循环依赖

spring中将循环依赖处理分为了两种情况

构造器循环依赖

使用构造器注入构成循环依赖，这种方式无法进行解决，抛出了BeanCurrentlyInCreationException异常

在创建bean之前会进行检测

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);
   }
}

在创建bean之后会进行移除

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

以TestA和TestB为例，由于构造器造成了循环依赖，所以在进行实例化TestA时(实例化TestA之前执行了beforeSingletonCreation方法，此时singletonsCurrentlyInCreation中添加TestA)，需要TestB实例，所以需要实例化TestB(实例化TestB之前执行了beforeSingletonCreation方法，此时singletonsCurrentlyInCreation中添加TestB)，然后在进行TestB时需要TestA实例，又需要进行实例化TestA，但是执行beforeSingletonCreation方法时，singletonsCurrentlyInCreation中已存在TestA，所以会抛出BeanCurrentlyInCreationException异常

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;
    }
}

setter循环依赖

对于setter注入造成的循环依赖，spring采用的是提前暴露刚完成的构造器实例化但未完成setter方法注入的bean来实现的，而且只能解决单例作用域的bean

// 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
               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);
				}
});

// 将实例化的bean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);
		}
	}

以TestA和TestB为例，创建单例TestA时，使用无参构造器进行创建，并将实例化的bean放入一个ObjectFactory中，存入到singletonFactories，用于返回提前暴露的bean，然后进行setter方法来注入TestB，此时还没有TestB，来创建单例TestB，使用无参构造器进行创建，并将实例化的bean放入一个ObjectFactory中，存入到singletonFactories，用于返回提前暴露的bean，然后进行setter来注入TestA，从ObjectFactory.getObject可以获取到testA，完成了TestB#setTestA方法，之后继续走TestA的逻辑，从而完成TestA#setTestB方法

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;
    }
}