springboot bean循环依赖实现以及源码分析

前言

本文基于springboot版本2.5.1

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.1</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

本文主要聚焦在循环依赖部分，主要用单例bean来进行讲解，其他bean实现的流程不会过多涉及。

1、什么叫循环依赖呢

简单来说就是springboot容器中的多个bean，如A、B两个bean，A有属性B需要注入，B有属性A需要注入，形成相互依赖的情况。

看下代码，就是类似下面这种情况

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class ServiceA {
    @Autowired
    private ServiceB serviceB;
}

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class ServiceB {
    @Autowired
    private ServiceA serviceA;
}

上面有两个bean，分别是ServiceA,ServiceB。ServiceA中需要注入ServiceB的实例，ServiceB中需要注入ServiceA的实例，这就是一种典型的循环依赖，其他还有方法参数循环依赖的场景等等，但是它们的内部实现基本是一样的。

2、具体出现循环依赖的代码逻辑

获取bean的方法

在springboot中默认的beanFactory是DefaultListableBeanFactory，在我们获取bean对象的时候，如果bean对象存在就直接返回，如果不存在，就先创建bean对象再返回。

我们先看下我们获取bean的常用方法都有哪些

public <T> T getBean(Class<T> requiredType) throws BeansException
public Object getBean(String name) throws BeansException
public <T> Map<String, T> getBeansOfType(@Nullable Class<T> type) throws BeansException
public Map<String, Object> getBeansWithAnnotation(Class<? extends Annotation> annotationType)
public void preInstantiateSingletons() throws BeansException

常用的获取bean的方法主要有上面几个和它们的重载版本，对于第3行、第4行、第5行最终都会调用到第2行的方法来获取bean。而它也会通过调用doGetBean(在AbstractBeanFactory这个类中)来获取bean

 public Object getBean(String name) throws BeansException {
  return doGetBean(name, null, null, false);
 }

第1行的方法也会调用doGetBean来获取bean

 public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
   throws BeansException {

  return doGetBean(name, requiredType, args, false);
 }

所有最终获取bean的方法都是

 protected <T> T doGetBean(
   String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
   throws BeansException {

这个方法，这个方法是protected的，是不对外提供的。所以我们不能直接调用它，只能通过上面提供的5个方法来获取bean对象。

下面我们从doGetBean这里来看下serviceA创建的过程

 protected <T> T doGetBean(
   String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
   throws BeansException {
   //如果bean之前存在，这里返回的shareInstance就是非空，就会从后面的if分支中返回，如果bean之前不存在，就会执行后面的bean创建及注入属性的过程
   Object sharedInstance = getSingleton(beanName);
   if (sharedInstance != null && args == null) {
   ......
   //如果当前不只是检查，而且是创建bean，这个参数就是false，在这里就会做个bean创建的标记，把beanName 加到alreadyCreated里面去
   if (!typeCheckOnly) {
    markBeanAsCreated(beanName);
   }
    //我们当前要创建的bean是单例的，就会走到这里去，下面我们走到里面的调用去看看
    // Create bean instance.
    if (mbd.isSingleton()) {
     sharedInstance = getSingleton(beanName, () -> {
      try {
       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;
      }
     });
     beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
    }

  }

 public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
  Assert.notNull(beanName, "Bean name must not be null");
  synchronized (this.singletonObjects) {
                ......
                //这里会把当前bean的名字加入到当前正在创建的单例对象集合singletonsCurrentlyInCreation中
    beforeSingletonCreation(beanName);
    ......
    try {
                    //这里就是调用上面的return createBean(beanName, mbd, args);这个方法，我们进这里面去看看
     singletonObject = singletonFactory.getObject();
     newSingleton = true;
    }
    ......
   }
   return singletonObject;
  }
 }

 @Override
 protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
   throws BeanCreationException {
  ......
  // Make sure bean class is actually resolved at this point, and
  // clone the bean definition in case of a dynamically resolved Class
  // which cannot be stored in the shared merged bean definition.
        //在这里获取要创建的bean的class对象
  Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
  ......
  try {
            //调用这里来创建，我们再走到这里面去看看
            //3个参数分别为
            //1、beanName  bean对象的名字
            //2、mbdToUseRootBeanDefinition对象，可以认为就是bean的元数据信息，包含bean的类对象，bean的类上注解，bean实际位置路径等等
            //3、args  bean对象的构造方法的实参，这里一般是空的
   Object beanInstance = doCreateBean(beanName, mbdToUse, args);
   if (logger.isTraceEnabled()) {
    logger.trace("Finished creating instance of bean '" + beanName + "'");
   }
   return beanInstance;
  }
  ......
 }

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {

   ......
   //真正创建bean对象是在这里，这里返回的instanceWrapper是bean对象的类实例的包装对象BeanWrapper
   if (instanceWrapper == null) {
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   //这里的bean就是实际创建的bean对象的类实例
   Object bean = instanceWrapper.getWrappedInstance();
   Class<?> beanType = instanceWrapper.getWrappedClass();
   if (beanType != NullBean.class) {
      mbd.resolvedTargetType = beanType;
   }
 ......
   // Eagerly cache singletons to be able to resolve circular references
   // even when triggered by lifecycle interfaces like BeanFactoryAware.
   //看上面的注释大概也能明白， 大概意思就是早期的单例缓存，为了解决由 BeanFactoryAware等等触发的循环依赖
   //mbd.isSingleton()  表示bean是单例的(这个是bean对应的类上的，默认就是单例)，
   //this.allowCircularReferences 允许循环引用,这个是beanFactory的成员属性，默认也是true
   //isSingletonCurrentlyInCreation(beanName) 表示是否在当前正在创建的bean集合中。beforeSingletonCreation(beanName);我们在前面执行过这句就加到正在创建的bean集合中了
   //这里earlySingletonExposure 就是true了，会进到if分支中
   boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
         isSingletonCurrentlyInCreation(beanName));
   if (earlySingletonExposure) {
      if (logger.isTraceEnabled()) {
         logger.trace("Eagerly caching bean '" + beanName +
               "' to allow for resolving potential circular references");
      }
      //这句主要是将将() -> getEarlyBeanReference(beanName, mbd, bean) 这个lambda表达式存储到this.singletonFactories集合中
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
   }

   // Initialize the bean instance.
   Object exposedObject = bean;
   try {
      //在这里就会进行属性填充，完成成员注入等等，也就是在这里serviceA这个bean会注入serviceB这个成员属性,我们走进这个方法去看看
      populateBean(beanName, mbd, instanceWrapper);
      ......
   }
  ......

   return exposedObject;
}

 protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
  ......
  if (hasInstAwareBpps) {
   if (pvs == null) {
    pvs = mbd.getPropertyValues();
   }
   //真正的属性注入是在这里完成的，aop也是在这里来完成的。这里是获取beanFactory中的InstantiationAwareBeanPostProcessor对bean对象进行增强
   //如果属性注入用的是@Resource，就会用CommonAnnotationBeanPostProcessor来完成
   //如果属性注入用的是@Autowired,就会用AutowiredAnnotationBeanPostProcessor来完成
   //如果是AOP 就会使用InfrastructureAdvisorAutoProxyCreator来生成对应的代理对象
   //我们这里使用的是@Autowired,所以会用AutowiredAnnotationBeanPostProcessor来完成注入。我们走到它的postProcessProperties的去看看
   for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
    PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
    ......
 }

 @Override
 public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
        //这里主要是获取bean的类属性和方法上的org.springframework.beans.factory.annotation.Autowired,org.springframework.beans.factory.annotation.Value注解来进行注入
  InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
  try {
            //继续进去看看
   metadata.inject(bean, beanName, pvs);
  }
  ......
 }

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
   ......
   //对每一个属性分别进行注入，继续进去
         element.inject(target, beanName, pvs);
      }
   }
}

    @Override
    protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
     Field field = (Field) this.member;
     Object value;
     //如果之前缓存过就从缓存取，我们是第一次注入，所以之前没有缓存，不会走这个分支
     if (this.cached) {
      try {
       value = resolvedCachedArgument(beanName, this.cachedFieldValue);
      }
      catch (NoSuchBeanDefinitionException ex) {
       // Unexpected removal of target bean for cached argument -> re-resolve
       value = resolveFieldValue(field, bean, beanName);
      }
     }
     else {
      //会走这里来解析字段的值，再进去
      value = resolveFieldValue(field, bean, beanName);
     }
     if (value != null) {
      ReflectionUtils.makeAccessible(field);
      field.set(bean, value);
     }
    }

  @Nullable
  private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
   //创建字段的包装类DependencyDescriptor
   DependencyDescriptor desc = new DependencyDescriptor(field, this.required);

   try {
    //调用这里完成对应字段值的查找,再进去
    value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
   }
   catch (BeansException ex) {
    throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
   }
   synchronized (this) {
    //获取到值之后，进行缓存
    if (!this.cached) {
      ......
     }
     this.cachedFieldValue = cachedFieldValue;
     this.cached = true;
    }
   }
   return value;
  }
 }

 public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
   @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

  descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
  if (Optional.class == descriptor.getDependencyType()) {
   return createOptionalDependency(descriptor, requestingBeanName);
  }
  else if (ObjectFactory.class == descriptor.getDependencyType() ||
    ObjectProvider.class == descriptor.getDependencyType()) {
   return new DependencyObjectProvider(descriptor, requestingBeanName);
  }
  else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
   return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
  }
  else {
   //当前的类是一个普通的class，会走到这里面，由于我们的bean没有Lazy注解，所以这里返回时null,走到下面的if分支
   Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
     descriptor, requestingBeanName);
   if (result == null) {
    //在这里我们看下这里的入参。
    //descriptor是包含了需要注入的字段的信息。
    //requestingBeanName是当前正在创建的bean的名字serviceA,
    //autowiredBeanNames是当前需要注入的字段的对应的bean的名字的集合，这里只有serviceB
    //typeConverter这个是进行注入时做类型转换的，这里我们可以不用关注这个
    result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
   }
   return result;
  }
 }

 @Nullable
 public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
   @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
   ......
   if (instanceCandidate instanceof Class) {
    //又会调用到这里,我们再进入到DependencyDescriptor的resolveCandidate去看看
                //注意：这里的autowiredBeanName是我们需要注入的属性名这里是serviceB
    instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
   }
   ......
 }

 public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
   throws BeansException {
  //看到没，到这里就出现循环调用了，到这里又会重新调用beanFactory.getBean("serviceB")去创建serviceB的bean对象，完成后注入到serivceA对应的Bean上的属性上来，这时代码又会从本节开头的位置开始执行，先创建serviceB对象实例，再去注入serviceB对象的serviceA属性。
        //最终会执行到beanFactory.getBean("serviceA")这里
  return beanFactory.getBean(beanName);
 }

就是下面图的样子

3、解决循环依赖的代码实现

接着上面的beanFactory.getBean("serviceA")这行代码我们继续往下看

这次又会走到这里

 protected <T> T doGetBean(
   String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
   throws BeansException {
  //我们第二部分就是从这里开始的，又走回来了，但这次又会有所不同
  String beanName = transformedBeanName(name);
  Object beanInstance;

  // Eagerly check singleton cache for manually registered singletons.
  //这次我们这里返回的就不是空了，sharedInstance对象的值就是对应serviceA的bean对象了，这次就会从if分支中返回，而之前我们不会进这里的if分支而是进入else分支导致后面出现了循环依赖的问题，这次我们进到这个方法看看
  Object sharedInstance = getSingleton(beanName);
  if (sharedInstance != null && args == null) {
   if (logger.isTraceEnabled()) {
    if (isSingletonCurrentlyInCreation(beanName)) {
     logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
       "' that is not fully initialized yet - a consequence of a circular reference");
    }
    else {
     logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
    }
   }
   beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
  }

 @Nullable
 public Object getSingleton(String beanName) {
  //再点进去
  return getSingleton(beanName, true);
 }

 protected Object getSingleton(String beanName, boolean allowEarlyReference) {
  // Quick check for existing instance without full singleton lock
  Object singletonObject = this.singletonObjects.get(beanName);
         //这里由于当前的serviceA bean还没完成创建，所以这里singletonObject返回的是空，
        //再看看 isSingletonCurrentlyInCreation(beanName)这里，由于我们在创建serviceA过程中有这么一句beforeSingletonCreation(beanName)(不清楚这句的搜索下本文，上面就有讲到)，所有这个条件是true。这时我们就会进入if分支中
  if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
   singletonObject = this.earlySingletonObjects.get(beanName);
           //由于我们是第一次进入这里，所以this.earlySingletonObjects.get(beanName)返回的也是null
           //我们的入参 allowEarlyReference是true,会继续进到这个if分支中
   if (singletonObject == null && allowEarlyReference) {
    synchronized (this.singletonObjects) {
     // Consistent creation of early reference within full singleton lock
     singletonObject = this.singletonObjects.get(beanName);
                    //这里的singletonObject还是null,继续进到if分支
     if (singletonObject == null) {
      singletonObject = this.earlySingletonObjects.get(beanName);
      if (singletonObject == null) {
                            //最终会走到这里，在创建serviceA对象之后，属性注入之前，执行了这句 addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean))(不清楚的搜索下本文，上面有说到)，所以这里返回的singletonFactory是个lamdba表达式，getEarlyBeanReference(beanName, mbd, bean))附带了3个参数，第一个beanName是serivceA,mdb是对应serviceA的附带serviceA元数据信息的RootBeanDefinition对象，bean就是创建出来的serviceA对象
       ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
       if (singletonFactory != null) {
                                //这里就会调用getEarlyBeanReference(beanName, mbd, bean)对serviceA对象进行一个getEarlyBeanReference增强后返回，返回后放置到earlySingletonObjects中，并从singletonFactories中删除
        singletonObject = singletonFactory.getObject();
        this.earlySingletonObjects中，并从.put(beanName, singletonObject);
        this.singletonFactories.remove(beanName);
       }
      }
     }
    }
   }
  }
  return singletonObject;
 }

最终在serviceA 这个bean创建完成后，就会从singletonsCurrentlyInCreation移除掉

 public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    ......
    finally {
     //在这里从singletonsCurrentlyInCreation中移除掉
     afterSingletonCreation(beanName);
    }
    if (newSingleton) {
     //将serviceA bean对象添加到singletonObjects，registeredSingletons中
     //从singletonFactories,earlySingletonObjects中移除掉
     addSingleton(beanName, singletonObject);
    }
   }
   return singletonObject;
  }
 }

所以整个获取serviceA的流程就是这样了，

1、首先去创建serviceA这个bean，

• 由于它有个属性serviceB,在创建完serviceA对象后，就会去进行serviceB的属性注入，
• 这时由于serviceB之前没有生成，这时又会去创建serviceB这个bean，
• 先创建serviceB对象，然后再进行serviceA这个属性的注入，
• 继续去获取serviceA这个bean，第二次进入获取serviceA的流程，这时从之前缓存的lambda表达式中获取到之前创建的serviceA的引用返回。

2、总结下关键的代码点

• 创建bean对象之前调用beforeSingletonCreation(beanName)将bean对象名字添加到singletonsCurrentlyInCreation集合中
• 创建bean对象对应的类实例后调用addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));添加到singletonFactories中
• 在循环依赖中第二次调用到创建bean对象时，调用getSingleton(beanName, true)时，从singletonFactories中返回对应的早期bean对象的引用,并添加到earlySingletonObjects中

总结

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