关于java:Spring源码之Bean的加载二

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bean 的加载(二)

之前文章次要对 getBean 办法进行简略的介绍,和 FactoryBean 的作用,以及是如何从缓存中获取 bean。本文持续解说 bean 的加载流程。

从 bean 的实例中获取对象

在 getBean 办法里,getObjectForBeanInstance()是个罕用的办法,无论是从缓存中获取 bean 还是依据不同 scope 策略来加载 bean。总而言之,咱们在获取到 bean 实例后第一步就是调用这个办法来检测正确性,其实就是检测以后 bean 是否为 FactoryBean 类型的 bean,如果是则调用 FactoryBean 实例的 getObject()作为返回值。

须要留神的是无论是缓存中获取到的 bean 还是通过 scope 策略加载的 bean 都是 原始的 bean 状态,咱们须要的是工厂 bean 中定义 factory-method 办法中返回的 bean,而 getObjectForBeanInstance 办法就是实现这个工作的。

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

   // 如果指定的 name 是工厂相干以 "&" 为前缀, 并且 beanInstance 又不是 FactoryBean 则校验不通过
   if (BeanFactoryUtils.isFactoryDereference(name)) {if (beanInstance instanceof NullBean) {return beanInstance;}
      if (!(beanInstance instanceof FactoryBean)) {throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
      }
      if (mbd != null) {mbd.isFactoryBean = true;}
      return beanInstance;
   }

   // 当初咱们有了这个 bean 实例, 这个实例可能是 FactoryBean, 也可能是失常的 bean
   // 如果是 FactoryBean 的话, 咱们应用它创立实例, 但如果是用户想要间接获取工厂实例而不是工厂的 getObject 办法则须要在 BeanName 前加上 "&"
   if (!(beanInstance instanceof FactoryBean)) {return beanInstance;}

   // 加载 FactoryBean
   Object object = null;
   if (mbd != null) {mbd.isFactoryBean = true;}
   else {
      // 尝试从缓存中加载 bean
      object = getCachedObjectForFactoryBean(beanName);
   }
   if (object == null) {
      // 执行到这里表明 beanInstance 肯定是一个 FactoryBean
      FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
      // containsBeanDefinition 检测 BeanDefinitionMap 中也就是在所有一键加载的类中检测是否定义 beanName
      if (mbd == null && containsBeanDefinition(beanName)) {
         // 将 GenericBeanDefinition 转换为 RootBeanDefinition, 如果指定 BeanName 是子 bean 的话同时会合并父类的属性
         mbd = getMergedLocalBeanDefinition(beanName);
      }
      // 是否是用户定义的而不是应用程序自身定义的
      boolean synthetic = (mbd != null && mbd.isSynthetic());
      object = getObjectFromFactoryBean(factory, beanName, !synthetic);
   }
   return object;
}

咱们先看一下 getObjectForBeanInstance 次要做了什么:

  • 对 FactoryBean 正确性进行验证
  • 对非 FactoryBean 不作任何解决
  • 对 bean 进行转换
  • 将从 Factory 中解析 bean 的工作委托给了getObjectFromFactoryBean

该段代码大多都是辅助代码,真正的外围代码委托给了getObjectFromFactoryBean

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {if (factory.isSingleton() && containsSingleton(beanName)) {synchronized (getSingletonMutex()) {Object object = this.factoryBeanObjectCache.get(beanName);
         if (object == null) {
               // 重点办法 doGetObjectFromFactoryBean
            object = doGetObjectFromFactoryBean(factory, beanName);
            //... 省略
   }
   else {
      // 重点办法 doGetObjectFromFactoryBean
      Object object = doGetObjectFromFactoryBean(factory, beanName);
      if (shouldPostProcess) {
         try {object = postProcessObjectFromFactoryBean(object, beanName);
         }
         catch (Throwable ex) {throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
         }
      }
      return object;
   }
}

这段代码就是返回的 bean 如果是单例的则须要保障全局惟一,也正因为是单例的所以不用反复创立,能够用缓存来进步性能。

咱们进入 doGetObjectFromFactoryBean 办法中。

private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
      throws BeanCreationException {

   Object object;
   try {
      // 是否须要权限校验
      if (System.getSecurityManager() != null) {AccessControlContext acc = getAccessControlContext();
         try {object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
         }
         catch (PrivilegedActionException pae) {throw pae.getException();
         }
      }
      else {// 间接调用 getObject 办法(重点)
         object = factory.getObject();}
   }
   catch (FactoryBeanNotInitializedException ex) {throw new BeanCurrentlyInCreationException(beanName, ex.toString());
   }
   catch (Throwable ex) {throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
   }

   // Do not accept a null value for a FactoryBean that's not fully
   // initialized yet: Many FactoryBeans just return null then.
   if (object == null) {if (isSingletonCurrentlyInCreation(beanName)) {
         throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject");
      }
      object = new NullBean();}
   return object;
}

在该办法中,咱们终于看到了想要看到的代码,也就是object = factory.getObject()。之前咱们曾经讲过了 FactoryBean 的调用办法,如果 bean 是 FactoryBean 类型,则当提取 bean 时提取的并不是 factoryBean 而是 factoryBean 的 getObject 办法的返回值。

获取单例

之前咱们说过如果缓存中不存在曾经加载的 bean 则须要重头开始 bean 的加载,在 Spring 中应用 getSingleton 的重载办法实现了 bean 的加载过程。

getBean 办法:

// 实例化依赖的 bean 后就能够实例化 mbd 自身了
// 如果 BeanDefinition 为单例
if (mbd.isSingleton()) {
   // 创立 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) {
         // 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,可能它曾经存在了,所以将其销毁
         destroySingleton(beanName);
         throw ex;
      }
   });
   // 如果是一般 bean,间接返回,如果是 FactoryBean,则返回它的 getObject
   bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

进入 getSingleton 办法:

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "Bean name must not be null");
   // 加锁
   synchronized (this.singletonObjects) {
      // 首先查看对应的 bean 是否曾经加载过,
      Object singletonObject = this.singletonObjects.get(beanName);
      // 如果为空则须要进行 singleton 的 bean 初始化
      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 + "'");
         }
         // 代码(1)
         beforeSingletonCreation(beanName);
         boolean newSingleton = false;
         boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
         if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<>();
         }
         try {
            // 通过回调形式获取 bean 实例。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;}
            // 代码(2)
            afterSingletonCreation(beanName);
         }
         if (newSingleton) {
            // 退出缓存 代码(3)addSingleton(beanName, singletonObject);
         }
      }
      return singletonObject;
   }
}

上述代码应用了回调办法,在单例创立的前后做了些筹备及解决操作,而真正获取单例 bean 的办法是在 ObjectFactory 类型的实例 singletonFactory 中。咱们先看一下 getSingleton 办法次要做了什么:

  1. 查看缓存是否曾经加载过
  2. 若没有加载,则记录 beanName 为正在加载状态
  3. 加载单例前记录加载状态,代码(1)
protected void beforeSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {throw new BeanCurrentlyInCreationException(beanName);
   }
}

这个办法次要做的就是记录加载状态,this.singletonsCurrentlyInCreation.add(beanName)将以后正要创立的 bean 记录在缓存中,这样就能够对循环依赖进行检测。

  1. 获取 bean 实例
  2. 调用加载单例后的解决办法,代码(2)
protected void afterSingletonCreation(String beanName) {if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {throw new IllegalStateException("Singleton'" + beanName + "'isn't currently in creation");
   }
}

和上述相似,只是这里是从缓存中移除 bean 的正在加载状态。

  1. 将 bean 退出缓存,并删除加载 bean 过程中所记录的各种辅助状态。
protected void addSingleton(String beanName, Object singletonObject) {synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, singletonObject);
      this.singletonFactories.remove(beanName);
      this.earlySingletonObjects.remove(beanName);
      this.registeredSingletons.add(beanName);
   }
}
  1. 返回处理结果

当初咱们曾经理解了 bean 的逻辑架构,然而 bean 的加载逻辑是在传入 ObjectFactory 类型的参数 singletonFactory 中定义的。

// 创立 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) {
      // 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,可能它曾经存在了,所以将其销毁
      destroySingleton(beanName);
      throw ex;
   }
});

咱们进入 createBean 办法中持续查看。

筹备创立 bean

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
      throws BeanCreationException {if (logger.isTraceEnabled()) {logger.trace("Creating instance of bean'" + beanName + "'");
   }
   RootBeanDefinition mbdToUse = mbd;

   // 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.
   // 锁定 class, 依据设置的 class 属性或者依据 className 来解析 Class
   Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
   if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {mbdToUse = new RootBeanDefinition(mbd);
      mbdToUse.setBeanClass(resolvedClass);
   }

   // Prepare method overrides.
   // 验证及筹备笼罩的办法
   try {mbdToUse.prepareMethodOverrides();
   }
   catch (BeanDefinitionValidationException ex) {throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
            beanName, "Validation of method overrides failed", ex);
   }

   try {
      // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
      // 给 BeanPostProcessors 一个机会来返回代理用于代替真正的实例
      Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
      if (bean != null) {return bean;}
   }
   catch (Throwable ex) {throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
            "BeanPostProcessor before instantiation of bean failed", ex);
   }

   try {// 代码(1)
      Object beanInstance = doCreateBean(beanName, mbdToUse, args);
      if (logger.isTraceEnabled()) {logger.trace("Finished creating instance of bean'" + beanName + "'");
      }
      return beanInstance;
   }
   catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
      // A previously detected exception with proper bean creation context already,
      // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
      throw ex;
   }
   catch (Throwable ex) {
      throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
   }
}
  1. 依据设置的 class 属性或者依据 className 来解析 Class
  2. 对 override 属性进行标记及验证

咱们晓得在 Spring 的配置中并没有相似于 override-methdo 之类的配置,那么该办法的作用是什么?

咱们之前说过 Spring 配置中存在 lookup-method 和 replace-method 的,这两个配置的加载就是将配置对立寄存在 BeanDefinition 中的 methodOverrides 属性中,而这个函数其实也就是针对这两个配置的。

  1. 利用初始化前的后处理器,解析指定 bean 是否存在初始化前的短路操作
  2. 创立 bean

咱们先看一下 override 属性标记及验证的逻辑实现

解决 override 属性

进入 prepareMethodOverrides 办法:

public void prepareMethodOverrides() throws BeanDefinitionValidationException {
   // Check that lookup methods exist and determine their overloaded status.
   if (hasMethodOverrides()) {getMethodOverrides().getOverrides().forEach(this::prepareMethodOverride);
   }
}
protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
   // 获取对应类中对应办法名的个数
   int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
   if (count == 0) {
      throw new BeanDefinitionValidationException("Invalid method override: no method with name'" + mo.getMethodName() +
            "'on class [" + getBeanClassName() + "]");
   }
   else if (count == 1) {
      // Mark override as not overloaded, to avoid the overhead of arg type checking.
      // 标记 MethodOverride 暂未被笼罩, 防止参数类型查看的开销
      mo.setOverloaded(false);
   }
}

方才说到 lookup-method 和 replace-method 两个配置性能是对立寄存在 BeanDefinition 中的 methodOverrides 属性中,这两个性能实现原理就是在 bean 实例化的时候如果检测到存在 methodOverrides 属性,就会动静地为以后 bean 生成代理并应用对应的拦截器对 bean 做加强解决。

须要提到的是,对于办法的匹配来说,如果一个类中有多个重载办法则须要依据参数类型进行匹配。如果类中只有办法只有一个那么就设置该办法没有被重载,这样在后续时候能够间接应用找到的办法,不须要进行办法的参数匹配验证,而且还能够提前对办法存在性进行验证,所谓一举两得。

实例化的前置解决

在调用 doCreateBean 之前,还应用了 resolveBeforeInstantiation(beanName, mbdToUse)办法对 BeanDefinition 中的属性做些前置解决。

Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {return bean;}

该段代码的重点就是这个 if 条件,当处理结果不为 null 的时候就会跳过后续 bean 的创立间接返回后果。咱们熟知的 AOP 性能就是基于这里判断的。

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
        Object bean = null;
        // 如果还没被解析过,则解析
        if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
            // Make sure bean class is actually resolved at this point.
            if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {Class<?> targetType = determineTargetType(beanName, mbd);
                if (targetType != null) {// 代码(1)
                    bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                    if (bean != null) {// 代码(2)
                        bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                    }
                }
            }
            mbd.beforeInstantiationResolved = (bean != null);
        }
        return bean;
}

该办法中的重点就是这两个办法:applyBeanPostProcessorsBeforeInstantiation、applyBeanPostProcessorsAfterInitialization。这两个办法实现都很简略,就是对后处理器中所有的 InstantiationAwareBeanPostProcessor 类型的后处理器进行 postProcessBeforeInstantiation 办法和 BeanPostProcessor 的 postProcessAfterInitialization 办法的调用。

1. 实例化前的后处理器利用

bean 的实例化前调用,也就是将 AbstractBeanDefinition 转换为 BeanWrapper 前的解决。给子类一个批改 BeanDefinition 的机会,也就是调用这个办法后 bean 就有可能产生变动,有可能是通过解决的代理 bean,也可能是 cglib 生成的。后续会具体介绍,当初只须要明确在 bean 实例化前会调用后处理器的办法进行解决。

protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
         Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
         if (result != null) {return result;}
      }
   }
   return null;
}

2. 实例化后的后处理器利用

Spring 的规定是在 bean 初始化后尽可能保障将注册的后处理器 postProcessAfterInitialization 办法利用到 bean 中,如果返回的 bean 不为空,则不须要再经验 bean 的创立过程。

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
      throws BeansException {

   Object result = existingBean;
   for (BeanPostProcessor processor : getBeanPostProcessors()) {Object current = processor.postProcessAfterInitialization(result, beanName);
      if (current == null) {return result;}
      result = current;
   }
   return result;
}

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