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);
        
  复制代码
  

  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 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