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！

巴特西

spring中BeanPostProcessor之一：InstantiationAwareBeanPostProcessor（02）

一、概述

二、详述

三、适用场合

最新文章

热门文章