConfigApplicationContext(springConfiguration.class);
           //父 RootBeanDefinition 
           RootBeanDefinition rootBeanDefinition = new RootBeanDefinition();
           rootBeanDefinition.setScope(AbstractBeanDefinition.SCOPE_DEFAULT);
           rootBeanDefinition.getPropertyValues().add("name","xiaoming");
           applicationContext.registerBeanDefinition("parent",rootBeanDefinition);
            //子 ChildBeanDefinition ，会继承使用父类已有的属性值，但是如果属性值冲突，不一样
            //那么合并过程中就会使用子类的
           ChildBeanDefinition childBeanDefinition = new ChildBeanDefinition("parent");
           childBeanDefinition.setBeanClass(A.class);
           childBeanDefinition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_NAME);
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  什么时候做合并

  在初始化bean工厂的过程中调用： PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors->DefaultListableBeanFactory.getBeanNamesForType
• DefaultListableBeanFactory.getBeanNamesForType

      public String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
      //....... 
      return doGetBeanNamesForType
      //......
       }
      复制代码
  
 

• DefaultListableBeanFactory#doGetBeanNamesForType

      private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) {
       List<String> result = new ArrayList<>();
      // 遍历所有的 beanDefinitionNames
      for (String beanName : this.beanDefinitionNames) {
      // Only consider bean as eligible if the bean name
      // is not defined as alias for some other bean.
      if (!isAlias(beanName)) {
      try {
      //获取合并的bd
       RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      //.....
       }
      复制代码
  
 

• AbstractBeanFactory#getMergedLocalBeanDefinition

      protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
      // 先尝试从缓存获取bd（就是一个map）
       RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
      if (mbd != null) {
      return mbd;
       }
      //合并bd
      return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
       }
      复制代码
  
 

• AbstractBeanFactory#getMergedBeanDefinition

  
          protected RootBeanDefinition getMergedBeanDefinition(
           String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
          throws BeanDefinitionStoreException {
          synchronized (this.mergedBeanDefinitions) {
           RootBeanDefinition mbd = null;
          // 先从缓存拿合并的bd
          if (containingBd == null) {
           mbd = this.mergedBeanDefinitions.get(beanName);
           }
          //合并的bd为空就进行bd合并
          if (mbd == null) {
          //如果父类为空，就不进行bd合并
          if (bd.getParentName() == null) {
          // Use copy of given root bean definition.
          if (bd instanceof RootBeanDefinition) {
           mbd = ((RootBeanDefinition) bd).cloneBeanDefinition();
           }
          else {
           mbd = new RootBeanDefinition(bd);
           }
           }
          else {
          // 进行bd合并
           BeanDefinition pbd;
          try {
           String parentBeanName = transformedBeanName(bd.getParentName());
          if (!beanName.equals(parentBeanName)) {
          //父类的bd可能也需要合并，所以调用 getMergedBeanDefinition
           pbd = getMergedBeanDefinition(parentBeanName);
           }
          else {
           BeanFactory parent = getParentBeanFactory();
          if (parent instanceof ConfigurableBeanFactory) {
           pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
           }
           }
           }
          //进行bd合并，子类覆盖父类的，如果子类没有，就用父类的属性
           mbd = new RootBeanDefinition(pbd);
           mbd.overrideFrom(bd);
           }
          // Set default singleton scope, if not configured before.
          if (!StringUtils.hasLength(mbd.getScope())) {
           mbd.setScope(RootBeanDefinition.SCOPE_SINGLETON);
           }
           }
          return mbd;
           }
          }
          复制代码
      
     
  

文章来源: javaedge.blog.csdn.net，作者：JavaEdge.，版权归原作者所有，如需转载，请联系作者。

原文链接：javaedge.blog.csdn.net/article/details/108177864