通过BeanPostProcessor理解Spring中Bean的生命周期

通过BeanPostProcessor理解Spring中Bean的生命周期及AOP原理

Spring源码解析(十一)Spring扩展接口InstantiationAwareBeanPostProcessor解析

Spring bean的生命周期

Spring作为一个优秀的框架，拥有良好的可扩展性。Spring对对象的可扩展性主要就是依靠InstantiationAwareBeanPostProcessor和BeanPostProcessor来实现的。

• InstantiationAwareBeanPostProcessor 主要是作用于实例化阶段。
• BeanPostProcessor 主要作用与 初始化阶段。

注册BeanPostProcessor

InstantiationAwareBeanPostProcessor代表了Spring的另外一段生命周期：实例化。先区别一下Spring Bean的实例化和初始化两个阶段的主要作用：

1、实例化—-实例化的过程是一个创建Bean的过程，即调用Bean的构造函数，单例的Bean放入单例池中

2、初始化—-初始化的过程是一个赋值的过程，即调用Bean的setter，设置Bean的属性

之前的BeanPostProcessor作用于过程（2）前后，现在的InstantiationAwareBeanPostProcessor则作用于过程（1）前后；

InstantiationAwareBeanPostProcessor接口继承BeanPostProcessor接口，它内部提供了3个方法，再加上BeanPostProcessor接口内部的2个方法，所以实现这个接口需要实现5个方法。InstantiationAwareBeanPostProcessor接口的主要作用在于目标对象的实例化过程中需要处理的事情，包括实例化对象的前后过程以及实例的属性设置

public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
   this();
   register(annotatedClasses);
   refresh();
}

applicationContext构造方法中调用refresh()方法

refresh() 方法中这里主要关心两个放

• registerBeanPostProcessors(beanFactory); 注册BeanPostProcessor
• finishBeanFactoryInitialization(beanFactory); 注册余下的Singletions Bean

public void refresh() throws BeansException, IllegalStateException {
    // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);
                    // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);
    }

public static void registerBeanPostProcessors(
      ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

   String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

   // Register BeanPostProcessorChecker that logs an info message when
   // a bean is created during BeanPostProcessor instantiation, i.e. when
   // a bean is not eligible for getting processed by all BeanPostProcessors.
   int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
   beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

   // Separate between BeanPostProcessors that implement PriorityOrdered,
   // Ordered, and the rest.
   List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
   List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
   List<String> orderedPostProcessorNames = new ArrayList<>();
   List<String> nonOrderedPostProcessorNames = new ArrayList<>();
   for (String ppName : postProcessorNames) {
      if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
         BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
         priorityOrderedPostProcessors.add(pp);
         if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
         }
      }
      else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
         orderedPostProcessorNames.add(ppName);
      }
      else {
         nonOrderedPostProcessorNames.add(ppName);
      }
   }

   // First, register the BeanPostProcessors that implement PriorityOrdered.
   sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

   // Next, register the BeanPostProcessors that implement Ordered.
   List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
   for (String ppName : orderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
      orderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
         internalPostProcessors.add(pp);
      }
   }
   sortPostProcessors(orderedPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, orderedPostProcessors);

   // Now, register all regular BeanPostProcessors.
   List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
   for (String ppName : nonOrderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
      nonOrderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
         internalPostProcessors.add(pp);
      }
   }
   registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

   // Finally, re-register all internal BeanPostProcessors.
   sortPostProcessors(internalPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, internalPostProcessors);

   // Re-register post-processor for detecting inner beans as ApplicationListeners,
   // moving it to the end of the processor chain (for picking up proxies etc).
   beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}

通过beanFactory.getBeanNamesForType来获取所有BeanPostProcessor。

BeanPostProcessor按优先级分为PriorityOrdered，Ordered和其他的，对他们分别进行操作。

• 先beanFactory.getBean进性实例化，
• 再使用sortPostProcessors() 进行排序，
• 最后registerBeanPostProcessors()进行注册。

BeanFactory.getBean()（注册Bean）

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

   final String beanName = transformedBeanName(name);
   Object bean;
   //缓存
   // Eagerly check singleton cache for manually registered singletons.
   Object sharedInstance = getSingleton(beanName);
   if (sharedInstance != null && args == null) {
     bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
   }

   else {
     // Fail if we're already creating this bean instance:
     // We're assumably within a circular reference.
     //判断循环引用,抛异常
     if (isPrototypeCurrentlyInCreation(beanName)) {
        throw new BeanCurrentlyInCreationException(beanName);
     }

     // Check if bean definition exists in this factory.
     BeanFactory parentBeanFactory = getParentBeanFactory();
     // this.beanDefinitionMap.containsKey(beanName); 就是判断有没有BeanDefinition
     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 {
           // No args -> delegate to standard getBean method.
           return parentBeanFactory.getBean(nameToLookup, requiredType);
        }
     }

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

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

        // Guarantee initialization of beans that the current bean depends on.
        // 获取bean的依赖，实例化bean前先实例化依赖。
        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);
              }
           }
        }
        //创建实例
        // 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;
              }
           });
           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);
           }
        }
     }
   }

   // 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;
     }
   }
   return (T) bean;
}

• 先getSingleton()从缓存中获取Bean，如果没有则创建。
• 创建过程先检查有无循环依赖，有则抛出异常。
• 实例化bean前先实例化所依赖的对象。

createBean，调用的开端

@Override
    protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
    // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
            if (bean != null) {
                return bean;
            }
    //省略....
    Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    return beanInstance;

    }

上面代码里面看到,在执行doCreateBean之前有resolveBeforeInstantiation方法；doCreateBean是创建bean的方法；
resolveBeforeInstantiation是 判断执行InstantiationAwareBeanPostProcessor.postProcessBeforeInstantiation的接方法实现;
下面看看执行的依据：

执行 postProcessBeforeInstantiation方法的时机

/**
     * Apply before-instantiation post-processors, resolving whether there is a
     * before-instantiation shortcut for the specified bean.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @return the shortcut-determined bean instance, or {@code null} if none
     */
    protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
        Object bean = null;
        //如果beforeInstantiationResolved还没有设置或者是false（说明还没有需要在实例化前执行的操作）
        if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
            // 判断是否有注册过InstantiationAwareBeanPostProcessor类型的bean
            if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
                Class<?> targetType = determineTargetType(beanName, mbd);
                if (targetType != null) {
                    //执行
                    bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                    if (bean != null) {
                        bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                    }
                }
            }
            mbd.beforeInstantiationResolved = (bean != null);
        }
        return bean;
    }

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);
                //只要有一个result不为null；后面的所有 后置处理器的方法就不执行了，直接返回(所以执行顺序很重要)
                if (result != null) {
                    return result;
                }
            }
        }
        return null;
    }

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

        Object result = existingBean;
        for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
            result = beanProcessor.postProcessAfterInitialization(result, beanName);
            //如果返回null；后面的所有 后置处理器的方法就不执行，直接返回(所以执行顺序很重要)
            if (result == null) {
                return result;
            }
        }
        return result;
    }

上面代码说明：

如果postProcessBeforeInstantiation方法返回了Object是null;那么就直接返回，调用doCreateBean方法();
如果postProcessBeforeInstantiation返回不为null;说明修改了bean对象;然后这个时候就立马执行postProcessAfterInitialization方法(注意这个是初始化之后的方法,也就是通过这个方法实例化了之后，直接执行初始化之后的方法;中间的实例化之后 和 初始化之前都不执行);
在调用postProcessAfterInitialization方法时候如果返回null;那么就直接返回，调用doCreateBean方法();(初始化之后的方法返回了null,那就需要调用doCreateBean生成对象了)
在调用postProcessAfterInitialization时返回不为null;那这个bean就直接返回给ioc容器了 初始化之后的操作 是这里面最后一个方法了；

通过上面的描述，我们其实可以在这里生成一个代理类：原文

postProcessAfterInstantiation调用的地方

代码往后面执行走到了populateBean里面；这个主要是给bean填充属性的;实例化已经在 pupulateBean之前已经完成了

//实例化bean；选择不同策略来实例化bean
    instanceWrapper = createBeanInstance(beanName, mbd, args);

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {


    //省略。。。。
    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    //执行postProcessAfterInstantiation方法
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        continueWithPropertyPopulation = false;
                        break;
                    }
                }
            }
        }
//省略....

//下面的代码是判断是否需要执行postProcessPropertyValues；改变bean的属性
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

        if (hasInstAwareBpps || needsDepCheck) {
            PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            if (hasInstAwareBpps) {
                for (BeanPostProcessor bp : getBeanPostProcessors()) {
                    if (bp instanceof InstantiationAwareBeanPostProcessor) {
                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvs == null) {
                            return;
                        }
                    }
                }
            }
            if (needsDepCheck) {
                checkDependencies(beanName, mbd, filteredPds, pvs);
            }
        }

//这里才是正在讲 属性值  真正的设置的我们的实例对象里面；之前postProcessPropertyValues这个还只是单纯的改变PropertyValues
//最后还是要通过PropertyValues 设置属性到实例对象里面的
        applyPropertyValues(beanName, mbd, bw, pvs);

}

这个postProcessAfterInstantiation返回值要注意，因为它的返回值是决定要不要调用postProcessPropertyValues方法的其中一个因素（因为还有一个因素是mbd.getDependencyCheck()）；如果该方法返回false,并且不需要check，那么postProcessPropertyValues就会被忽略不执行；如果返回true，postProcessPropertyValues就会被执行

postProcessPropertyValues调用的地方

原文

postProcessPropertyValues修改属性，但是要注意postProcessAfterInstantiation返回true；

InstantiationAwareBeanPostProcessor总结

1.   InstantiationAwareBeanPostProcessor接口继承BeanPostProcessor接口，它内部提供了3个方法，再加上BeanPostProcessor接口内部的2个方法，所以实现这个接口需要实现5个方法。InstantiationAwareBeanPostProcessor接口的主要作用在于目标对象的实例化过程中需要处理的事情，包括实例化对象的前后过程以及实例的属性设置
2.   postProcessBeforeInstantiation方法是最先执行的方法，它在目标对象实例化之前调用，该方法的返回值类型是Object，我们可以返回任何类型的值。由于这个时候目标对象还未实例化，所以这个返回值可以用来代替原本该生成的目标对象的实例(比如代理对象)。如果该方法的返回值代替原本该生成的目标对象，后续只有postProcessAfterInitialization方法会调用，其它方法不再调用；否则按照正常的流程走
3.   postProcessAfterInstantiation方法在目标对象实例化之后调用，这个时候对象已经被实例化，但是该实例的属性还未被设置，都是null。因为它的返回值是决定要不要调用postProcessPropertyValues方法的其中一个因素（因为还有一个因素是mbd.getDependencyCheck()）；如果该方法返回false,并且不需要check，那么postProcessPropertyValues就会被忽略不执行；如果返回true，postProcessPropertyValues就会被执行
4.   postProcessPropertyValues方法对属性值进行修改(这个时候属性值还未被设置，但是我们可以修改原本该设置进去的属性值)。如果postProcessAfterInstantiation方法返回false，该方法可能不会被调用。可以在该方法内对属性值进行修改
5.   父接口BeanPostProcessor的2个方法postProcessBeforeInitialization和postProcessAfterInitialization都是在目标对象被实例化之后，并且属性也被设置之后调用的
6.   Instantiation表示实例化，Initialization表示初始化。实例化的意思在对象还未生成，初始化的意思在对象已经生成