spring中BeanPostProcessor之一：InstantiationAwareBeanPostProcessor（02）

在上篇博客中写道了bean后置处理器InstantiationAwareBeanPostProcessor，只介绍了其中一个方法的作用及用法，现在来看postProcessBeforeInstantiation方法。

一、概述

postProcessBeforeInstantiation方法定义在InstantiationAwareBeanPostProcessor接口中，方法的定义如下，

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

从上面的代码中可以看到该方法默认返回null。

二、详述

postProcessBeforeInstantiation方法是用来做什么的，在看源码的过程中，在createBean方法中找到了该方法的调用，下面只贴出相关代码，

try {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            //1、调用beanPostProcessor即bean的后置处理器，这里会调用2次后置处理器
            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);
        }

在resolveBeforeInstantiation方法中进行了调用，resolveBeforeInstantiation方法返回值如果不为null，则该方法直接返回bean，也就是说resolveBeforeInstantiation方法至关重要，下面是resolveBeforeInstantiation方法，

@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) {
                    //如果是InstantiationAwareBeanPostProcessor的实例，则执行其postProcessBeforeInstantiation方法
                    bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                    //如果上面的postProcessBeforeInstantiation方法返回值不为null，则执行所有beanPostProcessor的postProcessAfterInitialization方法
                    //bean不为null，则说明postProcesBeforeInstantiation方法中的返回值是一个不为null的对象
                    if (bean != null) {
                        bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                    }
                }
            }
            mbd.beforeInstantiationResolved = (bean != null);
        }
        return bean;
    }

在resolveBeforeInstantiation方法中首先调用了applyBeanPostProcessorsBeforeInstantiation方法，该方法中便会调用InstantiationAwareBeanPostProcessor接口中的postProcessBeforeInstantiation方法，且如果applyBeanPostPorcessorsBeforeInstantiation方法返回值不为null，才会调用applyBeanPostProcessAfterIntialization方法，下面先看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;
    }

从上面方法的定义看到，该方法会遍历benaFactory中的beanPostProcessor，并且判断是否为InstantiationAwareBeanPostPrecessor的类型，如果是执行其postProcessBeforeInstantiation方法，这里默认注册的beanPostProcessor该方法的返回值均为null。稍后自定义一个BeanPostProcessor实现InstantiationAwareBeanPostProcessor接口。

下面看applyBeanPostProcessAfterIntializtion方法，

@Override
    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;
    }

改方法的逻辑是遍历beanFactory中的所有的beanPostProcessor，执行其postProcessAfterInitialization方法，该方法定义在BeanPostProcessor接口中，默认返回bean，如下，

@Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

从上面可以看出默认返回的是bean参数的值，也就是如果该方法未实现则默认返回参数中的bean。

再次回到resolveBeforeInstantiation方法，再看其逻辑有以下几种方法返回值的组合，

1、applyBeanPostProcessBeforeInstantiation返回值为null，则resolveBeforeInstantiation方法返回null；

2、applyBeanPostProcessBeforeInstantiation返回值bean不为null，applyBeanPostProcessAfterInitialization方法返回值为null，则resolveBeforeInstantiationf方法返回值为bean；

3、applyBeanPostProcessBeforeInstantiation返回值bean不为null，applyBeanPostProcessAfterInitialization方法返回值bean1不为为null，则resolveBeforeInstantiationf方法返回值为bean1；

从resolveBeforeInstantiation方法分析，该方法的返回值，直接决定了createBean方法的返回值，也就是说applyBeanPostProcessBeforeInstantiation方法返回的bean不为null，下面的方法不会执行。

再来看调用resolveBeforeInstantiation方法时的注释

// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);

大体意思时给BeanPostProcessor一个机会返回代理对象而不是目标对象的实例，所以这里resolveBeforeInstantiation方法返回的必然时一个代理对象（JDK和CGLib）。看下面的例子

自定义的BeanPostProcessor实现了InstantiationAwareBeanPostProcessor

package cn.com.my.test;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor;
import org.springframework.stereotype.Component;

import net.sf.cglib.proxy.Enhancer;
@Component
public class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        // TODO Auto-generated method stub
        
        if("userService".equals(beanName)) {
            
            UserService us=(UserService)bean;
            
            Enhancer enhancer = new Enhancer();
            //设置目标类的字节码文件
            enhancer.setSuperclass(UserService.class);
            //设置回调函数
            enhancer.setCallback(new MyMethodInterceptor());
            
            //这里的creat方法就是正式创建代理类
            UserService proxyUs = (UserService)enhancer.create();
            return proxyUs;
        }
        return InstantiationAwareBeanPostProcessor.super.postProcessBeforeInitialization(bean, beanName);
    }
}

当beanName等于userUservice时返回的是经过cglib代理后的对象。在MyInstantiationAwareBeanPostProcessor类中仅实现了postProcessBeforeInitialization方法，未实现postProcessAfterInitialization方法，所以resolveBeforeInstantiation方法的返回值即未postProcessBeforeInitialization方法的返回值，在上面的类中就是使用cglib代理后的UserService实例。

代理类MyMethodInterceptor，实现cglib的MethodInterceptor接口

package cn.com.my.test;

import java.lang.reflect.Method;

import org.springframework.cglib.proxy.MethodProxy;

import net.sf.cglib.proxy.MethodInterceptor;

public class MyMethodInterceptor implements MethodInterceptor{

   
    @Override
    public Object intercept(Object arg0, Method arg1, Object[] arg2, net.sf.cglib.proxy.MethodProxy arg3)
            throws Throwable {
        // TODO Auto-generated method stub
         Object object = arg3.invokeSuper(arg0, arg2);
         return object;
    }  
}

下面是测试类

package cn.com.my.test;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Test {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        AnnotationConfigApplicationContext ac=new AnnotationConfigApplicationContext(Config.class);
        
        UserService us=ac.getBean(UserService.class);
        System.out.println("us:"+us);
        
    }

}

看下面的结果，

us:cn.com.my.test.UserService$$EnhancerByCGLIB$$ffa582b4@5fe94a96

返回的是UserService的一个经过cglib代理后的对象。到这里发现真好强大，返回的一个代理对象。

三、适用场合

实现InstantiationAwareBeanPostProcessor接口的postProcessBeforeInitialization方法，通过返回一个代理对象的方式，达到改变目标类类型的目的。在不想改变现有类的逻辑而又想借助现有类实现其他功能，就可以使用这种方式。像AOP就是这种实现，AnnotationAwareAspectJAutoProxyCreator类便是InstantiationAwareBeanPostProcessor的一个实现。

原创不易，有不当之处，欢迎指正，谢谢！