序言

之前文章有介绍采用FactoryBean的方式创建对象，以及使用反射创建对象。
这篇文章继续介绍Spring中创建Bean的形式之一——自定义BeanPostProcessor。
之前在介绍BeanPostProcessor的文章中有提到，BeanPostProcessor接口的实现中有一个InstantiationAwareBeanPostProcessor接口
而在Spirng源码中的doGetBean方法中就有针对该接口的判断逻辑，如果有类实现了InstantiationAwareBeanPostProcessor则可以在对应的方法中进行Bean对象的创建。

源码
去除无用代码，我们这里主要看resolveBeforeInstantiation方法。如果通过resolveBeforeInstantiation创建了Bean示例，则return该Bean 不在继续往下走其他流程。

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		RootBeanDefinition mbdToUse = mbd;
		//锁定class，根据设置的class属性或者根据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);
		}
		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来替代真正的实例，
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
			if (bean != null) {
				return bean;
			}
		}
	}

resolveBeforeInstantiation
如果满足if条件，则会走applyBeanPostProcessorsBeforeInstantiation方法进行对象的创建。

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
		Object bean = null;
		//如果beforeInstantiationResolved值为null或者true，那么表示尚未被处理，进行后续的处理
		if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
			// Make sure bean class is actually resolved at this point.
			// 确保Bean确实在此处进行处理
			// 判断当前mbd是否是合成的，只有在实现aop的时候synthetic的值才为true，并且是否实现了InstantiationAwareBeanPostProcessor接口
			if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
				//确定目标类型
				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;
	}

applyBeanPostProcessorsBeforeInstantiation
获取所有实现了BeanPostProcessors的类，并强转成InstantiationAwareBeanPostProcessor 类型，调用postProcessBeforeInstantiation方法进行Bean的创建。

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;
	}

测试类

我们针对上面的源码流程，扩展自定义的BeanPostProcessor进行类的创建。我们这里依然采用上篇文章提到的Cglib动态代理的方式创建我们的对象。

BeforeInstantiation
BeforeInstantiation 是我们想要最终创建的实例对象。

public class BeforeInstantiation {

    public void doSomeThing(){
        System.out.println("执行do some thing....");
    }
}

MyMethodInterceptor
拦截器类，对目标方法进行中间拦截，上篇文章在讲lookup-method标签时，里面采用Cglib动态代理生成对象的固定写法，创建Enhancer对象并设置CallBack。
因为我们这里也采用Cglib的方式进行对象的创建，所以也需要设置CallBack。

public class MyMethodInterceptor implements MethodInterceptor {
    @Override
    public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
        System.out.println("目标方法执行之前：" + method);
        Object o1 = methodProxy.invokeSuper(o, objects);
        System.out.println("目标方法执行之后：" + method);
        return o1;
    }
}

MyInstantiationAwareBeanPostProcessor
自定义InstantiationAwareBeanPostProcessor接口的扩展类，调用postProcessBeforeInstantiation方法，采用Cglib的固定写法实现对BeforeInstantiation 对象的创建。
其余方法因为用不到，所以直接return对应类型，没有具体业务逻辑。

public class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor {
	@Override
	public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {

		if (beanClass == BeforeInstantiation.class) {
			Enhancer enhancer = new Enhancer();
			enhancer.setSuperclass(beanClass);
			enhancer.setCallback(new MyMethodInterceptor());
			BeforeInstantiation beforeInstantiation = (BeforeInstantiation) enhancer.create();
			return beforeInstantiation;
		}
		return null;
	}


	@Override
	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}

	@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}

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

	@Override
	public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) throws BeansException {
		return pvs;
	}

	@Override
	public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
		return pvs;
	}

resolveBeforeInstantiation.xml
xml中声明MyInstantiationAwareBeanPostProcessor对象和BeforeInstantiation对象。

<beans xmlns="http://www.springframework.org/schema/beans"
	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

	<bean id="myInstantiationAwareBeanPostProcessor" class="org.springframework.resolveBeforeInstantiation.MyInstantiationAwareBeanPostProcessor" />

	<bean id="beforeInstantiation" class="org.springframework.resolveBeforeInstantiation.BeforeInstantiation"/>
</beans>

main
执行refresh()主流程调用registerBeanPostProcessors方法创建MyInstantiationAwareBeanPostProcessor后，会调用addBeanPostProcessor方法将hasInstantiationAwareBeanPostProcessors()变量设置为true，当beforeInstantiation创建时，if 判断 整体为 true，调用我们自定义的扩展类创建beforeInstantiation对象。

	public static void main(String[] args) {
		ApplicationContext ac = new ClassPathXmlApplicationContext("resolveBeforeInstantiation.xml");
		BeforeInstantiation beanInstantiation = (BeforeInstantiation)ac.getBean("beforeInstantiation");
		beanInstantiation.doSomething();
	}