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springboot bean循环依赖实现以及源码分析

2021-09-16 12:11wang03 Java教程

最近在使用Springboot做项目的时候,遇到了一个循环依赖的 问题,所以下面这篇文章主要给大家介绍了关于springboot bean循环依赖实现以及源码分析的相关资料,需要的朋友可以参考下

前言

本文基于springboot版本2.5.1

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<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

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import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
 
@Component
public class ServiceA {
    @Autowired
    private ServiceB serviceB;
}
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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

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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

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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

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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

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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

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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);
   }
 
 }
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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;
 }
}
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@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;
 }
 ......
}
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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;
}
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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);
   ......
}
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@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);
 }
 ......
}
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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);
      }
   }
}
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@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);
 }
}
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@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;
 }
}
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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;
 }
}
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@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);
  }
  ......
}
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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

springboot bean循环依赖实现以及源码分析

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

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

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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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原文链接:https://www.cnblogs.com/wbo112/p/14938678.html

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