BeanDefinition 合併

 

BeanDefinition 的合併

• 1.BeanDefinition 有父子關係，有個屬性叫parentName,經過parentName能夠設置BeanDefinition的父子關係
• 2.爲何要設置BeanDefinition的父子關係，由於子類能夠繼承父類的屬性，抽出共性，跟java的繼承有點像
• 3.BeanDefinition 的合併過程就是將父子關係的 BeanDefinition 屬性合併起來，若是有相同的就使用子類的屬性值 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 ，會繼承使用父類已有的屬性值，可是若是屬性值衝突，不同
     //那麼合併過程當中就會使用子類的
         ChildBeanDefinition childBeanDefinition = new ChildBeanDefinition("parent");
         childBeanDefinition.setBeanClass(A.class);
         childBeanDefinition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_NAME);
        
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  何時作合併

  在初始化bean工廠的過程當中調用： PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors->DefaultListableBeanFactory.getBeanNamesForType
• DefaultListableBeanFactory.getBeanNamesForType

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

private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) {
        List<String> result = new ArrayList<>();

        // 遍歷全部的 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 {
                //獲取合併的bd
                    RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                //.....
    }
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• AbstractBeanFactory#getMergedLocalBeanDefinition

protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
        // 先嚐試從緩存獲取bd（就是一個map）
        RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
        if (mbd != null) {
            return mbd;
        }
        //合併bd
        return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
    }
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• AbstractBeanFactory#getMergedBeanDefinition

  protected RootBeanDefinition getMergedBeanDefinition(
          String beanName, BeanDefinition bd, @Nullable BeanDefinition containingBd)
          throws BeanDefinitionStoreException {
  
      synchronized (this.mergedBeanDefinitions) {
          RootBeanDefinition mbd = null;
  
          // 先從緩存拿合併的bd
          if (containingBd == null) {
              mbd = this.mergedBeanDefinitions.get(beanName);
          }
            //合併的bd爲空就進行bd合併
          if (mbd == null) {
             //若是父類爲空，就不進行bd合併
              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 {
                  // 進行bd合併
                  BeanDefinition pbd;
                  try {
                      String parentBeanName = transformedBeanName(bd.getParentName());
                      if (!beanName.equals(parentBeanName)) {
                         //父類的bd可能也須要合併，因此調用 getMergedBeanDefinition
                          pbd = getMergedBeanDefinition(parentBeanName);
                      }
                      else {
                          BeanFactory parent = getParentBeanFactory();
                          if (parent instanceof ConfigurableBeanFactory) {
                              pbd = ((ConfigurableBeanFactory) parent).getMergedBeanDefinition(parentBeanName);
                          }                       
                      }
                  }
                  //進行bd合併，子類覆蓋父類的，若是子類沒有，就用父類的屬性
                  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;
      }
  }
  
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