在 createBean 方法中, doCreateBean 方法前, 呼叫了這樣一句代碼:

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean

// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
// 在 bean實體化之前 應用后置處理,如果后置處理回傳的bean不為空,則直接回傳
// 回傳的是一個寡婦物件, 屬性什么的, spring不會去維護
// spring不推薦開發人員使用這個介面 InstantiationAwareBeanPostProcessor
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
    return bean;
}

這是一個擴展方法, 如果回傳了 bean , 那么就不走后面的創建流程了. 

要注意這里的執行時機: 物件實體化之前執行

@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
    Object bean = null;
    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) {
                bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                if (bean != null) {
                    bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                }
            }
        }
        mbd.beforeInstantiationResolved = (bean != null);
    }
    return bean;
}

InstantiationAwareBeanPostProcessor 這個后置處理器, 不但定義了 實體化之前的處理器, 還定義了  實體化之后的處理器. 

所以, 這里如果回傳的bean不為null, 還需要執行實體化之后的處理器, 來保證流程的完整性. 

在這里, 就不看實體化之后的回呼了.

直接看 applyBeanPostProcessorsBeforeInstantiation() 方法:

@Nullable
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;
}

首先來看一下, 這個介面:

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {

    @Nullable
    default Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        return null;
    }

 
    default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
        return true;
    }

   
    @Nullable
    default PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName)
            throws BeansException {

        return null;
    }

  
    @Deprecated
    @Nullable
    default PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {

        return pvs;
    }

}

注意到, 這個介面是 繼承了 BeanPostProcessor , 表示他是一個 bean 的后置處理器. 和前面章節出現過的 BeanFactory 后置處理器 是不同的

通過斷點除錯, 不難發現, 6個后置處理器中, 滿足條件的就三個, 按照執行順序排列為: 

1.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor

2.CommonAnnotationBeanPostProcessor

3.AutowiredAnnotationBeanPostProcessor

那接下來, 就需要看一下, 里面分別干了什么

ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor

org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessorAdapter#postProcessBeforeInstantiation

@Override
@Nullable
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
    return null;
}

從這里看, 他本身都沒有實作或重寫這個方法, 而是由它的父類去實作的.

里面啥也沒干, 直接回傳了 null.

CommonAnnotationBeanPostProcessor

@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
    return null;
}

啥也沒干

AutowiredAnnotationBeanPostProcessor

org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessorAdapter#postProcessBeforeInstantiation

@Override
@Nullable
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
    return null;
}

他本身也沒有實作這個方法, 而是父類去實作的.

里面也是啥也沒干.

到這里, 這個后置處理器就執行完了, 大家都沒干事情, 都是直接回傳的 null. 所以, spring后續流程還是要走的

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