spring如何解决循环依赖

java 代码解读复制代码protected void beforeSingletonCreation(String beanName) {
  // inCreationCheckExclusions中是否存在当前正在创建的bean
  // 并且singletonsCurrentlyInCreation是否可以添加成功(也就是singletonsCurrentlyInCreation中是否存在正在创建的bean)
   if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
      throw new BeanCurrentlyInCreationException(beanName);
   }
}

java 代码解读复制代码protected void afterSingletonCreation(String beanName) {
   if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
      throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
   }
}

java 代码解读复制代码public class TestA {
    
    private TestB testB;
    
    public TestA(TestB testB){
        this.testB = testB;
    }
}

public class TestB {
    private TestA testA;

    public TestB(TestA testA){
        this.testA = testA;
    }
}

java 代码解读复制代码// ①doGetBean
// 获取bean
// allowEarlyReference是true，表示允许早期依赖
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
  // 从singletonObjects缓存中取
  // singletonObjects中存的是beanName和bean实例
  // singletonObjects是一级缓存
   Object singletonObject = this.singletonObjects.get(beanName);
  // 缓存中没有
   if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
     // 锁定全局变量进行处理
      synchronized (this.singletonObjects) {
        // 如果该bean正在进行加载则不进行处理，直接返回
        // earlySingletonObjects中存的是beanName和bean实例,但是与singletonObjects不同的是,存储的是还没进行属性注入操作的Bean的实例，，目的是为了监测循环引用
        // earlySingletonObjects是二级缓存
         singletonObject = this.earlySingletonObjects.get(beanName);
         if (singletonObject == null && allowEarlyReference) {
           // 从singletonFactories中获取，对于某些方法需要提前进行初始化时则会调用addSingletonFactory方法将对应的ObjectFactory初始化策略存在singletonFactories中
           // singletonFactories中存的是beanName和创建bean的ObjectFactory工厂
           // singletonFactories是三级缓存
            ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
            if (singletonFactory != null) {
              // 调用ObjectFactory的getObject方法
               singletonObject = singletonFactory.getObject();
              // 放入到二级缓存中
              // 记录在缓存earlySingletonObjects中，earlySingletonObjects和singletonFactories互斥，earlySingletonObjects与singletonObjects的不同之处在于当一个单例bean被放入该缓存后，那么其他的bean在创建过程中就能通过getBean方法获取到，目的是用来监测循环引用
               this.earlySingletonObjects.put(beanName, singletonObject);
               this.singletonFactories.remove(beanName);
            }
         }
      }
   }
   return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

// ②doGetBean
// 第一次创建，缓存中不存在，则会进行创建
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
  @Override
  public Object getObject() throws BeansException {
    try {
      // 创建bean
      return createBean(beanName, mbd, args);
    }
    catch (BeansException ex) {
      // Explicitly remove instance from singleton cache: It might have been put there
      // eagerly by the creation process, to allow for circular reference resolution.
      // Also remove any beans that received a temporary reference to the bean.
      destroySingleton(beanName);
      throw ex;
    }
  }
});

// ③doCreateBean
if (instanceWrapper == null) {
  // 创建bean实例
   instanceWrapper = createBeanInstance(beanName, mbd, args);
}

// ④
addSingletonFactory(beanName, new ObjectFactory<Object>() {
				@Override
				public Object getObject() throws BeansException {
					return getEarlyBeanReference(beanName, mbd, bean);
				}
});

// 将ObjectFactory加入到三级缓存中
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);
			}
		}
	}
// ⑤
// 调用applyPropertyValues(beanName, mbd, bw, pvs)来set值
populateBean(beanName, mbd, instanceWrapper);

// ⑥
// 整个bean实例创建并且属性注入后执行
// if (newSingleton) {
//					addSingleton(beanName, singletonObject);
//				}
protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
      // 加入一级缓存
			this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
			this.singletonFactories.remove(beanName);
      // 移除二级缓存
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}

java 代码解读复制代码public class TestA {
    
    private TestB testB;

    public TestB getTestB() {
        return testB;
    }

    public void setTestB(TestB testB) {
        this.testB = testB;
    }
}

public class TestB {
    private TestA testA;

    public TestA getTestA() {
        return testA;
    }

    public void setTestA(TestA testA) {
        this.testA = testA;
    }
}

java 代码解读复制代码// 加入三级缓存
addSingletonFactory(beanName, new ObjectFactory<Object>() {
   @Override
   public Object getObject() throws BeansException {
      return getEarlyBeanReference(beanName, mbd, bean);
   }
});

java 代码解读复制代码protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
   Object exposedObject = bean;
   if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
         if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
            SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
           // 使用后置处理器，这里做了个代理，也就是AOP  (AbstractAutoProxyCreator继承的SmartInstantiationAwareBeanPostProcessor,也就是在getEarlyBeanReference方法中调用的wrapIfNecessary方法)
            exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
            if (exposedObject == null) {
               return null;
            }
         }
      }
   }
   return exposedObject;
}