Spring源碼閱讀（十）循環依賴的處理

循環依賴的處理流程

A和B産生循環依賴，處理過程如下：

源碼解讀

代碼段一

bean被執行個體化後，添加到三級緩存中，移除二級緩存。

// Class: DefaultSingletonBeanRegistry
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
   Assert.notNull(singletonFactory, "Singleton factory must not be null");
   synchronized (this.singletonObjects) {
      if (!this.singletonObjects.containsKey(beanName)) {
         this.singletonFactories.put(beanName, singletonFactory);
         this.earlySingletonObjects.remove(beanName);
         this.registeredSingletons.add(beanName);
      }
   }
}
           

代碼段二

1. 從一級緩存中擷取對象
2. 如果擷取不到，且該對象正在建立過程中，則從二級緩存中取。
3. 如果二級緩存中取不到，則使用三級緩存中的工程方法建立對象。
4. 放入到二級緩存中，并删除三級緩存中的内容。

// Class: DefaultSingletonBeanRegistry
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
   Object singletonObject = this.singletonObjects.get(beanName);
   if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
         singletonObject = this.earlySingletonObjects.get(beanName);
         if (singletonObject == null && allowEarlyReference) {
            ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
            if (singletonFactory != null) {
               singletonObject = singletonFactory.getObject();
               this.earlySingletonObjects.put(beanName, singletonObject);
               this.singletonFactories.remove(beanName);
            }
         }
      }
   }
   return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
           

代碼段三

将建立好的對象添加到一級緩存中，并删除二級和三級緩存。

// DefaultSingletonBeanRegistry
protected void addSingleton(String beanName, Object singletonObject) {
   synchronized (this.singletonObjects) {
      this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
      this.singletonFactories.remove(beanName);
      this.earlySingletonObjects.remove(beanName);
      this.registeredSingletons.add(beanName);
   }
}
           

代碼段四

如果二級緩存中存在對象，則直接使用二級緩存中的對象。

if (earlySingletonExposure) {
   Object earlySingletonReference = getSingleton(beanName, false);
   if (earlySingletonReference != null) {
      if (exposedObject == bean) {
         exposedObject = earlySingletonReference;
      }
      else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
         String[] dependentBeans = getDependentBeans(beanName);
         Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
         for (String dependentBean : dependentBeans) {
            if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
               actualDependentBeans.add(dependentBean);
            }
         }
         if (!actualDependentBeans.isEmpty()) {
            throw new BeanCurrentlyInCreationException(beanName,
                  "Bean with name '" + beanName + "' has been injected into other beans [" +
                  StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                  "] in its raw version as part of a circular reference, but has eventually been " +
                  "wrapped. This means that said other beans do not use the final version of the " +
                  "bean. This is often the result of over-eager type matching - consider using " +
                  "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
         }
      }
   }
}