SpringloC容器的依赖注入源码解析（3）—— doGetBean之剩余逻辑

doGetBean源码：

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

   // 先将方法传入的name转换成容器真实的beanName
   // 之前可以通过三种形式获取beanName
   // 一个是原始的beanName，一个是加了&的，一个是别名
   final String beanName = transformedBeanName(name);
   Object bean;

   // Eagerly check singleton cache for manually registered singletons.
   Object sharedInstance = getSingleton(beanName);
   // 加果先前已经创建过单例Bean的实例，并且调用的getBean方法传入的参数为空，则执行理面的逻辑
   // args之所以要求为空是因为如果有args，则需要做进一步赋值，因此无法直接返回
   if (sharedInstance != null && args == null) {
      if (logger.isTraceEnabled()) {
         // 如果Bean还在创建中，则说明是循环引用
         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 + "'");
         }
      }
      // 如果是普通bean，直接返回，如果是FactoryBean，则返回他的getObject
      bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
   }

   else {
      // Fail if we're already creating this bean instance:
      // We're assumably within a circular reference.
      if (isPrototypeCurrentlyInCreation(beanName)) {
         throw new BeanCurrentlyInCreationException(beanName);
      }

      // Check if bean definition exists in this factory.
      BeanFactory parentBeanFactory = getParentBeanFactory();
      if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
         // Not found -> check parent.
         String nameToLookup = originalBeanName(name);
         if (parentBeanFactory instanceof AbstractBeanFactory) {
            return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                  nameToLookup, requiredType, args, typeCheckOnly);
         }
         else if (args != null) {
            // Delegation to parent with explicit args.
            return (T) parentBeanFactory.getBean(nameToLookup, args);
         }
         else if (requiredType != null) {
            // No args -> delegate to standard getBean method.
            return parentBeanFactory.getBean(nameToLookup, requiredType);
         }
         else {
            return (T) parentBeanFactory.getBean(nameToLookup);
         }
      }

      if (!typeCheckOnly) {
         markBeanAsCreated(beanName);
      }

      try {
         final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
         checkMergedBeanDefinition(mbd, beanName, args);

         // Guarantee initialization of beans that the current bean depends on.
         String[] dependsOn = mbd.getDependsOn();
         if (dependsOn != null) {
            for (String dep : dependsOn) {
               if (isDependent(beanName, dep)) {
                  throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
               }
               registerDependentBean(dep, beanName);
               try {
                  getBean(dep);
               }
               catch (NoSuchBeanDefinitionException ex) {
                  throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
               }
            }
         }

         // Create bean instance.
         // 如果BeanDefinition为单例
         if (mbd.isSingleton()) {
            // 这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
            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;
               }
            });
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
         }

         else if (mbd.isPrototype()) {
            // It's a prototype -> create a new instance.
            Object prototypeInstance = null;
            try {
               beforePrototypeCreation(beanName);
               prototypeInstance = createBean(beanName, mbd, args);
            }
            finally {
               afterPrototypeCreation(beanName);
            }
            bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
         }

         else {
            String scopeName = mbd.getScope();
            final Scope scope = this.scopes.get(scopeName);
            if (scope == null) {
               throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
            }
            try {
               Object scopedInstance = scope.get(beanName, () -> {
                  beforePrototypeCreation(beanName);
                  try {
                     return createBean(beanName, mbd, args);
                  }
                  finally {
                     afterPrototypeCreation(beanName);
                  }
               });
               bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
            }
            catch (IllegalStateException ex) {
               throw new BeanCreationException(beanName,
                     "Scope '" + scopeName + "' is not active for the current thread; consider " +
                     "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                     ex);
            }
         }
      }
      catch (BeansException ex) {
         cleanupAfterBeanCreationFailure(beanName);
         throw ex;
      }
   }

   // Check if required type matches the type of the actual bean instance.
   if (requiredType != null && !requiredType.isInstance(bean)) {
      try {
         T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
         if (convertedBean == null) {
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
         }
         return convertedBean;
      }
      catch (TypeMismatchException ex) {
         if (logger.isTraceEnabled()) {
            logger.trace("Failed to convert bean '" + name + "' to required type '" +
                  ClassUtils.getQualifiedName(requiredType) + "'", ex);
         }
         throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
      }
   }
   return (T) bean;
}
复制代码

之前讨论了doGetBean里面从缓存获取bean的代码，doGetBean方法接下来的else处理Bean的scope为prototype或者单例模式但是缓存中还不存在bean的情况： Spring同样为scope为prototype的Bean设计了一个缓存列表

if (isPrototypeCurrentlyInCreation(beanName)) {
   throw new BeanCurrentlyInCreationException(beanName);
}
复制代码

protected boolean isPrototypeCurrentlyInCreation(String beanName) {
   Object curVal = this.prototypesCurrentlyInCreation.get();
   return (curVal != null &&
         (curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName))));
}
复制代码

相比单例的set，这里是ThreadLocal，只记录当前线程创建出来的scope为prototype的Bean，上面的if如果是true的话证明有循环依赖。

通过了循环依赖校验之后看容器是否存在父容器，如果存在且当前容器里没有包含此bean的BeanDefinition实例，尝试去从父类容器递归查询

为了防止之前的beanName已经被转换的不成样子，将&重新加上，再调用父类的doGetBean或者getBean方法，如果父类是AbstractBeanFactory，则调用doGetBean

如果当前容器里包含了此bean的BeanDefinition实例则继续执行

// typeCheckOnly 是用来判断调用 getBean() 是否仅仅是为了类型检查获取 bean，而不是为了创建Bean
if (!typeCheckOnly) {
   // 如果不是仅仅做类型检查则是创建bean
   markBeanAsCreated(beanName);
}
复制代码

进入markBeanAsCreated方法里，

protected void markBeanAsCreated(String beanName) {
   // 双重检查锁机制
   if (!this.alreadyCreated.contains(beanName)) {
      synchronized (this.mergedBeanDefinitions) {
         if (!this.alreadyCreated.contains(beanName)) {
            // Let the bean definition get re-merged now that we're actually creating
            // the bean... just in case some of its metadata changed in the meantime.
            // 将原先合并之后的RootBeanDefinition的需要重新合并的状态设置为true
            // 表示需要重新合并一遍，以防原数据的改动
            clearMergedBeanDefinition(beanName);
            // 将已经创建好的或者正在创建的Bean的名称加到alreadyCreated这个缓存中
            this.alreadyCreated.add(beanName);
         }
      }
   }
}
复制代码

有一个双重锁检查机制，创建的Bean的名称加到alreadyCreated（类型Set）这个缓存中，在加入缓存之前需要将原先的MergedBeanDefinition设置上一个需要清除的标识符，目的是让后续从容器中获取BeanDefinition时重新合并子类和父类的BeanDefinition，这样就可以防止元数据被改动后，BeanDefinition还是按照原来的数据去创建

protected void clearMergedBeanDefinition(String beanName) {
   RootBeanDefinition bd = this.mergedBeanDefinitions.get(beanName);
   if (bd != null) {
      bd.stale = true;
   }
}
复制代码

clearMergedBeanDefinition回去容器中获取RootBeanDefinition实例，然后把该实例需要重新合并的状态设为true（之前提到过，只要指定了parent属性，则两个BeanDefinition就合并成一个来使用）

---

回到doGetBean，接下来会调用getMergedLocalBeanDefinition方法来合并子类和父类的BeanDefinition，进入到该方法里：

protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
   // Quick check on the concurrent map first, with minimal locking.
   RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
   if (mbd != null && !mbd.stale) {
      return mbd;
   }
   return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
}
复制代码

先从mergedBeanDefinitions缓存里获取之前已经合并好的RootBeanDefinition实例，如果stale为true的话就会合并一遍BeanDefinition，随后返回。

---

回到doGetBean，获取到了BeanDefinition之后，就去对相关实例做合法性校验

checkMergedBeanDefinition(mbd, beanName, args);
复制代码

进入到checkMergedBeanDefinition方法里：

protected void checkMergedBeanDefinition(RootBeanDefinition mbd, String beanName, @Nullable Object[] args)
      throws BeanDefinitionStoreException {

   if (mbd.isAbstract()) {
      throw new BeanIsAbstractException(beanName);
   }
}
复制代码

看一下RootBeanDefinition是否是抽象的。

---

回到doGetBean，尝试从BeanDefinition里获取显式的依赖关系，如果有depends-on的话就检查是否有循环依赖关系 如果没有循环依赖的话则会将相关的依赖关系注册上

registerDependentBean(dep, beanName);
复制代码

public void registerDependentBean(String beanName, String dependentBeanName) {
   String canonicalName = canonicalName(beanName);

   synchronized (this.dependentBeanMap) {
      // computeIfAbsent：若key对应的value为空，会将第二个参数的返回值存入并返回
      // dependentBeanMap中存放着当前Bean被引用的Bean的集合
      // 比如当前需要实例化的是Bean的名字是userInfo，userInfo中有个Human类型的属性human
      // 那么就有human被userInfo引用的关系 human=[userInfo]
      Set<String> dependentBeans =
            this.dependentBeanMap.computeIfAbsent(canonicalName, k -> new LinkedHashSet<>(8));
      if (!dependentBeans.add(dependentBeanName)) {
         return;
      }
   }

   synchronized (this.dependenciesForBeanMap) {
      Set<String> dependenciesForBean =
            this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName, k -> new LinkedHashSet<>(8));
      dependenciesForBean.add(canonicalName);
   }
}
复制代码

先获取name，之后就进入两重注册，第一重注册往dependentBeanMap中写入键值对，key是被依赖的Bean名字，value是依赖他的Bean名字列表

第二重注册往dependenciesForBeanMap中写入键值对，键值对和上面正好是相反的

---

回到doGetBean，执行完显式依赖关系注册之后就会递归调用getBean(dep)来将依赖的bean创建出来，往后就是根据不同的scope进行不同的创建bean的操作了，分为Singleton、Prototype和其他。

除了Prototype是直接调用createBean(beanName, mbd, args)去创建Bean实例之外，scope=其他的将createBean封装到了一个匿名参数里 进入scope.get()方法

Object get(String name, ObjectFactory<?> objectFactory);
复制代码

可见后面的匿名函数实现的是ObjectFactory的getObject方法，调用createBean方法去创建适配scope的实例。

下面主要分析scope = singleton的：

if (mbd.isSingleton()) {
   // 这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
   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.
         // 显式从单例缓存中删除 bean实例
         // 因为单例模式下为了解决循环依赖，可能它已经存在了，所以将其销毁
         destroySingleton(beanName);
         throw ex;
      }
   });
   bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
复制代码

进入getSingleton方法： 也是接收一个ObjectFactory对象然后实现其getObject方法。

进入createBean：

protected abstract Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException;
复制代码

这是个抽象方法需要子类去实现。

再回到上面的getSingleton方法：

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
   Assert.notNull(beanName, "Bean name must not be null");
   synchronized (this.singletonObjects) {
      Object singletonObject = this.singletonObjects.get(beanName);
      if (singletonObject == null) {
         if (this.singletonsCurrentlyInDestruction) {
            throw new BeanCreationNotAllowedException(beanName,
                  "Singleton bean creation not allowed while singletons of this factory are in destruction " +
                  "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
         }
         if (logger.isDebugEnabled()) {
            logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
         }
         beforeSingletonCreation(beanName);
         boolean newSingleton = false;
         boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
         if (recordSuppressedExceptions) {
            this.suppressedExceptions = new LinkedHashSet<>();
         }
         try {
            singletonObject = singletonFactory.getObject();
            newSingleton = true;
         }
         catch (IllegalStateException ex) {
            // Has the singleton object implicitly appeared in the meantime ->
            // if yes, proceed with it since the exception indicates that state.
            singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
               throw ex;
            }
         }
         catch (BeanCreationException ex) {
            if (recordSuppressedExceptions) {
               for (Exception suppressedException : this.suppressedExceptions) {
                  ex.addRelatedCause(suppressedException);
               }
            }
            throw ex;
         }
         finally {
            if (recordSuppressedExceptions) {
               this.suppressedExceptions = null;
            }
            afterSingletonCreation(beanName);
         }
         if (newSingleton) {
            addSingleton(beanName, singletonObject);
         }
      }
      return singletonObject;
   }
}
复制代码

先将一级缓存singletonObjects上锁，获取到锁之后再次尝试从一级缓存里去获取实例，以防别的线程创建好了，如果获取不到实例就开始做真正的创建了：

先看一下目前容器是否正在销毁所有的单例： singletonsCurrentlyInDestruction是个flag。

通过验证之后会来到beforeSingletonCreation(beanName)，进入：

protected void beforeSingletonCreation(String beanName) {
    //inCreationCheckExclusions 直接缓存当前不能加载的bean，
    //主要用在web容器的拦截器里，所以这里可以忽略，因为肯定是不存在的
   if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
      throw new BeanCurrentlyInCreationException(beanName);
   }
}
复制代码

if 的第一个条件可以忽略，第二个条件是尝试将beanName放入正在创建的单例名字列表里，添加失败则会抛异常，因为bean的正常创建流程按理是进入不到这里的，如果beanName出现在正在创建的名字列表中则表明在本次操作之前对于同一个bean的创建已经在进行了，doGetBean的时候第一步从三级缓存中获取Bean实例的时候就应该已经获取到了。

在并发场景下，两个线程在三级缓存里都没获取到单例，就会来到这里。

再回到上面的getSingleton方法，接下来就可以去创建并获取bean实例了： newSingleton设置为true方便后续调用addSingleton来添加一级缓存。

异常处理跳过，看一下finally： 进入afterSingletonCreation：

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

由于bean实例已经创建完毕了，所以会从正在创建的bean名字列表singletonsCurrentlyInCreation中移除beanName

回到外面，接下来就会去判断是否是新创建的单例，之前标志位已经是true了，进入addSingleton：

protected void addSingleton(String beanName, Object singletonObject) {
   synchronized (this.singletonObjects) {
      // Bean实例完成创建之后，只保留一级缓存以及注册beanName的顺序，其余的清除
      this.singletonObjects.put(beanName, singletonObject);
      this.singletonFactories.remove(beanName);
      this.earlySingletonObjects.remove(beanName);
      this.registeredSingletons.add(beanName);
   }
}
复制代码

执行完addSingleton之后就会返回完整的bean实例了

---

回到doGetBean的创建单例的逻辑中，返回完整的bean实例之后就会执行

// 如果是普通bean，直接返回，是FactoryBean，返回他的getObject
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
复制代码

再看下scope为Prototype的创建： 进入beforePrototypeCreation：

protected void beforePrototypeCreation(String beanName) {
   Object curVal = this.prototypesCurrentlyInCreation.get();
   if (curVal == null) {
      this.prototypesCurrentlyInCreation.set(beanName);
   }
   else if (curVal instanceof String) {
      Set<String> beanNameSet = new HashSet<>(2);
      beanNameSet.add((String) curVal);
      beanNameSet.add(beanName);
      this.prototypesCurrentlyInCreation.set(beanNameSet);
   }
   else {
      Set<String> beanNameSet = (Set<String>) curVal;
      beanNameSet.add(beanName);
   }
}
复制代码

向类型为ThreadLocal的prototypesCurrentlyInCreation（当前线程正在创建的Prototype的bean名字列表）去添加该bean的记录，防止循环依赖，表示这个bean正在创建中。

回到doGetBean，之后也调用了prototypeInstance = createBean(beanName, mbd, args)，创建完之后将先前注册的正在创建中的Bean信息给抹除掉

afterPrototypeCreation(beanName);
复制代码

protected void afterPrototypeCreation(String beanName) {
   Object curVal = this.prototypesCurrentlyInCreation.get();
   if (curVal instanceof String) {
      this.prototypesCurrentlyInCreation.remove();
   }
   else if (curVal instanceof Set) {
      Set<String> beanNameSet = (Set<String>) curVal;
      beanNameSet.remove(beanName);
      if (beanNameSet.isEmpty()) {
         this.prototypesCurrentlyInCreation.remove();
      }
   }
}
复制代码

回到doGetBean，之后也会调用

bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
复制代码

获取bean实例或者beanFactory创建的bean实例

---

最后在看其他的情况， 根据生命周期范围选择实例化bean的合适方法来创建bean实例，比如request就是确保每个请求生成一个实例。

---

doGetBean最后会做一个类型检查，校验通过后返回实例：

if (requiredType != null && !requiredType.isInstance(bean)) {
   try {
      T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
      if (convertedBean == null) {
         throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
      }
      return convertedBean;
   }
   catch (TypeMismatchException ex) {
      if (logger.isTraceEnabled()) {
         logger.trace("Failed to convert bean '" + name + "' to required type '" +
               ClassUtils.getQualifiedName(requiredType) + "'", ex);
      }
      throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
   }
}
return (T) bean;
复制代码